2.4 Silicate Minerals

The vast bulk of the mineral that make up the rocks of Earth’s crust space silicate minerals. These incorporate minerals such together quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety that clay minerals. The structure block of every one of these mineral is the silica tetrahedron, a combination of 4 oxygen atoms and one silicon atom. These room arranged such the planes attracted through the oxygen atoms form a tetrahedron (Figure 2.6). Due to the fact that the silicon ion has a fee of +4 and also each of the four oxygen ions has a fee of –2, the silica tetrahedron has actually a net charge 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 facility frameworks (Figure 2.9). The most basic silicate structure, that of the mineral olivine, is composed of secluded tetrahedra external inspection to iron and/or magnesium ions. In olivine, the –4 fee of each silica tetrahedron is well balanced by 2 divalent (i.e., +2) steel or magnesium cations. Olivine can be either Mg2SiO4 or Fe2SiO4, or some mix of the two (Mg,Fe)2SiO4. The divalent cations that magnesium and also iron are fairly close in radius (0.73 matches 0.62 angstroms<1>). Because that this dimension similarity, and also because they room both divalent cations (both have a fee of +2), iron and magnesium deserve to readily substitute because that each other in olivine and also in countless 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 chain (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 exterior of the shape (solid lines and also dotted lines), and then fold follow me the solid currently to form a tetrahedron.

If you have actually glue or tape, certain the tabs come the tetrahedron to hold it together. If girlfriend don’t have actually glue or tape, make a slice along the slim grey line and also insert the spicy tab right into the slit.

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


In olivine, unlike most other silicate minerals, the silica tetrahedra room not bonded to every other. Lock are, however, external inspection to the steel and/or magnesium as displayed on figure 2.10.

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Figure 2.10 A explicate of the framework of olivine as checked out from above. The formula for this particular olivine, which has actually three Fe ion for every Mg ion, could be written: Mg0.5Fe1.5SiO4.

As currently noted, the +2 ions of iron and also magnesium are similar in dimension (although not quite the same). This enables them to substitute for each other in some silicate minerals. In fact, the usual ions in silicate minerals have a wide range of sizes, as presented in number 2.11. All of the ions displayed are cations, other than for oxygen. Note that iron can exist as both a +2 ion (if that loses 2 electrons during ionization) or a +3 ion (if it loser three). Fe2+ is known as ferrous iron. Fe3+ is well-known as ferric iron. Ionic radii are critical to the composition of silicate minerals, so we’ll be introduce to this diagram again.

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

The structure 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, wherein one oxygen ion from each tetrahedron is common with the nearby tetrahedron, thus there space fewer oxygens in the structure. The result is that the oxygen-to-silicon proportion is reduced than in olivine (3:1 rather of 4:1), and also the net fee per silicon atom is less (–2 instead of –4), since fewer cations are important to balance that charge. Pyroxene compositions space of the type MgSiO3, FeSiO3, and CaSiO3, or some mix of these. Pyroxene can additionally be written as (Mg,Fe,Ca)SiO3, whereby the facets in the brackets deserve to be present in any type of proportion. In various other words, pyroxene has actually one cation because that each silica tetrahedron (e.g., MgSiO3) while olivine has two (e.g., Mg2SiO4). Due to the fact that each silicon ion is +4 and each oxygen ion is –2, the three oxygens (–6) and the one silicon (+4) provide a net fee of –2 because that the single chain the silica tetrahedra. In pyroxene, the one divalent cation (2+) every tetrahedron balances that –2 charge. In olivine, that takes 2 divalent cations to balance the –4 fee of an diverted tetrahedron.

The framework of pyroxene is an ext “permissive” than that of olivine — meaning that cations through a broader range the ionic radii have the right to fit right into it. It is why pyroxenes have the right to have iron (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 framework of pyroxene. The tetrahedral chains proceed to left and also right and also each is interspersed through 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 four oxygen ion per silicon ion (SiO4). Component of a solitary chain that tetrahedra (right), wherein the oxygen atoms at the adjoining corners space shared between two tetrahedra (arrows). For a very long chain the resulting proportion of silicon come oxygen is 1 to 3 (SiO3).

