{"id":89,"date":"2021-09-16T19:28:28","date_gmt":"2021-09-16T19:28:28","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/accphysicalgeography\/chapter\/2-4-silicate-minerals-physical-geology-2nd-edition\/"},"modified":"2022-02-02T16:52:38","modified_gmt":"2022-02-02T16:52:38","slug":"2-4-silicate-minerals-physical-geology-2nd-edition","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/chapter\/2-4-silicate-minerals-physical-geology-2nd-edition\/","title":{"raw":"2.4 Silicate Minerals \u2014 Physical Geology \u2013 2nd Edition","rendered":"2.4 Silicate Minerals \u2014 Physical Geology \u2013 2nd Edition"},"content":{"raw":"<div>\r\n<div>\r\n<h1 class=\"entry-title\">2.4 Silicate Minerals<\/h1>\r\nThe vast majority of the minerals that make up the rocks of Earth\u2019s crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a variety of clay minerals. The building block of all of these minerals is the\u00a0<button class=\"glossary-term\" aria-describedby=\"92-1280\">silica tetrahedron<\/button>, a combination of four oxygen atoms and one silicon atom. As we\u2019ve seen, it\u2019s called a tetrahedron because planes drawn through the oxygen atoms form a shape with 4 surfaces (Figure 2.2.4). Since the silicon ion has a charge of 4 and each of the four oxygen ions has a charge of \u22122, the silica tetrahedron has a net charge of \u22124.\r\n\r\nIn silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to complex frameworks (Table 2.6). The simplest silicate structure, that of the mineral\u00a0<button class=\"glossary-term\" aria-describedby=\"92-1258\">olivine<\/button>, is composed of isolated tetrahedra bonded to iron and\/or magnesium ions. In olivine, the \u22124 charge of each silica tetrahedron is balanced by two\u00a0<button class=\"glossary-term\" aria-describedby=\"92-1256\">divalent<\/button>\u00a0(i.e., +2) iron or magnesium cations. Olivine can be either Mg<sub>2<\/sub>SiO<sub>4<\/sub>\u00a0or Fe<sub>2<\/sub>SiO<sub>4<\/sub>, or some combination of the two (Mg,Fe)<sub>2<\/sub>SiO<sub>4<\/sub>. The divalent cations of magnesium and iron are quite close in radius (0.73 versus 0.62 angstroms<a id=\"return-footnote-92-1\" class=\"footnote\" title=\"An angstrom is the unit commonly used for the expression of atomic-scale dimensions. One angstrom is 10\u221210 metres or 0.0000000001 metres. The symbol for an angstrom is \u00c5.\" href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/2-4-silicate-minerals\/#footnote-92-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a>).\u00a0Because of this size similarity, and because they are both divalent cations (both can have a charge of +2), iron and magnesium can readily substitute for each other in olivine and in many other minerals.\r\n<table class=\"aligncenter\" style=\"width: 100%\"><caption>Table 2.6 Silicate mineral configurations. The triangles represent silica tetrahedra.<\/caption>\r\n<thead>\r\n<tr>\r\n<td style=\"text-align: center\" colspan=\"3\"><a href=\"#exerciseq2.3\">[Skip Table]<\/a><\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"text-align: center;width: 50%\" scope=\"col\">Tetrahedron Configuration Picture<\/th>\r\n<th style=\"text-align: center;width: 25%\" scope=\"col\">Tetrahedron Configuration\u00a0Name<\/th>\r\n<th style=\"text-align: center;width: 25%\" scope=\"col\">Example Minerals<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>\u00a0<a><img src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/Isolated.png\" alt=\"One triangle\" width=\"91\" height=\"83\" \/><\/a><\/td>\r\n<td>Isolated (nesosilicates)<\/td>\r\n<td>Olivine, garnet, zircon, kyanite<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><a><img src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Pairs.png\" alt=\"Two triangles joined at their tips.\" width=\"121\" height=\"77\" \/><\/a><\/td>\r\n<td>Pairs (sorosilicates)<\/td>\r\n<td>Epidote, zoisite<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>\u00a0<a><img class=\"alignnone\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/rings.png\" alt=\"Six triangles joined together in a circle to form a star\" width=\"150\" height=\"168\" \/><\/a><\/td>\r\n<td>Rings (cyclosilicates)<\/td>\r\n<td>Tourmaline<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><a><img src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/single-chains.png\" alt=\"Five triangles joined together in a line.\" width=\"200\" height=\"74\" \/><\/a><\/td>\r\n<td>Single chains (inosilicates)<\/td>\r\n<td>Pyroxenes, wollastonite<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>\u00a0<a><img src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Double-chains-.png\" alt=\"Two rows of triangles joined together\" width=\"200\" height=\"87\" \/><\/a><\/td>\r\n<td>Double chains (inosilicates)<\/td>\r\n<td>Amphiboles<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>\u00a0<a><img src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Sheets.png\" alt=\"Multiple rows of triangles joined together\" width=\"200\" height=\"172\" \/><\/a><\/td>\r\n<td>Sheets (phyllosilicates)<\/td>\r\n<td>Micas, clay minerals, serpentine, chlorite<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>3-dimensional structure<\/td>\r\n<td>Framework (tectosilicates)<\/td>\r\n<td>Feldspars, quartz, zeolite<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<div>\r\n<div class=\"textbox textbox--exercises\">\r\n<div class=\"textbox__header\">\r\n\r\n<img class=\"alignnone size-large wp-image-80\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1.png\" alt=\"\" width=\"1024\" height=\"585\" \/>\r\n\r\nCut around the outside of the shape (solid lines and dotted lines), and then fold along the solid lines to form a tetrahedron. If you have glue or tape, secure the tabs to the tetrahedron to hold it together. If you don\u2019t have glue or tape, make a slice along the thin grey line and insert the pointed tab into the slit.\r\n\r\nIf you are doing this in a classroom, try joining your tetrahedron with others into pairs, rings, single and double chains, sheets, and even three-dimensional frameworks.