2.4 Silicate Minerals

The vast majority of the minerals that consist of the rocks of Earth’s crust space silicate minerals. These include minerals such together quartz, feldspar, mica, amphibole, pyroxene, olivine, and a an excellent variety of clay minerals. The structure block of all of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom. These space arranged such that planes drawn through the oxygen atoms type a tetrahedron (Figure 2.6). Since the silicon ion has actually a fee of +4 and 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 are arranged and also linked together in a variety of ways, from single units to facility frameworks (Figure 2.9). The most basic silicate structure, the of the mineral olivine, is composed of diverted tetrahedra bonded to steel 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 can be either Mg2SiO4 or Fe2SiO4, or some mix of the 2 (Mg,Fe)2SiO4. The divalent cations the magnesium and iron are quite close in radius (0.73 matches 0.62 angstroms<1>). Because that this size similarity, and also because they room both divalent cations (both have a fee of +2), iron and also magnesium can readily substitute because that each other in olivine and also in plenty of 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 chain (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 do a Tetrahedron

Cut about the outside of the form (solid lines and dotted lines), and then fold follow me the solid lines to form a tetrahedron.

If you have actually glue or tape, for sure the tabs to the tetrahedron to organize it together. If girlfriend don’t have glue or tape, make a part along the thin grey line and insert the sharp tab right into the slit.

If you space doing this in a classroom, shot joining her tetrahedron v others into pairs, rings, single and double chains, sheets, and even three-dimensional frameworks.


In olivine, unlike many other silicate minerals, the silica tetrahedra space not bonded to every other. Lock are, however, bonded to the iron and/or magnesium as shown on figure 2.10.

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

As already noted, the +2 ion of iron and also magnesium are similar in size (although not fairly the same). This enables them come substitute because that each various other in part silicate minerals. In fact, the typical ions in silicate minerals have a wide range of sizes, as displayed in figure 2.11. Every one of the ions presented are cations, other than for oxygen. Note that iron can exist as both a +2 ion (if the loses 2 electrons throughout ionization) or a +3 ion (if it loses three). Fe2+ is known as ferrous iron. Fe3+ is well-known as ferric iron. Ionic radii are vital to the ingredient 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 common 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 connected together in a single chain, whereby one oxygen ion from each tetrahedron is shared with the surrounding tetrahedron, hence there are fewer oxygens in the structure. The result is the the oxygen-to-silicon ratio is reduced than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is much less (–2 instead of –4), due to the fact that fewer cations are necessary to balance that charge. Pyroxene compositions space of the form MgSiO3, FeSiO3, and also CaSiO3, or some mix of these. Pyroxene can likewise be created as (Mg,Fe,Ca)SiO3, whereby the elements in the brackets have the right to be existing in any kind of proportion. In other words, pyroxene has actually one cation for each silica tetrahedron (e.g., MgSiO3) while olivine has actually two (e.g., Mg2SiO4). Due to the fact that each silicon ion is +4 and each oxygen ion is –2, the 3 oxygens (–6) and the one silicon (+4) offer a net fee of –2 for the single chain of silica tetrahedra. In pyroxene, the one divalent cation (2+) every tetrahedron balances the –2 charge. In olivine, that takes 2 divalent cations come balance the –4 charge of an isolated tetrahedron.

The structure of pyroxene is much more “permissive” 보다 that of olivine — meaning that cations with a more comprehensive range that ionic radii have the right to fit right into it. It is why pyroxenes can 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 structure of pyroxene. The tetrahedral chains proceed to left and also right and each is interspersed with a series of divalent cations. If these space Mg ions, climate the formula is MgSiO3.
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Figure 2.13 A single silica tetrahedron (left) with 4 oxygen ions per silicon ion (SiO4). Component of a solitary chain of tetrahedra (right), where the oxygen atoms at the adjoining corners room shared between two tetrahedra (arrows). For a an extremely long chain the resulting ratio 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 number of tetrahedra versus the number of oxygen ion (yellow spheres). Every tetrahedron has one silicon ion so this should give the ratio of Si come O in single-chain silicates (e.g., pyroxene).

