2.4 Silicate Minerals

The vast majority of the minerals that comprise the rocks the Earth’s crust are silicate minerals. These encompass minerals such together quartz, feldspar, mica, amphibole, pyroxene, olivine, and also a great variety of clay minerals. The structure block of all of these minerals is the silica tetrahedron, a mix of four oxygen atoms and one silicon atom. These are arranged such that planes attracted through the oxygen atoms kind a tetrahedron (Figure 2.6). Since the silicon ion has a fee of +4 and each that the 4 oxygen ions has a charge of –2, the silica tetrahedron has a net fee of –4.

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In silicate minerals, this tetrahedra room arranged and linked with each other in a selection of ways, from solitary units to complex frameworks (Figure 2.9). The simplest silicate structure, the of the mineral olivine, is composed of secluded tetrahedra bonded to stole and/or magnesium ions. In olivine, the –4 charge of each silica tetrahedron is well balanced by two divalent (i.e., +2) steel or magnesium cations. Olivine have the right to be one of two people Mg2SiO4 or Fe2SiO4, or some mix of the two (Mg,Fe)2SiO4. The divalent cations the magnesium and iron are quite close in radius (0.73 versus 0.62 angstroms<1>). Because of this dimension similarity, and because they are both divalent cations (both have a charge of +2), iron and magnesium have the right to readily substitute for each other in olivine and also in many other minerals.

Figure 2.9 Silicate mineral configurations. The triangles stand for 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 make a Tetrahedron

Cut around the outside of the form (solid lines and also dotted lines), and also then fold along the solid currently to kind a tetrahedron.

If you have glue or tape, secure the tabs to the tetrahedron to organize it together. If friend don’t have glue or tape, do a slice along the thin grey line and insert the pointed tab right into the slit.

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


In olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. Castle are, however, bonded to the iron and/or magnesium as shown on figure 2.10.

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Figure 2.10 A explicate of the structure of olivine as checked out from above. The formula because that this particular olivine, which has three Fe ions for each Mg ion, might be written: Mg0.5Fe1.5SiO4.

As already noted, the +2 ions of iron and magnesium are similar in dimension (although not rather the same). This allows them to substitute because that each various other in part silicate minerals. In fact, the usual ions in silicate minerals have actually a wide variety of sizes, as presented in figure 2.11. All of the ions shown are cations, except for oxygen. Note that iron deserve to exist as both a +2 ion (if the loses 2 electrons during ionization) or a +3 ion (if it loses three). Fe2+ is recognized as ferrous iron. Fe3+ is recognized as ferric iron. Ionic radii are critical to the composition of silicate minerals, so fine be referring to this chart again.

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

The structure of the single-chain silicate pyroxene is displayed on figures 2.12 and 2.13. In pyroxene, silica tetrahedra are attached together in a single chain, where one oxygen ion from each tetrahedron is mutual with the adjacent tetrahedron, therefore there are fewer oxygens in the structure. The result is the the oxygen-to-silicon ratio is lower than in olivine (3:1 rather of 4:1), and also the net fee per silicon atom is less (–2 rather of –4), due to the fact that fewer cations are important to balance the charge. Pyroxene compositions room of the type MgSiO3, FeSiO3, and also CaSiO3, or some combination of these. Pyroxene can likewise be written as (Mg,Fe,Ca)SiO3, where the elements in the brackets can be current in any proportion. In other words, pyroxene has one cation because that each silica tetrahedron (e.g., MgSiO3) while olivine has two (e.g., Mg2SiO4). Since each silicon ion is +4 and also each oxygen ion is –2, the three oxygens (–6) and also the one silicon (+4) provide a net charge of –2 because that the single chain the silica tetrahedra. In pyroxene, the one divalent cation (2+) every tetrahedron balances the –2 charge. In olivine, it takes two divalent cations come balance the –4 fee of an secluded tetrahedron.

The structure of pyroxene is an ext “permissive” than that the olivine — an interpretation that cations through a more comprehensive range the ionic radii have the right to fit right into it. It is why pyroxenes can 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 explicate of the structure of pyroxene. The tetrahedral chains continue to left and also right and also each is interspersed with a collection of divalent cations. If these room Mg ions, then the formula is MgSiO3.
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Figure 2.13 A solitary silica tetrahedron (left) with 4 oxygen ion per silicon ion (SiO4). Part of a single chain of tetrahedra (right), wherein the oxygen atom at the adjoining corners space shared between two tetrahedra (arrows). Because that a an extremely long chain the resulting ratio of silicon to oxygen is 1 to 3 (SiO3).