Exercise 2.4 Oxygen Deprivation

The diagram listed below represents a single chain in a silicate mineral. Counting the variety of tetrahedra matches the number of oxygen ion (yellow spheres). Every tetrahedron has actually one silicon ion so this should provide the proportion 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 matches the number of oxygen ions. This should give 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 double chain that has actually an oxygen-to-silicon ratio lower than the of pyroxene, and also hence still under cations are necessary to balance the charge. Amphibole is even an ext permissive than pyroxene and also its compositions have the right to be very complex. Hornblende, for example, can encompass sodium, potassium, calcium, magnesium, iron, aluminum, silicon, oxygen, fluorine, and also the hydroxyl ion (OH–).


In mica structures, the silica tetrahedra space arranged in continuous sheets, whereby each tetrahedron shares 3 oxygen anions with adjacent tetrahedra. Over there is even an ext sharing the oxygens between nearby tetrahedra and also hence fewer charge-balancing cations are needed for sheet silicate minerals. Bonding in between sheets is fairly weak, and also this accounts because that the well-developed one-directional cleavage (Figure 2.14). Biotite mica have the right to have iron and/or magnesium in it and that provides it a ferromagnesian silicate mineral (like olivine, pyroxene, and also amphibole). Chlorite is another comparable mineral that frequently includes magnesium. In muscovite mica, the just cations present are aluminum and 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 are sheet silicates and also split conveniently into slim layers along planes parallel come the sheets. Biotite is dark like the other iron- and/or magnesium-bearing silicates (e.g., olivine, pyroxene, and also amphibole), if muscovite is light coloured. (Each sample is around 3 cm across.)

Apart indigenous muscovite, biotite, and chlorite, there are countless other sheet silicates (or phyllosilicates), which generally exist as clay-sized fragments (i.e., less than 0.004 mm). These include the clay minerals kaolinite, illite, and also smectite, and also although castle are difficult to study since of their very little size, they are extremely important components of rocks and also especially the soils.

All of the paper silicate minerals additionally have water in your structure.

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

The three main feldspar minerals room 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). Together is the case for iron and also magnesium in olivine, over there is a continuous range of compositions (solid equipment series) in between albite and also anorthite in plagioclase. This is because the calcium and sodium ions are virtually identical in dimension (1.00 Å matches 0.99 Å). Any kind of intermediate compositions between CaAl2Si3O8 and NaAlSi3O8 can exist (Figure 2.15). This is a little bit how amazing because, return they room very similar in size, calcium and sodium ions don’t have the very same charge (Ca2+ versus Na+). This difficulty is accounted for by corresponding substitution that Al3+ for Si4+. Therefore, albite is NaAlSi3O8 (one Al and also three Si) when anorthite is CaAl2Si2O8 (two Al and two Si), and plagioclase feldspars of intermediary composition have actually intermediate proportions of Al and also Si. This is dubbed 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 bytownite (70% come 90% Ca). K-feldspar (KAlSi3O8) has actually a slightly different structure than that that plagioclase, fan to the larger size of the potassium ion (1.37 Å) and also because the this large size, potassium and sodium perform not conveniently substitute because that each other, except at high temperatures. These high-temperature feldspars are most likely to be found only in volcano rocks since intrusive igneous rocks cool slowly enough to short temperatures because that the feldspars to readjust 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 are bonded in a “perfect” three-dimensional framework. Every tetrahedron is bonded to 4 other tetrahedra (with one oxygen mutual at every edge of each tetrahedron), and also 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 –2 charge, the charge is balanced. There is no require for aluminum or any kind of of the various other cations such 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 depending upon whether or no they have actually iron (Fe) and/or magnesium (Mg) in your formula. A variety of minerals and their formulas are noted below. For each one, suggest whether or not 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, especially the more facility ones, have actually been simplified.