\r\n\r\nSee Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea2.3\">Exercise 2.3 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\nIn olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. Instead they are bonded to the iron and\/or magnesium ions, in the configuration shown on Figure 2.4.1.\r\n<div id=\"attachment_81\" class=\"wp-caption aligncenter\" style=\"width: 650px\"><img class=\"wp-image-81\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine.png\" alt=\"\" width=\"650\" height=\"554\" \/>\r\n<div id=\"caption-attachment-85\" class=\"wp-caption-text\">Figure 2.4.1 A depiction of the structure of olivine as seen from above. The formula for this particular olivine, which has three Fe ions for each Mg ion, could be written: Mg<sub>0.5<\/sub>Fe<sub>1.5<\/sub>SiO<sub>4<\/sub>.<\/div>\r\n<\/div>\r\nAs already noted, the 2 ions of iron and magnesium are similar in size (although not quite the same). This allows them to substitute for each other in some silicate minerals. In fact, the ions that are common in silicate minerals have a wide range of sizes, as depicted in Figure 2.4.2. All of the ions shown are cations, except for oxygen. Note that iron can exist as both a +2 ion (if it loses two electrons during ionization) or a +3 ion (if it loses three). Fe<sup>2+\u00a0<\/sup> is known as <strong><span class=\"glossary-term\">ferrous<\/span><\/strong> iron. Fe<sup>3+\u00a0<\/sup> is known as <strong><span class=\"glossary-term\">ferric<\/span><\/strong> iron. Ionic radii are critical to the composition of silicate minerals, so we\u2019ll be referring to this diagram again.<a id=\"retfig2.4.2\"><\/a>\r\n<div class=\"wp-caption aligncenter\" style=\"width: 700px\"><img class=\"wp-image-82\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii.png\" alt=\"\" width=\"700\" height=\"514\" \/>\r\n<div class=\"wp-caption-text\">Figure 2.4.2 The ionic radii (effective sizes) in angstroms, of some of the common ions in silicate minerals. <a href=\"#fig2.4.2\">[Image Description]<\/a><\/div>\r\n<\/div>\r\nThe structure of the single-chain silicate pyroxene is shown on Figures 2.4.3 and 2.4.4. In <strong><span class=\"glossary-term\">pyroxene<\/span><\/strong>, silica tetrahedra are linked together in a single chain, where one oxygen ion from each tetrahedron is shared with the adjacent tetrahedron, hence there are fewer oxygens in the structure. The result is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is less (\u22122 instead of \u22124).\u00a0 Therefore, fewer cations are necessary to balance that charge. Pyroxene compositions are of the type MgSiO<sub>3<\/sub>, FeSiO<sub>3<\/sub>, and CaSiO<sub>3<\/sub>, or some combination of these. Pyroxene can also be written as (Mg,Fe,Ca)SiO<sub>3<\/sub>, where the elements in the brackets can be present in any proportion. In other words, pyroxene has one cation for each silica tetrahedron (e.g., MgSiO<sub>3<\/sub>) while olivine has two (e.g., Mg<sub>2<\/sub>SiO<sub>4<\/sub>). Because each silicon ion is +4 and each oxygen ion is \u22122, the three oxygens (\u22126) and the one silicon (+4) give a net charge of \u22122 for the single chain of silica tetrahedra. In pyroxene, the one divalent cation (2) per tetrahedron balances that \u22122 charge. In olivine, it takes two divalent cations to balance the \u22124 charge of an isolated tetrahedron. The structure of pyroxene is more \u201cpermissive\u201d than that of olivine\u2014meaning that cations with a wider range of ionic radii can fit into it. That\u2019s why pyroxenes can have iron (radius 0.63 \u00c5) or magnesium (radius 0.72 \u00c5) or calcium (radius 1.00 \u00c5) cations (see Figure 2.4.2 above).\r\n<div class=\"wp-caption aligncenter\" style=\"width: 500px\"><a>\r\n<img class=\"wp-image-83\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/pyroxene.png\" alt=\"Three parallel chains with a rows of positive 2 cations in between them\" width=\"500\" height=\"403\" \/>\r\n<\/a>\r\n<div class=\"wp-caption-text\">Figure 2.4.3 A depiction of the structure of pyroxene. The tetrahedral chains continue to left and right and each is interspersed with a series of divalent cations. If these are Mg ions, then the formula is MgSiO<sub>3<\/sub>.<\/div>\r\n<\/div>\r\n<div class=\"wp-caption aligncenter\" style=\"width: 500px\"><a>\r\n<img class=\"wp-image-84\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron-.png\" alt=\"&quot;&quot;\" width=\"500\" height=\"150\" \/>\r\n<\/a>\r\n<div class=\"wp-caption-text\">Figure 2.4.4 A single silica tetrahedron (left) with four oxygen ions per silicon ion (SiO<sub>4<\/sub>). Part of a single chain of tetrahedra (right), where the oxygen atoms at the adjoining corners are shared between two tetrahedra (arrows). For a very long chain the resulting ratio of silicon to oxygen is 1 to 3 (SiO<sub>3<\/sub>).<\/div>\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--exercises\">\r\n<div class=\"textbox__header\">\r\n\r\nThe diagram below represents a single chain in a silicate mineral. Count the number of tetrahedra versus the number of oxygen ions (yellow spheres). Each tetrahedron has one silicon ion so this should give you the ratio of Si to O in single-chain silicates (e.g., pyroxene).\r\n\r\n<a>\r\n<img class=\"aligncenter\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/diagram1.png\" alt=\"A chain of six tetrahedra and 21 oxygen ions\" width=\"506\" height=\"138\" \/>\r\n<\/a>\r\n\r\nThe diagram below represents a double chain in a silicate mineral. Again, count the number of tetrahedra versus the number of oxygen ions. This should give you the ratio of Si to O in double-chain silicates (e.g., amphibole).\r\n\r\n<a>\r\n<img class=\"aligncenter\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/diagram2.png\" alt=\"A double chain of 14 tetrahedra and 48 oxygen ions\" width=\"509\" height=\"247\" \/>\r\n<\/a>\r\n\r\nSee Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea2.4\">Exercise 2.4 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\n<div>\r\n\r\nIn <strong><span class=\"glossary-term\">amphibole<\/span><\/strong> structures, the silica tetrahedra are linked in a double chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and hence still fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene and its compositions can be very complex. Hornblende, for example, can include sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and the hydroxyl ion (OH<sup>\u2212<\/sup>).