Exercise 2.4 Oxygen Deprivation

The diagram listed below represents a solitary chain in a silicate mineral. Counting the number of tetrahedra matches the variety of oxygen ion (yellow spheres). Every tetrahedron has actually one silicon ion for this reason this should offer the ratio of Si come O in single-chain silicates (e.g., pyroxene).

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The diagram below represents a twin chain in a silicate mineral. Again, counting 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 attached in a twin chain that has an oxygen-to-silicon ratio reduced than the of pyroxene, and hence still fewer cations are vital to balance the charge. Amphibole is even an ext permissive 보다 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 the hydroxyl ion (OH–).


In mica structures, the silica tetrahedra room arranged in continuous sheets, whereby each tetrahedron shares three oxygen anions with adjacent tetrahedra. Over there is even much more sharing of oxygens between surrounding tetrahedra and hence fewer charge-balancing cations are essential for sheet silicate minerals. Bonding in between sheets is relatively weak, and this accounts for the well-developed one-directional cleavage (Figure 2.14). Biotite mica have the right to have iron and/or magnesium in it and also that renders it a ferromagnesian silicate mineral (like olivine, pyroxene, and amphibole). Chlorite is another similar mineral that commonly includes magnesium. In muscovite mica, the just cations current are aluminum and also potassium; for this reason it is a non-ferromagnesian silicate mineral.

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Figure 2.14 Biotite mica (left) and also muscovite mica (right). Both space sheet silicates and split easily into slim layers along planes parallel to the sheets. Biotite is dark prefer the other iron- and/or magnesium-bearing silicates (e.g., olivine, pyroxene, and also amphibole), when muscovite is light coloured. (Each sample is about 3 cm across.)

Apart from muscovite, biotite, and also chlorite, over there are numerous other sheet silicates (or phyllosilicates), which commonly exist as clay-sized pieces (i.e., much less than 0.004 mm). These include the clay mineral kaolinite, illite, and smectite, and also although they are challenging to study since of their very small size, lock are very important materials of rocks and also especially that soils.

All that the sheet silicate minerals likewise have water in your structure.

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

The three key feldspar minerals space potassium feldspar, (a.k.a. K-feldspar or K-spar) and also two types of plagioclase feldspar: albite (sodium only) and also anorthite (calcium only). As is the instance for iron and also magnesium in olivine, there is a continuous range that compositions (solid solution series) between albite and anorthite in plagioclase. This is due to the fact that the calcium and sodium ions are virtually identical in dimension (1.00 Å matches 0.99 Å). Any type of intermediate compositions in between CaAl2Si3O8 and NaAlSi3O8 deserve to exist (Figure 2.15). This is a small bit how amazing because, back they are very similar in size, calcium and also sodium ions don’t have the very same charge (Ca2+ versus Na+). This trouble is accounted for by corresponding substitution the Al3+ for Si4+. Therefore, albite is NaAlSi3O8 (one Al and also three Si) if anorthite is CaAl2Si2O8 (two Al and also two Si), and plagioclase feldspars of intermediate composition have actually intermediate proportions of Al and also Si. This is called a “coupled-substitution.”

The intermediate-composition plagioclase feldspars room 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 different structure than that of plagioclase, owing to the larger size of the potassium ion (1.37 Å) and also because the this huge size, potassium and also sodium execute not easily substitute because that each other, other than at high temperatures. This high-temperature feldspars are likely to be discovered only in volcanic rocks since intrusive igneous rocks cool slowly enough to low temperatures for the feldspars to change into one of 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. Each tetrahedron is external inspection to four other tetrahedra (with an oxygen mutual at every edge of every tetrahedron), and as a result, the proportion of silicon come oxygen is 1:2. Because the one silicon cation has actually a +4 charge and the two oxygen anions each have a –2 charge, the fee is balanced. There is no require for aluminum or any type of of the various other cations together as sodium or potassium. The hardness and also lack of cleavage in quartz result from the strong covalent/ionic bonds characteristic of the silica tetrahedron.


Exercise 2.5 Ferromagnesian Silicates?

Silicate minerals space classified together being either ferromagnesian or non-ferromagnesian relying 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, suggest 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 of the formulas, especially the more facility ones, have been simplified.