2.4 Silicate Minerals

The vast majority of the minerals that consist of the rocks the Earth’s crust are silicate minerals. These encompass minerals such together quartz, feldspar, mica, amphibole, pyroxene, olivine, and a variety of clay minerals. The building block of every one of these minerals is the silica tetrahedron, a mix of four oxygen atoms and one silicon atom. Together we’ve seen, it’s dubbed a tetrahedron due to the fact that planes attracted through the oxygen atoms kind a shape with 4 surface (Figure 2.2.4). Due to the fact that the silicon ion has actually a charge of 4 and each of the 4 oxygen ions has actually a fee of −2, the silica tetrahedron has a net charge of −4.

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In silicate minerals, this tetrahedra room arranged and linked with each other in a variety of ways, from single units to facility frameworks (Table 2.6). The simplest silicate structure, that of the mineral olivine, is created of secluded tetrahedra external inspection to iron and/or magnesium ions. In olivine, the −4 fee of every silica tetrahedron is well balanced by two divalent (i.e., +2) iron or magnesium cations. Olivine deserve to be one of two people Mg2SiO4 or Fe2SiO4, or some combination of the two (Mg,Fe)2SiO4. The divalent cations the magnesium and iron are fairly close in radius (0.73 versus 0.62 angstroms<1>). Because the this dimension similarity, and because they room both divalent cations (both can have a charge of +2), iron and magnesium have the right to readily substitute for each other in olivine and also in plenty of other minerals.

Table 2.6 Silicate mineral configurations. The triangles stand for silica tetrahedra.Tetrahedron configuration PictureTetrahedron Configuration NameExample Minerals
Isolated (nesosilicates)Olivine, garnet, zircon, kyanite
Pairs (sorosilicates)Epidote, zoisite
Rings (cyclosilicates)Tourmaline
Single chain (inosilicates)Pyroxenes, wollastonite
Double chains (inosilicates)Amphiboles
Sheets (phyllosilicates)Micas, clay minerals, serpentine, chlorite
3-dimensional structureFramework (tectosilicates)Feldspars, quartz, zeolite

Exercise 2.3 make a Tetrahedron


Cut about the outside of the shape (solid lines and also dotted lines), and also then fold along the solid lines to form a tetrahedron. If you have glue or tape, certain the tabs come the tetrahedron to organize it together. If girlfriend don’t have actually glue or tape, do a part along the thin grey line and also insert the spicy tab right into the slit.

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

See attachment 3 for exercise 2.3 answers.

In olivine, unlike most other silicate minerals, the silica tetrahedra room not external inspection to every other. Instead they room bonded come the stole and/or magnesium ions, in the configuration shown on number 2.4.1.

Figure 2.4.1 A depiction of the structure of olivine as seen from above. The formula because that this particular olivine, which has three Fe ion for every Mg ion, can be written: Mg0.5Fe1.5SiO4.

As currently noted, the 2 ions of iron and also magnesium are comparable in size (although not fairly the same). This allows them come substitute because that each other in some silicate minerals. In fact, the ions that are typical in silicate minerals have actually a wide variety of sizes, as illustrated in number 2.4.2. Every one of the ions shown are cations, except for oxygen. Keep in mind that iron can exist together both a +2 ion (if the loses two electrons during ionization) or a +3 ion (if it loses three). Fe2+  is well-known as ferrous iron. Fe3+  is well-known as ferric iron. Ionic radii are vital to the composition of silicate minerals, so we’ll be introduce to this chart again.

Figure 2.4.2 The ionic radii (effective sizes) in angstroms, of few of the typical ions in silicate minerals.

The structure of the single-chain silicate pyroxene is shown on figures 2.4.3 and also 2.4.4. In pyroxene, silica tetrahedra are attached together in a single chain, where one oxygen ion from each tetrahedron is common with the nearby tetrahedron, therefore there are fewer oxygens in the structure. The an outcome is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and also the net fee per silicon atom is less (−2 instead of −4). Therefore, under cations are essential to balance the charge. Pyroxene compositions room of the kind MgSiO3, FeSiO3, and also CaSiO3, or some combination of these. Pyroxene can additionally be created as (Mg,Fe,Ca)SiO3, whereby the aspects in the brackets deserve to be existing in any proportion. In other words, pyroxene has actually one cation for each silica tetrahedron (e.g., MgSiO3) when olivine has two (e.g., Mg2SiO4). Because each silicon ion is +4 and each oxygen ion is −2, the 3 oxygens (−6) and also the one silicon (+4) provide a net charge of −2 for the solitary chain that silica tetrahedra. In pyroxene, the one divalent cation (2) every tetrahedron balances that −2 charge. In olivine, it takes two divalent cations come balance the −4 charge of an diverted tetrahedron.The framework of pyroxene is more “permissive” than that of olivine—meaning the cations v a broader range that ionic radii have the right to fit into it. That’s why pyroxenes can have stole (radius 0.63 Å) or magnesium (radius 0.72 Å) or calcium (radius 1.00 Å) cations (see figure 2.4.2 above).