\r\n\r\n<\/div>\r\nIn <strong><span class=\"glossary-term\">mica<\/span><\/strong> structures, the silica tetrahedra are arranged in continuous sheets, where each tetrahedron shares three oxygen anions with adjacent tetrahedra. There is even more sharing of oxygens between adjacent tetrahedra and hence fewer cations are needed to balance the charge of the silica-tetrahedra structure in sheet silicate minerals. Bonding between sheets is relatively weak, and this accounts for the well-developed one-directional cleavage in micas (Figure 2.4.5). <strong><span class=\"glossary-term\">Biotite<\/span><\/strong> mica can have iron and\/or magnesium in it and that makes it a <strong><span class=\"glossary-term\">ferromagnesian<\/span><\/strong> silicate mineral (like olivine, pyroxene, and amphibole). <strong><span class=\"glossary-term\">Chlorite<\/span><\/strong> is another similar mineral that commonly includes magnesium. In <strong><span class=\"glossary-term\">muscovite<\/span><\/strong> mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral.\r\n<div id=\"attachment_87\" class=\"wp-caption aligncenter\" style=\"width: 750px\"><img class=\"wp-image-87\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1.png\" alt=\"\" width=\"750\" height=\"240\" \/>\r\n<div id=\"caption-attachment-91\" class=\"wp-caption-text\">Figure 2.4.5 Biotite mica (left) and muscovite mica (right). Both are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite is dark like the other iron- and\/or magnesium-bearing silicates (e.g., olivine, pyroxene, and amphibole), while muscovite is light colored. (Each sample is about 3 cm across.)<\/div>\r\n<\/div>\r\nApart from muscovite, biotite, and chlorite, there are many other <strong><span class=\"glossary-term\">sheet silicates<\/span><\/strong> (a.k.a.\u00a0<strong><span class=\"glossary-term\">phyllosilicates<\/span><\/strong>), many of which exist as clay-sized fragments (i.e., less than 0.004 millimeters). These include the clay minerals <strong><span class=\"glossary-term\">kaolinite<\/span><\/strong>, <strong><span class=\"glossary-term\">illite<\/span><\/strong>, and <strong><span class=\"glossary-term\">smectite<\/span><\/strong>, and although they are difficult to study because of their very small size, they are extremely important components of rocks and especially of soils.\r\n\r\nAll of the sheet silicate minerals also have water molecules within their structure.\r\n\r\nSilica tetrahedra are bonded in three-dimensional frameworks in both the <strong><span class=\"glossary-term\">feldspars<\/span><\/strong> and <strong><span class=\"glossary-term\">quartz<\/span><\/strong>. These are <strong><span class=\"glossary-term\">non-ferromagnesian minerals<\/span><\/strong>\u2014they don\u2019t contain any iron or magnesium. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. Quartz contains only silica tetrahedra.\r\n\r\nThe three main <strong><span class=\"glossary-term\">feldspar<\/span><\/strong> minerals are <strong><span class=\"glossary-term\">potassium feldspar<\/span><\/strong>, (a.k.a. K-feldspar or K-spar) and two types of plagioclase feldspar: <strong><span class=\"glossary-term\">albite<\/span><\/strong> (sodium only) and <strong><span class=\"glossary-term\">anorthite<\/span><\/strong> (calcium only). As is the case for iron and magnesium in olivine, there is a continuous range of compositions (solid solution series) between albite and anorthite in plagioclase. Because the calcium and sodium ions are almost identical in size (1.00 \u00c5 versus 0.99 \u00c5) any intermediate compositions between CaAl<sub>2<\/sub>Si<sub>3<\/sub>O<sub>8<\/sub> and NaAlSi<sub>3<\/sub>O<sub>8<\/sub> can exist (Figure 2.4.6). This is a little bit surprising because, although they are very similar in size, calcium and sodium ions don\u2019t have the same charge (Ca<sup>2+<\/sup> versus Na<sup>+<\/sup> ). This problem is accounted for by the corresponding substitution of Al<sup>+3\u00a0<\/sup> for Si<sup>+4\u00a0<\/sup>. Therefore, albite is NaAlSi<sub>3<\/sub>O<sub>8<\/sub> (1 Al and 3 Si) while anorthite is CaAl<sub>2<\/sub>Si<sub>2<\/sub>O<sub>8<\/sub> (2 Al and 2 Si), and plagioclase feldspars of intermediate composition have intermediate proportions of Al and Si. This is called a \u201ccoupled-substitution.\u201d\r\n\r\nThe intermediate-composition plagioclase feldspars are oligoclase (10% to 30% Ca), andesine (30% to 50% Ca), labradorite (50% to 70% Ca), and bytownite (70% to 90% Ca). <strong><span class=\"glossary-term\">K-feldspar <\/span><\/strong>(KAlSi<sub>3<\/sub>O<sub>8<\/sub>) has a slightly different structure than that of plagioclase, owing to the larger size of the potassium ion (1.37 \u00c5) and because of this large size, potassium and sodium do not readily substitute for each other, except at high temperatures. These high-temperature feldspars are likely to be found only in volcanic rocks because intrusive igneous rocks cool slowly enough to low temperatures for the feldspars to change into one of the lower-temperature forms.\r\n<div class=\"wp-caption aligncenter\" style=\"width: 700px\"><img class=\"wp-image-88\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals.png\" alt=\"\" width=\"700\" height=\"560\" \/>\r\n<div class=\"wp-caption-text\">Figure 2.4.6 Compositions of the feldspar minerals.<\/div>\r\n<\/div>\r\nIn <strong><span class=\"glossary-term\">quartz <\/span><\/strong>(SiO<sub>2<\/sub>)<strong>,<\/strong> the silica tetrahedra are bonded in a \u201cperfect\u201d three-dimensional framework. Each tetrahedron is bonded to four other tetrahedra (with an oxygen shared at every corner of each tetrahedron), and as a result, the ratio of silicon to oxygen is 1:2. Since the one silicon cation has a +4 charge and the two oxygen anions each have a \u22122 charge, the charge is balanced. There is no need for aluminum or any of the other cations such as sodium or potassium. The hardness and lack of cleavage in quartz result from the strong covalent\/ionic bonds characteristic of the silica tetrahedron.