2.4 Silicate Minerals

The vast bulk of the mineral that comprise the rocks the Earth’s crust room silicate minerals. These incorporate minerals such together quartz, feldspar, mica, amphibole, pyroxene, olivine, and a variety of clay minerals. The building block of every one of these mineral is the silica tetrahedron, a mix of four oxygen atoms and also one silicon atom. As we’ve seen, it’s dubbed a tetrahedron since planes drawn through the oxygen atoms form a form with 4 surface (Figure 2.2.4). Due to the fact that the silicon ion has actually a fee of 4 and also each the the 4 oxygen ions has a fee of −2, the silica tetrahedron has a net charge of −4.

You are watching: Quartz, by definition, is a dark silicate mineral.

In silicate minerals, this tetrahedra room arranged and also linked together in a range of ways, from solitary units to complex frameworks (Table 2.6). The easiest silicate structure, the of the mineral olivine, is written of secluded tetrahedra external inspection to iron and/or magnesium ions. In olivine, the −4 fee of each silica tetrahedron is well balanced by two divalent (i.e., +2) stole or magnesium cations. Olivine have the right to be either Mg2SiO4 or Fe2SiO4, or some mix of the two (Mg,Fe)2SiO4. The divalent cations of magnesium and iron are rather close in radius (0.73 matches 0.62 angstroms<1>). Because that this dimension similarity, and because they space both divalent cations (both can have a charge of +2), iron and also magnesium have the right to readily substitute for each other in olivine and in plenty of other minerals.

Table 2.6 Silicate mineral configurations. The triangles represent 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 around the exterior of the form (solid lines and dotted lines), and then fold follow me the solid present to form a tetrahedron. If you have glue or tape, for sure the tabs come the tetrahedron to organize it together. If friend don’t have actually glue or tape, make a part along the slim grey line and insert the spicy tab right into the slit.

If you are doing this in a classroom, try joining her tetrahedron through others into pairs, rings, single and double 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 space not bonded to every other. Instead they are bonded to the stole and/or magnesium ions, in the configuration displayed on number 2.4.1.

Figure 2.4.1 A relenten of the framework of olivine as watched from above. The formula because that this specific olivine, which has actually three Fe ions for every Mg ion, can be written: Mg0.5Fe1.5SiO4.

As currently noted, the 2 ions of iron and also magnesium are similar in size (although not quite the same). This allows them come substitute for each various other in some silicate minerals. In fact, the ions that are common in silicate minerals have a wide selection of sizes, as illustrated in number 2.4.2. All of the ions shown are cations, except for oxygen. Keep in mind that iron deserve to exist together both a +2 ion (if that loses two electrons during ionization) or a +3 ion (if it loses three). Fe2+  is well-known as ferrous iron. Fe3+  is known as ferric iron. Ionic radii are an important to the ingredient of silicate minerals, so we’ll be introduce to this diagram again.

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

The structure of the single-chain silicate pyroxene is presented on numbers 2.4.3 and 2.4.4. In pyroxene, silica tetrahedra are connected together in a solitary chain, where one oxygen ion from every tetrahedron is shared with the nearby tetrahedron, therefore there room fewer oxygens in the structure. The an outcome is the the oxygen-to-silicon ratio is lower than in olivine (3:1 rather of 4:1), and the net fee per silicon atom is less (−2 rather of −4). Therefore, fewer cations are necessary to balance that charge. Pyroxene compositions room of the form MgSiO3, FeSiO3, and also CaSiO3, or some mix of these. Pyroxene can likewise be written as (Mg,Fe,Ca)SiO3, where the facets in the brackets can be existing in any kind of proportion. In other words, pyroxene has actually one cation for each silica tetrahedron (e.g., MgSiO3) when olivine has actually two (e.g., Mg2SiO4). Because each silicon ion is +4 and also each oxygen ion is −2, the three oxygens (−6) and the one silicon (+4) provide a net fee of −2 because that the solitary chain of silica tetrahedra. In pyroxene, the one divalent cation (2) every tetrahedron balances that −2 charge. In olivine, it takes 2 divalent cations come balance the −4 fee of an isolated tetrahedron.The structure of pyroxene is an ext “permissive” than that of olivine—meaning the cations with a wider range that ionic radii can 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 (see figure 2.4.2 above).

