2.4 Silicate Minerals

The vast majority of the minerals that make up the rocks the Earth’s crust room silicate minerals. These encompass minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals. The structure block of every one of these minerals is the silica tetrahedron, a mix of 4 oxygen atoms and one silicon atom. These space arranged such that planes drawn through the oxygen atoms kind a tetrahedron (Figure 2.6). Because the silicon ion has actually a charge of +4 and also each the the 4 oxygen ions has a charge of –2, the silica tetrahedron has actually a net fee of –4.

You are watching: In the silicon-oxygen tetrahedron there are more silicon atoms than oxygen atoms.

In silicate minerals, these tetrahedra are arranged and also linked together in a selection of ways, from solitary units to complicated frameworks (Figure 2.9). The easiest silicate structure, the of the mineral olivine, is created of secluded tetrahedra bonded to iron and/or magnesium ions. In olivine, the –4 fee of each silica tetrahedron is balanced by two divalent (i.e., +2) iron or magnesium cations. Olivine have the right to be either Mg2SiO4 or Fe2SiO4, or some combination of the two (Mg,Fe)2SiO4. The divalent cations that magnesium and also iron are rather close in radius (0.73 versus 0.62 angstroms<1>). Because that this size similarity, and because they space both divalent cations (both have actually a fee of +2), iron and magnesium can readily substitute for each various other in olivine and also in many other minerals.

Figure 2.9 Silicate mineral configurations. The triangles represent silica tetrahedra.Tetrahedron ConfigurationExample Minerals
 
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Isolated (nesosilicates)Olivine, garnet, zircon, kyanite
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Pairs (sorosilicates)Epidote, zoisite
 
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Rings (cyclosilicates)Tourmaline
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Single chains (inosilicates)Pyroxenes, wollastonite
 
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Double chains (inosilicates)Amphiboles
 
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Sheets (phyllosilicates)Micas, clay minerals, serpentine, chlorite
3-dimensional structureFramework (tectosilicates)Feldspars, quartz, zeolite

Exercises


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Exercise 2.3 do a Tetrahedron

Cut roughly the exterior of the shape (solid lines and also dotted lines), and also then fold along the solid lines to type a tetrahedron.

If you have actually glue or tape, certain the tabs to the tetrahedron to hold it together. If you don’t have glue or tape, do a part along the thin grey line and also insert the sharp tab into the slit.

If you space doing this in a classroom, try joining your tetrahedron through others into pairs, rings, single and twin chains, sheets, and even three-dimensional frameworks.


In olivine, unlike most other silicate minerals, the silica tetrahedra room not external inspection to each other. Castle are, however, bonded to the iron and/or magnesium as presented on number 2.10.

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Figure 2.10 A explicate of the structure of olivine as seen from above. The formula for this certain olivine, which has three Fe ion for each Mg ion, could be written: Mg0.5Fe1.5SiO4.

As already noted, the +2 ions of iron and also magnesium are comparable in size (although not rather the same). This allows them to substitute because that each other in part silicate minerals. In fact, the typical ions in silicate minerals have actually a wide range of sizes, as presented in number 2.11. Every one of the ions displayed are cations, except for oxygen. Keep in mind that iron deserve to exist as both a +2 ion (if the loses 2 electrons during ionization) or a +3 ion (if it loser three). Fe2+ is well-known as ferrous iron. Fe3+ is known as ferric iron. Ionic radii are critical to the composition of silicate minerals, so fine be introduce to this diagram again.

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Figure 2.11 The ionic radii (effective sizes) in angstroms, of few of the typical ions in silicate minerals

The framework of the single-chain silicate pyroxene is displayed on figures 2.12 and also 2.13. In pyroxene, silica tetrahedra are attached together in a single chain, where one oxygen ion from every tetrahedron is mutual with the surrounding tetrahedron, therefore there space fewer oxygens in the structure. The an outcome is the the oxygen-to-silicon ratio is reduced than in olivine (3:1 rather of 4:1), and also the net charge per silicon atom is much less (–2 instead of –4), due to the fact that fewer cations are necessary to balance the charge. Pyroxene compositions are of the form MgSiO3, FeSiO3, and also CaSiO3, or some mix of these. Pyroxene can likewise be composed as (Mg,Fe,Ca)SiO3, wherein the elements in the brackets have the right to be existing in any proportion. In various other words, pyroxene has one cation for each silica tetrahedron (e.g., MgSiO3) when olivine has two (e.g., Mg2SiO4). Since each silicon ion is +4 and also each oxygen ion is –2, the three oxygens (–6) and the one silicon (+4) provide a net charge of –2 for the single chain of silica tetrahedra. In pyroxene, the one divalent cation (2+) per tetrahedron balances that –2 charge. In olivine, the takes two divalent cations to balance the –4 fee of an diverted tetrahedron.

The framework of pyroxene is an ext “permissive” 보다 that the olivine — meaning that cations through a wider range the ionic radii have the right to fit into it. It is why pyroxenes deserve to have stole (radius 0.63 Å) or magnesium (radius 0.72 Å) or calcium (radius 1.00 Å) cations.

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Figure 2.12 A depiction of the structure of pyroxene. The tetrahedral chains proceed to left and right and also each is interspersed v a series of divalent cations. If these space Mg ions, then the formula is MgSiO3.
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Figure 2.13 A solitary silica tetrahedron (left) with 4 oxygen ions per silicon ion (SiO4). Part of a solitary chain of tetrahedra (right), where the oxygen atom at the adjoining corners space shared between two tetrahedra (arrows). For a very long chain the resulting ratio of silicon come oxygen is 1 come 3 (SiO3).