\r\n<div class=\"textbox textbox--exercises\">\r\n<div class=\"textbox__header\">\r\n\r\nSilicate minerals are classified as being either ferromagnesian or non-ferromagnesian depending on whether or not they have iron (Fe) and\/or magnesium (Mg) in their formula. A number of minerals and their formulas are listed below. For each one, indicate whether or not it is a <em>ferromagnesian silicate<\/em>.\r\n<table style=\"width: 100%\">\r\n<tbody>\r\n<tr>\r\n<td><strong>Mineral<\/strong><\/td>\r\n<td><strong>Formula<\/strong><\/td>\r\n<td><strong>Ferromagnesian silicate?<\/strong><\/td>\r\n<\/tr>\r\n<tr>\r\n<td>olivine<\/td>\r\n<td>(Mg,Fe)<sub>2<\/sub>SiO<sub>4<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>pyrite<\/td>\r\n<td>FeS<sub>2<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>plagioclase feldspar<\/td>\r\n<td>CaAl<sub>2<\/sub>Si<sub>2<\/sub>O<sub>8<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>pyroxene<\/td>\r\n<td>MgSiO<sub>3<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>hematite<\/td>\r\n<td>Fe<sub>2<\/sub>O<sub>3<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>orthoclase feldspar<\/td>\r\n<td>KAlSi<sub>3<\/sub>O<sub>8<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>quartz<\/td>\r\n<td>SiO<sub>2<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>amphibole<\/td>\r\n<td>Fe<sub>7<\/sub>Si<sub>8<\/sub>O<sub>22<\/sub>(OH)<sub>2<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>muscovite<\/td>\r\n<td>K<sub>2<\/sub>Al<sub>4<\/sub>Si<sub>6<\/sub>Al<sub>2<\/sub>O<sub>20<\/sub>(OH)<sub>4<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>magnetite<\/td>\r\n<td>Fe<sub>3<\/sub>O<sub>4<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>biotite<\/td>\r\n<td>K<sub>2<\/sub>Fe<sub>4<\/sub>Al<sub>2<\/sub>Si<sub>6<\/sub>Al<sub>4<\/sub>O<sub>20<\/sub>(OH)<sub>4<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>dolomite<\/td>\r\n<td>(Ca,Mg)CO<sub>3<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>garnet<\/td>\r\n<td>Fe<sub>2<\/sub>Al<sub>2<\/sub>Si<sub>3<\/sub>O<sub>12<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>serpentine<\/td>\r\n<td>Mg<sub>3<\/sub>Si<sub>2<\/sub>O<sub>5<\/sub>(OH)<sub>4<\/sub><\/td>\r\n<td>.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nSee Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea2.5\">Exercise 2.5 answers<\/a>.*Some of the formulas, especially the more complicated ones, have been simplified.\r\n\r\n<\/div>\r\n<\/div>\r\n<h3>Image Descriptions<\/h3>\r\n<table id=\"fig2.11\" class=\"aligncenter\" style=\"width: 100%\"><caption><a id=\"fig2.4.2\"><\/a>Figure 2.4.2 image description: The ionic radii of elements in angstroms and their charges.<\/caption>\r\n<thead>\r\n<tr>\r\n<th>Element<\/th>\r\n<th>Ionic Radii (in angstroms)<\/th>\r\n<th>Charge<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>Oxygen<\/td>\r\n<td>1.4<\/td>\r\n<td>\u22122 (Anion)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Potassium<\/td>\r\n<td>1.37<\/td>\r\n<td>1 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Calcium<\/td>\r\n<td>1.00<\/td>\r\n<td>2 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Sodium<\/td>\r\n<td>0.99<\/td>\r\n<td>1 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Magnesium<\/td>\r\n<td>0.72<\/td>\r\n<td>2 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td rowspan=\"2\">Iron<\/td>\r\n<td>0.63<\/td>\r\n<td>2 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>0.49<\/td>\r\n<td>3 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Aluminum<\/td>\r\n<td>0.39<\/td>\r\n<td>3 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Silicon<\/td>\r\n<td>0.26<\/td>\r\n<td>4 (Cation)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Carbon<\/td>\r\n<td>0.15<\/td>\r\n<td>4 (Cation)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<a href=\"#retfig2.4.2\">[Return to Figure 2.4.2]<\/a>\r\n\r\n<hr class=\"before-footnotes clear\" \/>\r\n\r\n<div class=\"footnotes\">\r\n<ol>\r\n \t<li id=\"footnote-92-1\">An angstrom is the unit commonly used for the expression of atomic-scale dimensions. One angstrom is 10<sup>\u221210<\/sup> metres or 0.0000000001 metres. The symbol for an angstrom is \u00c5. <a class=\"return-footnote\" href=\"#return-footnote-92-1\">\u21b5<\/a><\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<!-- pb_fixme -->","rendered":"<div>\n<div>\n<h1 class=\"entry-title\">2.4 Silicate Minerals<\/h1>\n<p>The vast majority of the minerals that make up the rocks of Earth\u2019s crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a variety of clay minerals. The building block of all of these minerals is the\u00a0<button class=\"glossary-term\" aria-describedby=\"92-1280\">silica tetrahedron<\/button>, a combination of four oxygen atoms and one silicon atom. As we\u2019ve seen, it\u2019s called a tetrahedron because planes drawn through the oxygen atoms form a shape with 4 surfaces (Figure 2.2.4). Since the silicon ion has a charge of 4 and each of the four oxygen ions has a charge of \u22122, the silica tetrahedron has a net charge of \u22124.<\/p>\n<p>In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to complex frameworks (Table 2.6). The simplest silicate structure, that of the mineral\u00a0<button class=\"glossary-term\" aria-describedby=\"92-1258\">olivine<\/button>, is composed of isolated tetrahedra bonded to iron and\/or magnesium ions. In olivine, the \u22124 charge of each silica tetrahedron is balanced by two\u00a0<button class=\"glossary-term\" aria-describedby=\"92-1256\">divalent<\/button>\u00a0(i.e., +2) iron or magnesium cations. Olivine can be either Mg<sub>2<\/sub>SiO<sub>4<\/sub>\u00a0or Fe<sub>2<\/sub>SiO<sub>4<\/sub>, or some combination of the two (Mg,Fe)<sub>2<\/sub>SiO<sub>4<\/sub>. The divalent cations of magnesium and iron are quite close in radius (0.73 versus 0.62 angstroms<a id=\"return-footnote-92-1\" class=\"footnote\" title=\"An angstrom is the unit commonly used for the expression of atomic-scale dimensions. One angstrom is 10\u221210 metres or 0.0000000001 metres. The symbol for an angstrom is \u00c5.