Figure 2.4.3 A relenten of the framework of pyroxene. The tetrahedral chains proceed to left and right and each is interspersed with a series of divalent cations. If these room Mg ions, then the formula is MgSiO3.
Figure 2.4.4 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 are shared between two tetrahedra (arrows). For a very long chain the resulting ratio of silicon come oxygen is 1 come 3 (SiO3).

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


The diagram below represents a twin chain in a silicate mineral. Again, count the variety of tetrahedra versus the variety of oxygen ions. This should provide you the ratio of Si to O in double-chain silicates (e.g., amphibole).


See attachment 3 for practice 2.4 answers.

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

In mica structures, the silica tetrahedra space arranged in constant sheets, wherein each tetrahedron shares 3 oxygen anions with surrounding tetrahedra. Over there is even much more sharing of oxygens between adjacent tetrahedra and hence fewer cations are required to balance the fee of the silica-tetrahedra framework in paper silicate minerals. Bonding in between sheets is fairly weak, and also this accounts because that the well-developed one-directional cleavage in micas (Figure 2.4.5). Biotite mica deserve to have steel 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 only cations current 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 space sheet silicates and split quickly into thin layers along planes parallel come the sheets. Biotite is dark like 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 chlorite, there are plenty of other sheet silicates (a.k.a. phyllosilicates), countless of which exist together clay-sized fragments (i.e., less than 0.004 millimetres). These encompass the clay minerals kaolinite, illite, and smectite, and although castle are difficult to study because of your very little size, they are very important materials of rocks and especially the soils.

All of the paper silicate minerals additionally have water molecule within 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 iron or magnesium. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations. Quartz consists of only silica tetrahedra.

The three main feldspar minerals room potassium feldspar, (a.k.a. K-feldspar or K-spar) and two varieties of plagioclase feldspar: albite (sodium only) and also anorthite (calcium only). As is the case for iron and magnesium in olivine, over there is a constant range that compositions (solid systems series) between albite and also anorthite in plagioclase. Since the calcium and also sodium ions are practically identical in size (1.00 Å versus 0.99 Å) any type of intermediate compositions in between CaAl2Si3O8 and also NaAlSi3O8 have the right to exist (Figure 2.4.6). This is a little bit how amazing because, although they room very similar in size, calcium and also sodium ions don’t have the same charge (Ca2+ matches Na+ ). This trouble is accounted for by the equivalent substitution that Al+3  for Si+4 . Therefore, albite is NaAlSi3O8 (1 Al and 3 Si) if anorthite is CaAl2Si2O8 (2 Al and also 2 Si), and also plagioclase feldspars of intermediary composition have intermediate proportions that Al and 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 actually a slightly various structure than that the plagioclase, fan to the bigger size the the potassium ion (1.37 Å) and also because the this huge size, potassium and sodium execute not conveniently substitute because that each other, except at high temperatures. These high-temperature feldspars are most likely to be uncovered only in volcano rocks since intrusive igneous rocks cool slowly enough to low temperatures for the feldspars to adjust into one of 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. Every tetrahedron is bonded to four other tetrahedra (with an oxygen common at every edge of each tetrahedron), and as a result, the ratio of silicon come oxygen is 1:2. Since the one silicon cation has a +4 charge and also the two oxygen anions each have a −2 charge, the fee is balanced. There is no need for aluminum or any kind of of the other cations such as sodium or potassium. The hardness and lack of cleavage in quartz result from the solid covalent/ionic bond characteristic the the silica tetrahedron.

Silicate minerals are classified as being one of two people ferromagnesian or non-ferromagnesian depending on whether or no they have actually iron (Fe) and/or magnesium (Mg) in their formula. A number of minerals and also their recipe are listed below. Because that each one, show whether or not it is a ferromagnesian silicate.

MineralFormulaFerromagnesian silicate?
plagioclase feldsparCaAl2Si2O8.
orthoclase feldsparKAlSi3O8.

See appendix 3 for exercise 2.5 answers.*Some that the formulas, particularly the more complicated ones, have actually been simplified.

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Image Descriptions

Figure 2.4.2 photo description: The ionic radii of aspects in angstroms and also 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)