![]() In other words, pyroxene has one cation for each silica tetrahedron (e.g., MgSiO 3) while olivine has two (e.g., Mg 2SiO 4). Pyroxene can also be written as (Mg,Fe,Ca)SiO 3, where the elements in the brackets can be present in any proportion. Pyroxene compositions are of the type MgSiO 3, FeSiO 3, and CaSiO 3, or some combination of these. The result is that the oxygen-to-silicon ratio is lower than in olivine (3:1 instead of 4:1), and the net charge per silicon atom is less (–2 instead of –4), since fewer cations are necessary to balance that charge. ![]() In pyroxene, silica tetrahedra are linked together in a single chain, where one oxygen ion from each tetrahedron is shared with the adjacent tetrahedron, hence there are fewer oxygens in the structure. The structure of the single-chain silicate pyroxene is shown on Figures 2.12 and 2.13. Figure 2.11 The ionic radii (effective sizes) in angstroms, of some of the common ions in silicate minerals Ionic radii are critical to the composition of silicate minerals, so we’ll be referring to this diagram again. Note that iron can exist as both a +2 ion (if it loses two electrons during ionization) or a +3 ion (if it loses three). All of the ions shown are cations, except for oxygen. In fact, the common ions in silicate minerals have a wide range of sizes, as shown in Figure 2.11. This allows them to substitute for each other in some silicate minerals. The formula for this particular olivine, which has three Fe ions for each Mg ion, could be written: Mg0.5Fe1.5SiO4.Īs already noted, the +2 ions of iron and magnesium are similar in size (although not quite the same). Figure 2.10 A depiction of the structure of olivine as seen from above. They are, however, bonded to the iron and/or magnesium as shown on Figure 2.10. In olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. If you are doing this in a classroom, try joining your tetrahedron with others into pairs, rings, single and double chains, sheets, and even three-dimensional frameworks. If you don’t have glue or tape, make a slice along the thin grey line and insert the pointed tab into the slit. If you have glue or tape, secure the tabs to the tetrahedron to hold it together. Micas, clay minerals, serpentine, chloriteĬut around the outside of the shape (solid lines and dotted lines), and then fold along the solid lines to form a tetrahedron. The triangles represent silica tetrahedra. Figure 2.9 Silicate mineral configurations. The divalent cations of magnesium and iron are quite close in radius (0.73 versus 0.62 angstroms ). Because of this size similarity, and because they are both divalent cations (both have a charge of +2), iron and magnesium can readily substitute for each other in olivine and in many other minerals. Olivine can be either Mg 2SiO 4 or Fe 2SiO 4, or some combination of the two (Mg,Fe) 2SiO 4. In olivine, the –4 charge of each silica tetrahedron is balanced by two divalent (i.e., +2) iron or magnesium cations. The simplest silicate structure, that of the mineral olivine, is composed of isolated tetrahedra bonded to iron and/or magnesium ions. In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to complex frameworks (Figure 2.9). Since the silicon ion has a charge of +4 and each of the four oxygen ions has a charge of –2, the silica tetrahedron has a net charge of –4. ![]() These are arranged such that planes drawn through the oxygen atoms form a tetrahedron (Figure 2.6). The building block of all of these minerals is the silica tetrahedron, a combination of four oxygen atoms and one silicon atom. These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals. The vast majority of the minerals that make up the rocks of Earth’s crust are silicate minerals.
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