\" href=\"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/2-4-silicate-minerals\/#footnote-92-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a>).\u00a0Because of this size similarity, and because they are both divalent cations (both can have a charge of +2), iron and magnesium can readily substitute for each other in olivine and in many other minerals.<\/p>\n<table class=\"aligncenter\" style=\"width: 100%\">\n<caption>Table 2.6 Silicate mineral configurations. The triangles represent silica tetrahedra.<\/caption>\n<thead>\n<tr>\n<td style=\"text-align: center\" colspan=\"3\"><a href=\"#exerciseq2.3\">[Skip Table]<\/a><\/td>\n<\/tr>\n<tr>\n<th style=\"text-align: center;width: 50%\" scope=\"col\">Tetrahedron Configuration Picture<\/th>\n<th style=\"text-align: center;width: 25%\" scope=\"col\">Tetrahedron Configuration\u00a0Name<\/th>\n<th style=\"text-align: center;width: 25%\" scope=\"col\">Example Minerals<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>\u00a0<a><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2021\/09\/Isolated.png\" alt=\"One triangle\" width=\"91\" height=\"83\" \/><\/a><\/td>\n<td>Isolated (nesosilicates)<\/td>\n<td>Olivine, garnet, zircon, kyanite<\/td>\n<\/tr>\n<tr>\n<td><a><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Pairs.png\" alt=\"Two triangles joined at their tips.\" width=\"121\" height=\"77\" \/><\/a><\/td>\n<td>Pairs (sorosilicates)<\/td>\n<td>Epidote, zoisite<\/td>\n<\/tr>\n<tr>\n<td>\u00a0<a><img loading=\"lazy\" decoding=\"async\" class=\"alignnone\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/rings.png\" alt=\"Six triangles joined together in a circle to form a star\" width=\"150\" height=\"168\" \/><\/a><\/td>\n<td>Rings (cyclosilicates)<\/td>\n<td>Tourmaline<\/td>\n<\/tr>\n<tr>\n<td><a><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/single-chains.png\" alt=\"Five triangles joined together in a line.\" width=\"200\" height=\"74\" \/><\/a><\/td>\n<td>Single chains (inosilicates)<\/td>\n<td>Pyroxenes, wollastonite<\/td>\n<\/tr>\n<tr>\n<td>\u00a0<a><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Double-chains-.png\" alt=\"Two rows of triangles joined together\" width=\"200\" height=\"87\" \/><\/a><\/td>\n<td>Double chains (inosilicates)<\/td>\n<td>Amphiboles<\/td>\n<\/tr>\n<tr>\n<td>\u00a0<a><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Sheets.png\" alt=\"Multiple rows of triangles joined together\" width=\"200\" height=\"172\" \/><\/a><\/td>\n<td>Sheets (phyllosilicates)<\/td>\n<td>Micas, clay minerals, serpentine, chlorite<\/td>\n<\/tr>\n<tr>\n<td>3-dimensional structure<\/td>\n<td>Framework (tectosilicates)<\/td>\n<td>Feldspars, quartz, zeolite<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div>\n<div class=\"textbox textbox--exercises\">\n<div class=\"textbox__header\">\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-large wp-image-80\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1.png\" alt=\"\" width=\"1024\" height=\"585\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1-300x171.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1-768x439.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1-65x37.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1-225x129.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/Tetrahedron-e1560901154442-1024x585-1-350x200.png 350w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><\/p>\n<p>Cut around the outside of the shape (solid lines and dotted lines), and then fold along the solid lines to form a tetrahedron. If you have glue or tape, secure the tabs to the tetrahedron to hold it together. If you don\u2019t have glue or tape, make a slice along the thin grey line and insert the pointed tab into the slit.<\/p>\n<p>If you are doing this in a classroom, try joining your tetrahedron with others into pairs, rings, single and double chains, sheets, and even three-dimensional frameworks.<\/p>\n<p>See Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea2.3\">Exercise 2.3 answers<\/a>.<\/p>\n<\/div>\n<\/div>\n<p>In olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. Instead they are bonded to the iron and\/or magnesium ions, in the configuration shown on Figure 2.4.1.<\/p>\n<div id=\"attachment_81\" class=\"wp-caption aligncenter\" style=\"width: 650px\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-81\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine.png\" alt=\"\" width=\"650\" height=\"554\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine-300x256.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine-768x655.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine-65x55.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine-225x192.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/structure-of-olivine-350x298.png 350w\" sizes=\"auto, (max-width: 650px) 100vw, 650px\" \/><\/p>\n<div id=\"caption-attachment-85\" class=\"wp-caption-text\">Figure 2.4.1 A depiction of the structure of olivine as seen from above. The formula for this particular olivine, which has three Fe ions for each Mg ion, could be written: Mg<sub>0.5<\/sub>Fe<sub>1.5<\/sub>SiO<sub>4<\/sub>.<\/div>\n<\/div>\n<p>As already noted, the 2 ions of iron and magnesium are similar in size (although not quite the same). This allows them to substitute for each other in some silicate minerals. In fact, the ions that are common in silicate minerals have a wide range of sizes, as depicted in Figure 2.4.2. All of the ions shown are cations, except for oxygen. Note that iron can exist as both a +2 ion (if it loses two electrons during ionization) or a +3 ion (if it loses three). Fe<sup>2+\u00a0<\/sup> is known as <strong><span class=\"glossary-term\">ferrous<\/span><\/strong> iron. Fe<sup>3+\u00a0<\/sup> is known as <strong><span class=\"glossary-term\">ferric<\/span><\/strong> iron. Ionic radii are critical to the composition of silicate minerals, so we\u2019ll be referring to this diagram again.