2.4 Silicate Minerals

The large majority of the minerals that comprise the rocks of Earth’s crust are silicate minerals. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals. The building block of all of these minerals is the silica tetrahedron, a mix of four oxygen atoms and also one silsymbol atom. These are arranged such that planes attracted with the oxygen atoms develop a tetrahedron (Figure 2.6). Because the silsymbol ion has actually a charge of +4 and each of the 4 oxygen ions has actually a charge of –2, the silica tetrahedron has actually a net charge of –4.

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In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single systems to facility frameworks (Figure 2.9). The most basic silicate framework, that of the mineral olivine, is written of isolated tetrahedra bonded to iron and/or magnesium ions. In olivine, the –4 charge of each silica tetrahedron is balanced by two divalent (i.e., +2) iron or magnesium cations. Olivine deserve to be either Mg2SiO4 or Fe2SiO4, or some combination of the two (Mg,Fe)2SiO4. The divalent cations of magnesium and also iron are quite cshed in radius (0.73 versus 0.62 angstroms<1>). Thus dimension similarity, and bereason they are both divalent cations (both have a charge of +2), iron and also magnesium can readily substitute for each various other in olivine and in many type of other minerals.

Figure 2.9 Silicate mineral configurations. The triangles reexisting 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 structureFrameoccupational (tectosilicates)Feldspars, quartz, zeolite

Exercises


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Exercise 2.3 Make a Tetrahedron

Cut around the external of the form (solid lines and dotted lines), and also then fold alengthy the solid lines to create a tetrahedron.

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

If you are doing this in a classroom, try joining your tetrahedron via others into pairs, rings, single and double chains, sheets, and also even three-dimensional framefunctions.


In olivine, unchoose most other silicate minerals, the silica tetrahedra are not bonded to each various other. They are, but, bonded to the iron and/or magnesium as shown on Figure 2.10.

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Figure 2.10 A depiction of the framework of olivine as watched from over. The formula for this particular olivine, which has actually three Fe ions for each Mg ion, could be written: Mg0.5Fe1.5SiO4.

As currently provided, the +2 ions of iron and magnesium are similar in dimension (although not rather the same). This permits them to substitute for each various other in some silicate minerals. In truth, the common ions in silicate minerals have a wide variety of sizes, as displayed in Figure 2.11. All of the ions displayed are cations, other than for oxygen. Keep in mind that iron have the right to exist as both a +2 ion (if it loses 2 electrons throughout ionization) or a +3 ion (if it loses three). Fe2+ is recognized as ferrous iron. Fe3+ is known as ferric iron. Ionic radii are critical to the complace of silicate minerals, so we’ll be referring to this diagram aget.

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

The structure of the single-chain silicate pyroxene is shown on Figures 2.12 and 2.13. In pyroxene, silica tetrahedra are linked together in a solitary chain, wbelow one oxygen ion from each tetrahedron is common via the adjacent tetrahedron, for this reason tright here are fewer oxygens in the framework. The outcome is that the oxygen-to-silsymbol proportion is lower than in olivine (3:1 instead of 4:1), and also the net charge per silicon atom is less (–2 rather of –4), given that fewer cations are essential to balance that charge. Pyroxene compositions are of the kind MgSiO3, FeSiO3, and CaSiO3, or some combicountry of these. Pyroxene deserve to additionally be written as (Mg,Fe,Ca)SiO3, wbelow the aspects in the brackets deserve to be current in any propercent. In other words, pyroxene has actually one cation for each silica tetrahedron (e.g., MgSiO3) while olivine has actually 2 (e.g., Mg2SiO4). Due to the fact that each silsymbol ion is +4 and 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, it takes 2 divalent cations to balance the –4 charge of an isolated tetrahedron.

The framework of pyroxene is more “permissive” than that of olivine — definition that cations via a more comprehensive range of ionic radii deserve to fit into it. That’s 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 depiction of the framework of pyroxene. The tetrahedral chains proceed to left and also right and also each is interspersed with a collection of divalent cations. If these are Mg ions, then the formula is MgSiO3.
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Figure 2.13 A single silica tetrahedron (left) via 4 oxygen ions per silsymbol ion (SiO4). Part of a solitary chain of tetrahedra (right), wbelow the oxygen atoms at the adjoining corners are common between two tetrahedra (arrows). For an extremely long chain the resulting proportion of silsymbol to oxygen is 1 to 3 (SiO3).