Figure 2.4.3 A explicate of the structure of pyroxene. The tetrahedral chains continue to left and right and each is interspersed with a series of divalent cations. If these space Mg ions, climate the formula is MgSiO3.
Figure 2.4.4 A solitary silica tetrahedron (left) with 4 oxygen ions per silicon ion (SiO4). Part of a single chain of tetrahedra (right), where the oxygen atoms 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).

The diagram below represents a single chain in a silicate mineral. Count the number of tetrahedra versus the number of oxygen ion (yellow spheres). Each tetrahedron has one silicon ion therefore this should offer you the proportion of Si come O in single-chain silicates (e.g., pyroxene).


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


See attachment 3 for exercise 2.4 answers.

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

In mica structures, the silica tetrahedra are arranged in constant sheets, where each tetrahedron shares three oxygen anions with adjacent tetrahedra. Over there is even more sharing the oxygens between adjacent tetrahedra and also hence fewer cations are required to balance the charge of the silica-tetrahedra framework in sheet silicate minerals. Bonding between sheets is fairly weak, and this accounts because that the well-developed one-directional cleavage in micas (Figure 2.4.5). Biotite mica have the right to have steel and/or magnesium in it and that renders 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 existing are aluminum and also potassium; thus it is a non-ferromagnesian silicate mineral.

Figure 2.4.5 Biotite mica (left) and also muscovite mica (right). Both are sheet silicates and also split quickly into thin layers along planes parallel come the sheets. Biotite is dark favor the various other iron- and/or magnesium-bearing silicates (e.g., olivine, pyroxene, and amphibole), while muscovite is light coloured. (Each sample is about 3 centimeter across.)

Apart indigenous muscovite, biotite, and also chlorite, over there are many other sheet silicates (a.k.a. phyllosilicates), numerous of i m sorry exist together clay-sized fragments (i.e., less than 0.004 millimetres). These incorporate the clay mineral kaolinite, illite, and also smectite, and also although lock are challenging to study since of your very tiny size, lock are very important materials of rocks and especially of soils.

All of the sheet silicate minerals additionally have water molecules within your structure.

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

The three main feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and also two species of plagioclase feldspar: albite (sodium only) and also anorthite (calcium only). Together is the instance for iron and magnesium in olivine, there is a continuous range that compositions (solid equipment series) in between albite and anorthite in plagioclase. Due to the fact that the calcium and sodium ions are nearly identical in size (1.00 Å matches 0.99 Å) any type of intermediate compositions in between CaAl2Si3O8 and NaAlSi3O8 can exist (Figure 2.4.6). This is a tiny bit how amazing because, back they space very similar in size, calcium and sodium ions don’t have actually the very same charge (Ca2+ matches Na+ ). This difficulty is accounted for by the matching substitution of Al+3  for Si+4 . Therefore, albite is NaAlSi3O8 (1 Al and 3 Si) when anorthite is CaAl2Si2O8 (2 Al and 2 Si), and also plagioclase feldspars of intermediary composition have actually intermediate proportions the Al and also Si. This is dubbed a “coupled-substitution.”

The intermediate-composition plagioclase feldspars room oligoclase (10% come 30% Ca), andesine (30% come 50% Ca), labradorite (50% to 70% Ca), and bytownite (70% come 90% Ca). K-feldspar (KAlSi3O8) has a slightly different structure 보다 that the plagioclase, fan to the larger size the the potassium ion (1.37 Å) and because that this big size, potassium and sodium carry out not easily substitute for each other, other than at high temperatures. These high-temperature feldspars are most likely to be found only in volcano 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.

Figure 2.4.6 Compositions of the feldspar minerals.

In quartz (SiO2), the silica tetrahedra are bonded in a “perfect” three-dimensional framework. Each tetrahedron is external inspection to 4 other tetrahedra (with one oxygen mutual at every corner of each tetrahedron), and as a result, the proportion of silicon to oxygen is 1:2. Because the one silicon cation has a +4 charge and also 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 other cations together as salt or potassium. The hardness and also lack of cleavage in quartz result from the solid covalent/ionic bond characteristic that the silica tetrahedron.

Silicate minerals room classified as being one of two people ferromagnesian or non-ferromagnesian depending on whether or not they have iron (Fe) and/or magnesium (Mg) in your formula. A variety of minerals and their recipe are detailed below. For each one, indicate whether or no it is a ferromagnesian silicate.

MineralFormulaFerromagnesian silicate?
plagioclase feldsparCaAl2Si2O8.
orthoclase feldsparKAlSi3O8.

See postposition 3 for practice 2.5 answers.*Some that the formulas, especially the more facility ones, have been simplified.

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Image Descriptions

Figure 2.4.2 photo description: The ionic radii of elements in angstroms and their charges.ElementIonic Radii (in angstroms)Charge
Oxygen1.4−2 (Anion)
Potassium1.371 (Cation)
Calcium1.002 (Cation)
Sodium0.991 (Cation)
Magnesium0.722 (Cation)
Iron0.632 (Cation)
0.493 (Cation)
Aluminum0.393 (Cation)
Silicon0.264 (Cation)
Carbon0.154 (Cation)