<a id=\"retfig2.4.2\"><\/a><\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 700px\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-82\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii.png\" alt=\"\" width=\"700\" height=\"514\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii.png 944w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii-300x220.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii-768x564.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii-65x48.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii-225x165.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/ionic-radii-350x257.png 350w\" sizes=\"auto, (max-width: 700px) 100vw, 700px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 2.4.2 The ionic radii (effective sizes) in angstroms, of some of the common ions in silicate minerals. <a href=\"#fig2.4.2\">[Image Description]<\/a><\/div>\n<\/div>\n<p>The structure of the single-chain silicate pyroxene is shown on Figures 2.4.3 and 2.4.4. In <strong><span class=\"glossary-term\">pyroxene<\/span><\/strong>, silica tetrahedra are linked together in a single chain, where one oxygen ion from each tetrahedron is shared with the adjacent tetrahedron, hence there are fewer oxygens in the structure. The result is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is less (\u22122 instead of \u22124).\u00a0 Therefore, fewer cations are necessary to balance that charge. Pyroxene compositions are of the type MgSiO<sub>3<\/sub>, FeSiO<sub>3<\/sub>, and CaSiO<sub>3<\/sub>, or some combination of these. Pyroxene can also be written as (Mg,Fe,Ca)SiO<sub>3<\/sub>, where the elements in the brackets can be present in any proportion. In other words, pyroxene has one cation for each silica tetrahedron (e.g., MgSiO<sub>3<\/sub>) while olivine has two (e.g., Mg<sub>2<\/sub>SiO<sub>4<\/sub>). Because each silicon ion is +4 and each oxygen ion is \u22122, the three oxygens (\u22126) and the one silicon (+4) give a net charge of \u22122 for the single chain of silica tetrahedra. In pyroxene, the one divalent cation (2) per tetrahedron balances that \u22122 charge. In olivine, it takes two divalent cations to balance the \u22124 charge of an isolated tetrahedron. The structure of pyroxene is more \u201cpermissive\u201d than that of olivine\u2014meaning that cations with a wider range of ionic radii can fit into it. That\u2019s why pyroxenes can have iron (radius 0.63 \u00c5) or magnesium (radius 0.72 \u00c5) or calcium (radius 1.00 \u00c5) cations (see Figure 2.4.2 above).<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 500px\"><a><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-83\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/pyroxene.png\" alt=\"Three parallel chains with a rows of positive 2 cations in between them\" width=\"500\" height=\"403\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/pyroxene.png 760w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/pyroxene-300x242.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/pyroxene-65x52.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/pyroxene-225x181.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/pyroxene-350x282.png 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><br \/>\n<\/a><\/p>\n<div class=\"wp-caption-text\">Figure 2.4.3 A depiction of the structure of pyroxene. The tetrahedral chains continue to left and right and each is interspersed with a series of divalent cations. If these are Mg ions, then the formula is MgSiO<sub>3<\/sub>.<\/div>\n<\/div>\n<div class=\"wp-caption aligncenter\" style=\"width: 500px\"><a><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"wp-image-84\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron-.png\" alt=\"&quot;&quot;\" width=\"500\" height=\"150\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron-.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron--300x90.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron--768x231.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron--65x20.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron--225x68.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/silica-tetrahedron--350x105.png 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><br \/>\n<\/a><\/p>\n<div class=\"wp-caption-text\">Figure 2.4.4 A single silica tetrahedron (left) with four oxygen ions per silicon ion (SiO<sub>4<\/sub>). Part of a single chain of tetrahedra (right), where the oxygen atoms at the adjoining corners are shared between two tetrahedra (arrows). For a very long chain the resulting ratio of silicon to oxygen is 1 to 3 (SiO<sub>3<\/sub>).<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<div class=\"textbox__header\">\n<p>The diagram below represents a single chain in a silicate mineral. Count the number of tetrahedra versus the number of oxygen ions (yellow spheres). Each tetrahedron has one silicon ion so this should give you the ratio of Si to O in single-chain silicates (e.g., pyroxene).<\/p>\n<p><a><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"aligncenter\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/diagram1.png\" alt=\"A chain of six tetrahedra and 21 oxygen ions\" width=\"506\" height=\"138\" \/><br \/>\n<\/a><\/p>\n<p>The diagram below represents a double chain in a silicate mineral. Again, count the number of tetrahedra versus the number of oxygen ions. This should give you the ratio of Si to O in double-chain silicates (e.g., amphibole).<\/p>\n<p><a><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"aligncenter\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/diagram2.png\" alt=\"A double chain of 14 tetrahedra and 48 oxygen ions\" width=\"509\" height=\"247\" \/><br \/>\n<\/a><\/p>\n<p>See Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea2.4\">Exercise 2.4 answers<\/a>.<\/p>\n<\/div>\n<\/div>\n<div>\n<p>In <strong><span class=\"glossary-term\">amphibole<\/span><\/strong> structures, the silica tetrahedra are linked in a double chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and hence still fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene and its compositions can be very complex. Hornblende, for example, can include sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and the hydroxyl ion (OH<sup>\u2212<\/sup>).