Exercise 2.4 Oxygen Deprivation

The diagram listed below represents a single chain in a silicate mineral. Count the variety of tetrahedra versus the variety of oxygen ions (yellow spheres). Each tetrahedron has actually one silicon ion so this have to give the ratio of Si to O in single-chain silicates (e.g., pyroxene).

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The diagram below represents a twin chain in a silicate mineral. Again, count the number of tetrahedra versus the number of oxygen ions. This have to offer you the ratio 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 dual chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and also thus still fewer cations are essential to balance the charge. Amphibole is even even more permissive than pyroxene and also its compositions can be extremely complicated. Hornblende, for example, have the right to encompass sodium, potassium, calcium, magnesium, iron, aluminum, silsymbol, oxygen, fluorine, and also the hydroxyl ion (OH–).


In mica structures, the silica tetrahedra are arranged in continuous sheets, wright here each tetrahedron shares 3 oxygen anions with nearby tetrahedra. Tright here is even more sharing of oxygens between surrounding tetrahedra and thus fewer charge-balancing cations are necessary for sheet silicate minerals. Bonding between sheets is relatively weak, and this accounts for the well-emerged one-directional cleavage (Figure 2.14). Biotite mica can have iron and/or magnesium in it and also that renders it a ferromagnesian silicate mineral (choose olivine, pyroxene, and also amphibole). Chlorite is another comparable mineral that commonly consists of magnesium. In muscovite mica, the only cations existing are aluminum and potassium; thus 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 also separation conveniently right into thin layers alengthy 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 coloured. (Each sample is around 3 cm across.)

Acomponent from muscovite, biotite, and also chlorite, tright here are many other sheet silicates (or phyllosilicates), which typically exist as clay-sized pieces (i.e., much less than 0.004 mm). These incorporate the clay minerals kaolinite, illite, and smectite, and also although they are hard to research bereason of their extremely tiny size, they are very important components of rocks and particularly of soils.

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

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

The three primary feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and also two forms of plagioclase feldspar: albite (sodium only) and also anorthite (calcium only). As is the instance for iron and magnesium in olivine, there is a continuous range of compositions (solid solution series) in between albite and also anorthite in plagioclase. This is because the calcium and also sodium ions are practically identical in dimension (1.00 Å versus 0.99 Å). Any intermediate compositions between CaAl2Si3O8 and NaAlSi3O8 deserve to exist (Figure 2.15). This is a small little bit surpclimbing because, although they are incredibly equivalent in size, calcium and also sodium ions don’t have the same charge (Ca2+ versus Na+). This trouble is accounted for by equivalent substitution of Al3+ for Si4+. As such, albite is NaAlSi3O8 (one Al and also 3 Si) while anorthite is CaAl2Si2O8 (two Al and two Si), and plagioclase feldspars of intermediate composition have actually intermediate prosections of Al and also Si. This is called a “coupled-substitution.”

The intermediate-complace plagioclase feldspars are oligoclase (10% to 30% Ca), andesine (30% to 50% Ca), labradorite (50% to 70% Ca), and bytownite (70% to 90% Ca). K-feldspar (KAlSi3O8) has actually a slightly different structure than that of plagioclase, owing to the bigger size of the potassium ion (1.37 Å) and bereason of this huge dimension, potassium and also sodium do not readily substitute for each other, other than at high temperatures. These high-temperature feldspars are most likely to be uncovered just in volcanic rocks bereason intrusive igneous rocks cool gradually enough to low temperatures for the feldspars to adjust right into among the lower-temperature creates.

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Figure 2.15 Compositions of the feldspar minerals

In quartz (SiO2), the silica tetrahedra are bonded in a “perfect” three-dimensional structure. Each tetrahedron is bonded to four other tetrahedra (through an oxygen shared at every corner of each tetrahedron), and as an outcome, the proportion of silsymbol to oxygen is 1:2. Since the one silicon cation has actually 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 type of of the various other cations such as sodium or potassium. The hardness and also lack of cleavage in quartz result from the solid covalent/ionic bonds characteristic of the silica tetrahedron.


Exercise 2.5 Ferromagnesian Silicates?

Silicate minerals are classified as being either ferromagnesian or non-ferromagnesian depending on whether or not they have actually iron (Fe) and/or magnesium (Mg) in their formula. A number of minerals and their formulas are listed listed below. For each one, indicate 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

*Several of the formulas, particularly the more complex ones, have been simplified.