What makes silicate minerals

In this course, we will focus on just the isolated, single chain, double chain, sheet, and framework silicates. In olivine, unlike most other silicate minerals, the silica tetrahedra are not bonded to each other. As already noted, the 2 ions of iron and magnesium are similar in size although not quite the same. This allows them to substitute for each other in some silicate minerals. In fact, the ions that are common in silicate minerals have a wide range of sizes, as depicted in Figure 3.

All of the ions shown are cations, except for oxygen. The structure of the single-chain silicate pyroxene is shown on Figures 3. 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.

Therefore, fewer cations are necessary to balance that charge. In amphibole structures, the silica tetrahedra are linked in a double chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and hence still fewer cations are necessary to balance the charge.

Amphibole is even more permissive than pyroxene and its compositions can be very complex. In mica minerals, the silica tetrahedra are arranged in continuous sheets. There is even more sharing of oxygens between adjacent tetrahedra and hence fewer cations are needed to balance the charge of the silica-tetrahedra structure in sheet silicate minerals.

Bonding between sheets is relatively weak, and this accounts for the well-developed one-directional cleavage in micas. Chlorite is another similar mineral that commonly includes magnesium. In muscovite mica, the only cations present are aluminum and potassium; hence it is a non-ferromagnesian silicate mineral. Apart from muscovite, biotite, and chlorite, there are many other sheet silicates a. These include the clay minerals kaolinite , illite , and smectite , and although they are difficult to study because of their very small size, they are extremely important components of rocks and especially of soils.

Silica tetrahedra are bonded in three-dimensional frameworks in both the feldspars and quartz. In addition to silica tetrahedra, feldspars include the cations aluminum, potassium, sodium, and calcium in various combinations.

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. Therefore, fewer cations are necessary to balance that charge.

Pyroxene can also be written as Mg,Fe,Ca SiO 3 , where the elements in the brackets can be present in any proportion.

In other words, pyroxene has one cation for each silica tetrahedron e. The diagram below represents a single chain in a silicate mineral.

Count the number of tetrahedra versus the number of oxygen ions yellow spheres. Each tetrahedron has one silicon ion so this should give you the ratio of Si to O in single-chain silicates e.

The diagram below represents a double chain in a silicate mineral. Again, count the number of tetrahedra versus the number of oxygen ions. This should give you the ratio of Si to O in double-chain silicates e. In amphibole structures, the silica tetrahedra are linked in a double chain that has an oxygen-to-silicon ratio lower than that of pyroxene, and hence still fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene and its compositions can be very complex.

In mica structures, the silica tetrahedra are arranged in continuous sheets, where each tetrahedron shares three oxygen anions with adjacent tetrahedra. There is even more sharing of oxygens between adjacent tetrahedra and hence fewer cations are needed to balance the charge of the silica-tetrahedra structure in sheet silicate minerals. Pyroxene Figure 5. The structure of chain silicates is shown in Figure 5. In pyroxene , silica tetrahedra form a chain because one oxygen from each tetrahedron is shared with the adjacent tetrahedron.

This means there are fewer oxygens in the structure. This can be expressed as an oxygen-to-silicon ratio O:Si. The O:Si is lower than in olivine instead of , and the net charge per silicon atom is less —2 instead of —4 , because fewer cations are necessary to balance that charge. Pyroxene compositions have the silica tetrahedra represented as SiO 3 e. In other words, pyroxene has one cation for each silica tetrahedron e. In mica structures the silica tetrahedra are arranged in continuous sheets Figure 5.

Because even more oxygens are shared between adjacent tetrahedra, fewer charge-balancing cations are needed for sheet silicate minerals.

Bonding between sheets is relatively weak, and this accounts for the tendency of mica minerals to split apart in sheets Figure 5. Two common micas in silicate rocks are biotite Figure 5. All of the sheet silicate minerals have water in their structure, in the form of the hydroxyl OH- anion.

Some sheet silicates typically occur in clay-sized fragments i. Not to be confused with a liquid solution, a solid solution occurs when two or more elements have similar properties and can freely substitute for each other in the same location in the crystal structure. Olivine is referred to as a mineral family because of the ability of iron and magnesium to substitute for each other. Iron and magnesium in the olivine family indicate a solid solution forming a compositional series within the mineral group which can form crystals of all iron as one end member and all mixtures of iron and magnesium in between to all magnesium at the other end member.

Different mineral names are applied to compositions between these end members. In the olivine series of minerals, the iron and magnesium ions in the solid solution are about the same size and charge, so either atom can fit into the same location in the growing crystals. Within the cooling magma, the mineral crystals continue to grow until they solidify into igneous rock. The relative amounts of iron and magnesium in the parent magma determine which minerals in the series form.

Other rarer elements with similar properties to iron or magnesium, like manganese Mn , can substitute into the olivine crystalline structure in small amounts. Such ionic substitutions in mineral crystals give rise to the great variety of minerals and are often responsible for differences in color and other properties within a group or family of minerals.

Olivine has a pure iron end-member called fayalite and a pure magnesium end-member called forsterite. Mafic minerals are also referred to as dark-colored ferromagnesian minerals. Ferro means iron and magnesian refers to magnesium. The crystal structure of olivine is built from independent silica tetrahedra. Minerals with independent tetrahedral structures are called neosilicates or orthosilicates. In addition to olivine, other common neosilicate minerals include garnet, topaz, kyanite, and zircon.

Two other similar arrangements of tetrahedra are close in structure to the neosilicates and grade toward the next group of minerals, the pyroxenes. In a variation on independent tetrahedra called sorosilicates, there are minerals that share one oxygen between two tetrahedra and include minerals like pistachio-green epidote, a gemstone.

Another variation are the cyclosilicates, which as the name suggests, consist of tetrahedral rings, and include gemstones such as beryl, emerald, aquamarine, and tourmaline.

Pyroxene is another family of dark ferromagnesian minerals, typically black or dark green in color. Members of the pyroxene family have a complex chemical composition that includes iron, magnesium, aluminum, and other elements bonded to polymerized silica tetrahedra. Polymers are chains, sheets, or three-dimensional structures, and are formed by multiple tetrahedra covalently bonded via their corner oxygen atoms. Pyroxenes are commonly found in mafic igneous rocks such as peridotite, basalt, and gabbro, as well as metamorphic rocks like eclogite and blue-schist.

Pyroxenes are built from long, single chains of polymerized silica tetrahedra in which tetrahedra share two corner oxygens.