<\/p>\n<\/div>\n<p>In <strong><span class=\"glossary-term\">mica<\/span><\/strong> structures, the silica tetrahedra are arranged in continuous sheets, where each tetrahedron shares three oxygen anions with adjacent tetrahedra. There is even more sharing of oxygens between adjacent tetrahedra and hence fewer cations are needed to balance the charge of the silica-tetrahedra structure in sheet silicate minerals. Bonding between sheets is relatively weak, and this accounts for the well-developed one-directional cleavage in micas (Figure 2.4.5). <strong><span class=\"glossary-term\">Biotite<\/span><\/strong> mica can have iron and\/or magnesium in it and that makes it a <strong><span class=\"glossary-term\">ferromagnesian<\/span><\/strong> silicate mineral (like olivine, pyroxene, and amphibole). <strong><span class=\"glossary-term\">Chlorite<\/span><\/strong> is another similar mineral that commonly includes magnesium. In <strong><span class=\"glossary-term\">muscovite<\/span><\/strong> mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral.<\/p>\n<div id=\"attachment_87\" class=\"wp-caption aligncenter\" style=\"width: 750px\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-87\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1.png\" alt=\"\" width=\"750\" height=\"240\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1.png 1024w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1-300x96.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1-768x245.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1-65x21.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1-225x72.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/mica2-1024x327-1-350x112.png 350w\" sizes=\"auto, (max-width: 750px) 100vw, 750px\" \/><\/p>\n<div id=\"caption-attachment-91\" class=\"wp-caption-text\">Figure 2.4.5 Biotite mica (left) and muscovite mica (right). Both are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite is dark like the other iron- and\/or magnesium-bearing silicates (e.g., olivine, pyroxene, and amphibole), while muscovite is light colored. (Each sample is about 3 cm across.)<\/div>\n<\/div>\n<p>Apart from muscovite, biotite, and chlorite, there are many other <strong><span class=\"glossary-term\">sheet silicates<\/span><\/strong> (a.k.a.\u00a0<strong><span class=\"glossary-term\">phyllosilicates<\/span><\/strong>), many of which exist as clay-sized fragments (i.e., less than 0.004 millimeters). These include the clay minerals <strong><span class=\"glossary-term\">kaolinite<\/span><\/strong>, <strong><span class=\"glossary-term\">illite<\/span><\/strong>, and <strong><span class=\"glossary-term\">smectite<\/span><\/strong>, and although they are difficult to study because of their very small size, they are extremely important components of rocks and especially of soils.<\/p>\n<p>All of the sheet silicate minerals also have water molecules within their structure.<\/p>\n<p>Silica tetrahedra are bonded in three-dimensional frameworks in both the <strong><span class=\"glossary-term\">feldspars<\/span><\/strong> and <strong><span class=\"glossary-term\">quartz<\/span><\/strong>. These are <strong><span class=\"glossary-term\">non-ferromagnesian minerals<\/span><\/strong>\u2014they don\u2019t contain any iron or magnesium. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. Quartz contains only silica tetrahedra.<\/p>\n<p>The three main <strong><span class=\"glossary-term\">feldspar<\/span><\/strong> minerals are <strong><span class=\"glossary-term\">potassium feldspar<\/span><\/strong>, (a.k.a. K-feldspar or K-spar) and two types of plagioclase feldspar: <strong><span class=\"glossary-term\">albite<\/span><\/strong> (sodium only) and <strong><span class=\"glossary-term\">anorthite<\/span><\/strong> (calcium only). As is the case for iron and magnesium in olivine, there is a continuous range of compositions (solid solution series) between albite and anorthite in plagioclase. Because the calcium and sodium ions are almost identical in size (1.00 \u00c5 versus 0.99 \u00c5) any intermediate compositions between CaAl<sub>2<\/sub>Si<sub>3<\/sub>O<sub>8<\/sub> and NaAlSi<sub>3<\/sub>O<sub>8<\/sub> can exist (Figure 2.4.6). This is a little bit surprising because, although they are very similar in size, calcium and sodium ions don\u2019t have the same charge (Ca<sup>2+<\/sup> versus Na<sup>+<\/sup> ). This problem is accounted for by the corresponding substitution of Al<sup>+3\u00a0<\/sup> for Si<sup>+4\u00a0<\/sup>. Therefore, albite is NaAlSi<sub>3<\/sub>O<sub>8<\/sub> (1 Al and 3 Si) while anorthite is CaAl<sub>2<\/sub>Si<sub>2<\/sub>O<sub>8<\/sub> (2 Al and 2 Si), and plagioclase feldspars of intermediate composition have intermediate proportions of Al and Si. This is called a \u201ccoupled-substitution.\u201d<\/p>\n<p>The intermediate-composition plagioclase feldspars are oligoclase (10% to 30% Ca), andesine (30% to 50% Ca), labradorite (50% to 70% Ca), and bytownite (70% to 90% Ca). <strong><span class=\"glossary-term\">K-feldspar <\/span><\/strong>(KAlSi<sub>3<\/sub>O<sub>8<\/sub>) has a slightly different structure than that of plagioclase, owing to the larger size of the potassium ion (1.37 \u00c5) and because of this large size, potassium and sodium do not readily substitute for each other, except at high temperatures. These high-temperature feldspars are likely to be found only in volcanic rocks because intrusive igneous rocks cool slowly enough to low temperatures for the feldspars to change into one of the lower-temperature forms.<\/p>\n<div class=\"wp-caption aligncenter\" style=\"width: 700px\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-88\" src=\"https:\/\/pressbooks.ccconline.org\/physicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals.png\" alt=\"\" width=\"700\" height=\"560\" srcset=\"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals.png 974w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals-300x240.png 300w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals-768x614.png 768w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals-65x52.png 65w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals-225x180.png 225w, https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-content\/uploads\/sites\/48\/2022\/01\/feldspar-minerals-350x280.png 350w\" sizes=\"auto, (max-width: 700px) 100vw, 700px\" \/><\/p>\n<div class=\"wp-caption-text\">Figure 2.4.6 Compositions of the feldspar minerals.