Exercise 2.4 Oxygen Deprivation

The diagram below represents a single chain in a silicate mineral. Counting the number of tetrahedra matches the number of oxygen ion (yellow spheres). Every tetrahedron has one silicon ion so this should give the ratio of Si to O in single-chain silicates (e.g., pyroxene).

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The diagram listed below represents a dual chain in a silicate mineral. Again, count the number of tetrahedra versus the variety of oxygen ions. This should offer you the proportion of Si to O in double-chain silicates (e.g., amphibole).

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In amphibole structures, the silica tetrahedra are connected in a double chain that has actually an oxygen-to-silicon ratio reduced than that of pyroxene, and also hence still fewer cations are essential to balance the charge. Amphibole is even much more permissive than pyroxene and also its compositions deserve to be very complex. Hornblende, because that example, can include sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and also the hydroxyl ion (OH–).


In mica structures, the silica tetrahedra room arranged in constant sheets, wherein each tetrahedron shares three oxygen anions with adjacent tetrahedra. There is even more sharing that oxygens between adjacent tetrahedra and hence fewer charge-balancing cations are needed for paper silicate minerals. Bonding in between sheets is fairly weak, and also this accounts for the well-developed one-directional cleavage (Figure 2.14). Biotite mica deserve to have stole and/or magnesium in it and that makes it a ferromagnesian silicate mineral (like olivine, pyroxene, and also amphibole). Chlorite is another similar mineral that frequently includes magnesium. In muscovite mica, the only cations current are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral.

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Figure 2.14 Biotite mica (left) and muscovite mica (right). Both are sheet silicates and split quickly into thin layers follow me planes parallel to the sheets. Biotite is dark favor the other iron- and/or magnesium-bearing silicates (e.g., olivine, pyroxene, and also amphibole), while muscovite is irradiate coloured. (Each sample is around 3 centimeter across.)

Apart native muscovite, biotite, and also chlorite, over there are many other sheet silicates (or phyllosilicates), which generally exist as clay-sized fragments (i.e., less than 0.004 mm). These encompass the clay minerals kaolinite, illite, and also smectite, and also although they are daunting to study since of their very tiny size, lock are exceptionally important materials of rocks and especially of soils.

All of the sheet silicate minerals additionally have water in their structure.

Silica tetrahedra space bonded in three-dimensional frameworks in both the feldspars and also quartz. These space non-ferromagnesian minerals — they don’t contain any type of iron or magnesium. In addition to silica tetrahedra, feldspars encompass the cations aluminum, potassium, sodium, and also calcium in various combinations. Quartz includes only silica tetrahedra.

The three key feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and also two varieties of plagioclase feldspar: albite (sodium only) and also anorthite (calcium only). Together is the case for iron and magnesium in olivine, over there is a consistent range of compositions (solid equipment series) in between albite and also anorthite in plagioclase. This is because the calcium and also sodium ion are nearly identical in size (1.00 Å versus 0.99 Å). Any kind of intermediate compositions between CaAl2Si3O8 and NaAlSi3O8 can exist (Figure 2.15). This is a small bit how amazing because, return they are very comparable in size, calcium and sodium ions don’t have actually the very same charge (Ca2+ versus Na+). This trouble is accounted because that by corresponding substitution that Al3+ for Si4+. Therefore, albite is NaAlSi3O8 (one Al and three Si) when anorthite is CaAl2Si2O8 (two Al and also two Si), and also plagioclase feldspars of intermediate composition have intermediate proportions that Al and also Si. This is referred to as a “coupled-substitution.”

The intermediate-composition plagioclase feldspars are oligoclase (10% to 30% Ca), andesine (30% to 50% Ca), labradorite (50% to 70% Ca), and also bytownite (70% come 90% Ca). K-feldspar (KAlSi3O8) has a slightly various structure 보다 that the plagioclase, owing to the bigger size of the potassium ion (1.37 Å) and also because of this big size, potassium and sodium perform not easily substitute because that each other, other than at high temperatures. This high-temperature feldspars are most likely to be uncovered only in volcanic rocks due to the fact that intrusive igneous rocks cool slowly enough to low temperatures because that the feldspars to change into among the lower-temperature forms.

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Figure 2.15 Compositions that the feldspar minerals

In quartz (SiO2), the silica tetrahedra space bonded in a “perfect” three-dimensional framework. Every tetrahedron is external inspection to four other tetrahedra (with an oxygen common at every corner of every tetrahedron), and as a result, the ratio of silicon come oxygen is 1:2. Due to the fact that the one silicon cation has actually a +4 charge and the two oxygen anions each have actually a –2 charge, the fee is balanced. Over there is no require for aluminum or any kind of of the other cations together as salt or potassium. The hardness and also lack the cleavage in quartz result from the strong covalent/ionic bond characteristic the the silica tetrahedron.


Exercise 2.5 Ferromagnesian Silicates?

Silicate minerals are classified as being one of two people ferromagnesian or non-ferromagnesian relying on whether or no they have iron (Fe) and/or magnesium (Mg) in your formula. A variety of minerals and also their formulas are listed below. For each one, indicate whether or no it is a ferromagnesian silicate.

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MineralFormulaFerromagnesian Silicate?
olivine(Mg,Fe)2SiO4
pyriteFeS2
plagioclaseCaAl2Si2O8
pyroxeneMgSiO3
hematiteFe2O3
orthoclaseKAlSi3O8
quartzSiO2
MineralFormula*Ferromagnesian Silicate?
amphiboleFe7Si8O22(OH)2
muscoviteK2Al4 Si6Al2O20(OH)4
magnetiteFe3O4
biotiteK2Fe4Al2Si6Al4O20(OH)4
dolomite(Ca,Mg)CO3
garnetFe2Al2Si3O12
serpentineMg3Si2O5(OH)4

*Some the the formulas, specifically the more complicated ones, have actually been simplified.