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The diagram below represents a double chain in a silicate mineral. Again, count the variety of tetrahedra matches the variety of oxygen ions. This should provide you the ratio of Si come 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 reduced than that of pyroxene, and hence still fewer cations are vital to balance the charge. Amphibole is even an ext permissive 보다 pyroxene and its compositions can 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 space arranged in continuous sheets, where each tetrahedron shares 3 oxygen anions with nearby tetrahedra. Over there is even an ext sharing that oxygens between surrounding tetrahedra and hence under charge-balancing cations are necessary for paper silicate minerals. Bonding in between sheets is relatively weak, and this accounts because that the well-developed one-directional cleavage (Figure 2.14). Biotite mica have the right to have stole and/or magnesium in it and that provides it a ferromagnesian silicate mineral (like olivine, pyroxene, and also amphibole). Chlorite is another similar mineral that commonly includes magnesium. In muscovite mica, the only cations existing are aluminum and also 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 split easily into slim layers follow me planes parallel to the sheets. Biotite is dark favor the various other iron- and/or magnesium-bearing silicates (e.g., olivine, pyroxene, and also amphibole), when muscovite is light coloured. (Each sample is around 3 centimeter across.)

Apart indigenous muscovite, biotite, and also chlorite, there are countless other sheet silicates (or phyllosilicates), which usually exist together clay-sized pieces (i.e., much less than 0.004 mm). These include the clay mineral kaolinite, illite, and also smectite, and although castle are challenging to study because of your very tiny size, lock are very important contents of rocks and also especially the soils.

All the the paper silicate minerals likewise have water in your structure.

Silica tetrahedra space 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 addition to silica tetrahedra, feldspars incorporate the cations aluminum, potassium, sodium, and also calcium in various combinations. Quartz contains only silica tetrahedra.

The three main feldspar minerals space potassium feldspar, (a.k.a. K-feldspar or K-spar) and also two types of plagioclase feldspar: albite (sodium only) and anorthite (calcium only). As is the instance for iron and magnesium in olivine, over there is a constant range of compositions (solid systems series) in between albite and also anorthite in plagioclase. This is since the calcium and sodium ions are practically identical in size (1.00 Å versus 0.99 Å). Any kind of intermediate compositions between CaAl2Si3O8 and also NaAlSi3O8 deserve to exist (Figure 2.15). This is a tiny bit how amazing because, back they room very comparable in size, calcium and sodium ions don’t have actually the same charge (Ca2+ versus Na+). This problem is accounted for by matching substitution of Al3+ because that Si4+. Therefore, albite is NaAlSi3O8 (one Al and three Si) while anorthite is CaAl2Si2O8 (two Al and also two Si), and also plagioclase feldspars of intermediary composition have actually intermediate proportions that Al and also Si. This is called a “coupled-substitution.”

The intermediate-composition plagioclase feldspars space oligoclase (10% to 30% Ca), andesine (30% come 50% Ca), labradorite (50% come 70% Ca), and bytownite (70% come 90% Ca). K-feldspar (KAlSi3O8) has actually a slightly various structure than that that plagioclase, fan to the bigger size the the potassium ion (1.37 Å) and also because the this huge size, potassium and also sodium do not readily substitute for each other, other than at high temperatures. These high-temperature feldspars are likely to be discovered only in volcanic rocks because intrusive igneous rocks cool slowly sufficient to short temperatures for the feldspars to change into among the lower-temperature forms.

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Figure 2.15 Compositions of the feldspar minerals

In quartz (SiO2), the silica tetrahedra room bonded in a “perfect” three-dimensional framework. Each tetrahedron is external inspection to 4 other tetrahedra (with one oxygen mutual at every edge of every 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 also the two oxygen anions each have actually a –2 charge, the charge is balanced. Over there is no require for aluminum or any type of of the other cations such as salt or potassium. The hardness and lack that cleavage in quartz result from the solid covalent/ionic binding characteristic of the silica tetrahedron.


Exercise 2.5 Ferromagnesian Silicates?

Silicate minerals are classified as being either ferromagnesian or non-ferromagnesian depending upon whether or not they have actually iron (Fe) and/or magnesium (Mg) in their formula. A number of minerals and their formulas are detailed 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.