<\/div>\n<\/div>\n<p>In <strong><span class=\"glossary-term\">quartz <\/span><\/strong>(SiO<sub>2<\/sub>)<strong>,<\/strong> the silica tetrahedra are bonded in a \u201cperfect\u201d three-dimensional framework. Each tetrahedron is bonded to four other tetrahedra (with an oxygen shared at every corner of each tetrahedron), and as a result, the ratio of silicon to oxygen is 1:2. Since the one silicon cation has a +4 charge and the two oxygen anions each have a \u22122 charge, the charge is balanced. There is no need for aluminum or any of the other cations such as sodium or potassium. The hardness and lack of cleavage in quartz result from the strong covalent\/ionic bonds characteristic of the silica tetrahedron.<\/p>\n<div class=\"textbox textbox--exercises\">\n<div class=\"textbox__header\">\n<p>Silicate minerals are classified as being either ferromagnesian or non-ferromagnesian depending on whether or not they have iron (Fe) and\/or magnesium (Mg) in their formula. A number of minerals and their formulas are listed below. For each one, indicate whether or not it is a <em>ferromagnesian silicate<\/em>.<\/p>\n<table style=\"width: 100%\">\n<tbody>\n<tr>\n<td><strong>Mineral<\/strong><\/td>\n<td><strong>Formula<\/strong><\/td>\n<td><strong>Ferromagnesian silicate?<\/strong><\/td>\n<\/tr>\n<tr>\n<td>olivine<\/td>\n<td>(Mg,Fe)<sub>2<\/sub>SiO<sub>4<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>pyrite<\/td>\n<td>FeS<sub>2<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>plagioclase feldspar<\/td>\n<td>CaAl<sub>2<\/sub>Si<sub>2<\/sub>O<sub>8<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>pyroxene<\/td>\n<td>MgSiO<sub>3<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>hematite<\/td>\n<td>Fe<sub>2<\/sub>O<sub>3<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>orthoclase feldspar<\/td>\n<td>KAlSi<sub>3<\/sub>O<sub>8<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>quartz<\/td>\n<td>SiO<sub>2<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>amphibole<\/td>\n<td>Fe<sub>7<\/sub>Si<sub>8<\/sub>O<sub>22<\/sub>(OH)<sub>2<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>muscovite<\/td>\n<td>K<sub>2<\/sub>Al<sub>4<\/sub>Si<sub>6<\/sub>Al<sub>2<\/sub>O<sub>20<\/sub>(OH)<sub>4<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>magnetite<\/td>\n<td>Fe<sub>3<\/sub>O<sub>4<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>biotite<\/td>\n<td>K<sub>2<\/sub>Fe<sub>4<\/sub>Al<sub>2<\/sub>Si<sub>6<\/sub>Al<sub>4<\/sub>O<sub>20<\/sub>(OH)<sub>4<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>dolomite<\/td>\n<td>(Ca,Mg)CO<sub>3<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>garnet<\/td>\n<td>Fe<sub>2<\/sub>Al<sub>2<\/sub>Si<sub>3<\/sub>O<sub>12<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<tr>\n<td>serpentine<\/td>\n<td>Mg<sub>3<\/sub>Si<sub>2<\/sub>O<sub>5<\/sub>(OH)<sub>4<\/sub><\/td>\n<td>.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>See Appendix 3 for <a href=\"back-matter-005-appendix-3-answers-to-exercises.html#exercisea2.5\">Exercise 2.5 answers<\/a>.*Some of the formulas, especially the more complicated ones, have been simplified.<\/p>\n<\/div>\n<\/div>\n<h3>Image Descriptions<\/h3>\n<table id=\"fig2.11\" class=\"aligncenter\" style=\"width: 100%\">\n<caption><a id=\"fig2.4.2\"><\/a>Figure 2.4.2 image description: The ionic radii of elements in angstroms and their charges.<\/caption>\n<thead>\n<tr>\n<th>Element<\/th>\n<th>Ionic Radii (in angstroms)<\/th>\n<th>Charge<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Oxygen<\/td>\n<td>1.4<\/td>\n<td>\u22122 (Anion)<\/td>\n<\/tr>\n<tr>\n<td>Potassium<\/td>\n<td>1.37<\/td>\n<td>1 (Cation)<\/td>\n<\/tr>\n<tr>\n<td>Calcium<\/td>\n<td>1.00<\/td>\n<td>2 (Cation)<\/td>\n<\/tr>\n<tr>\n<td>Sodium<\/td>\n<td>0.99<\/td>\n<td>1 (Cation)<\/td>\n<\/tr>\n<tr>\n<td>Magnesium<\/td>\n<td>0.72<\/td>\n<td>2 (Cation)<\/td>\n<\/tr>\n<tr>\n<td rowspan=\"2\">Iron<\/td>\n<td>0.63<\/td>\n<td>2 (Cation)<\/td>\n<\/tr>\n<tr>\n<td>0.49<\/td>\n<td>3 (Cation)<\/td>\n<\/tr>\n<tr>\n<td>Aluminum<\/td>\n<td>0.39<\/td>\n<td>3 (Cation)<\/td>\n<\/tr>\n<tr>\n<td>Silicon<\/td>\n<td>0.26<\/td>\n<td>4 (Cation)<\/td>\n<\/tr>\n<tr>\n<td>Carbon<\/td>\n<td>0.15<\/td>\n<td>4 (Cation)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><a href=\"#retfig2.4.2\">[Return to Figure 2.4.2]<\/a><\/p>\n<hr class=\"before-footnotes clear\" \/>\n<div class=\"footnotes\">\n<ol>\n<li id=\"footnote-92-1\">An angstrom is the unit commonly used for the expression of atomic-scale dimensions. One angstrom is 10<sup>\u221210<\/sup> metres or 0.0000000001 metres. The symbol for an angstrom is \u00c5. <a class=\"return-footnote\" href=\"#return-footnote-92-1\">\u21b5<\/a><\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<p><!-- pb_fixme --><\/p>\n","protected":false},"author":32,"menu_order":19,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-89","chapter","type-chapter","status-publish","hentry"],"part":17,"_links":{"self":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/89","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/users\/32"}],"version-history":[{"count":3,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/89\/revisions"}],"predecessor-version":[{"id":1046,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/89\/revisions\/1046"}],"part":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/parts\/17"}],"metadata":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapters\/89\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/media?parent=89"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/pressbooks\/v2\/chapter-type?post=89"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/contributor?post=89"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.ccconline.org\/accphysicalgeology\/wp-json\/wp\/v2\/license?post=89"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}