Figure 3. The formula for this particular olivine, which has three Fe ions for each Mg ion, could be written: Mg 0. The tetrahedral chains continue to left and right and each is interspersed with a series of divalent cations.
If these are Mg ions, then the formula is MgSiO 3. Part of a single chain of tetrahedra right , where the oxygen atoms at the adjoining corners are shared between two tetrahedra arrows. For a very long chain the resulting ratio of silicon to oxygen is 1 to 3 SiO 3. Augite is one of many minerals within the pyroxene family. Mineral Formula Ferromagnesian silicate? An angstrom is the unit commonly used for the expression of atomic-scale dimensions.
A mineral that includes silica tetrahedra. A combination of 1 silicon atom and 4 oxygen atoms that form a tetrahedron. A flexible synthetic material made up of Si—O chains with attached organic molecules. A double-chain ferromagnesian silicate mineral e. A sheet silicate mineral e. Referring to a silicate mineral that contains iron and or magnesium. A potassium-bearing non-ferromagnesian mica. A silicate mineral in which the silica tetrahedra are combined within sheets. A silicate mineral in which the silica tetrahedra are made up of sheets.
A clay mineral that does not have cations other than Al and Si. A clay mineral with a composition similar to that of muscovite mica. A very common family of framework silicate minerals. A silicate mineral with the formula SiO2. A silicate mineral that does not contain iron or magnesium e.
Previous: Lab 3: Silicate Minerals. Next: Lab 3 Exercises. Share This Book Share on Twitter. A crystal of pure silicon sliced very thinly and used for electronics. Although the cations may freely substitute for each other in the crystal, they carry different ionic charges that must be balanced out in the final crystalline structure.
Note that ionic size is more important than ionic charge for substitutions to occur in solid solution series in crystals. Amphibole minerals are built from polymerized double silica chains and they are also referred to as inosilicates.
Imagine two pyroxene chains that connect together by sharing the third oxygen on each tetrahedron. Amphiboles are usually found in igneous and metamorphic rocks and typically have a long-bladed crystal habit.
The most common amphibole, hornblende, is usually black; however, they come in a variety of colors depending on their chemical composition. The metamorphic rock, amphibolite, is primarily composed of amphibole minerals. Amphiboles are composed of iron, magnesium, aluminum, and other cations bonded with silica tetrahedra. These dark ferromagnesian minerals are commonly found in gabbro, basalt, diorite, and often form the black specks in granite.
Their chemical formula is very complex and generally written as RSi 4 O 11 2 , where R represents many different cations. The substitutions create a wide variety of colors such as green, black, colorless, white, yellow, blue, or brown.
Amphibole crystals can also include hydroxide ions OH — , which occurs from an interaction between the growing minerals and water dissolved in the magma. Sheet silicates are built from tetrahedra which share all three of their bottom corner oxygens thus forming sheets of tetrahedra with their top corners available for bonding with other atoms.
Micas and clays are common types of sheet silicates, also known as phyllosilicates. Mica minerals are usually found in igneous and metamorphic rocks, while clay minerals are more often found in sedimentary rocks. Two frequently found micas are dark-colored biotite, frequently found in granite, and light-colored muscovite, found in the metamorphic rock called schist. Chemically, sheet silicates usually contain silicon and oxygen in a ratio Si 4 O Micas contain mostly silica, aluminum, and potassium.
Biotite mica has more iron and magnesium and is considered a ferromagnesian silicate mineral. Muscovite micas belong to the felsic silicate minerals. Felsic is a contraction formed from feldspar, the dominant mineral in felsic rocks. The illustration of the crystalline structure of mica shows the corner O atoms bonded with K, Al, Mg, Fe, and Si atoms, forming polymerized sheets of linked tetrahedra, with an octahedral layer of Fe, Mg, or Al, between them.
The yellow potassium ions form Van der Waals bonds attraction and repulsion between atoms, molecules, and surfaces and hold the sheets together. Van der Waals bonds differ from covalent and ionic bonds, and exist here between the sandwiches, holding them together into a stack of sandwiches.
The Van der Waals bonds are weak compared to the bonds within the sheets, allowing the sandwiches to be separated along the potassium layers. This gives mica its characteristic property of easily cleaving into sheets.
Clays minerals occur in sediments formed by the weathering of rocks and are another family of silicate minerals with a tetrahedral sheet structure. Clay minerals form a complex family and are an important component of many sedimentary rocks. Other sheet silicates include serpentine and chlorite, found in metamorphic rocks.
Clay minerals are composed of hydrous aluminum silicates. One type of clay, kaolinite, has a structure like an open-faced sandwich, with the bread being a single layer of silicon-oxygen tetrahedra and a layer of aluminum as the spread in an octahedral configuration with the top oxygens of the sheets.
Thus, it could either fit in 6-fold coordination with oxygen or 4-fold coordination with oxygen. If such a substitution takes place, it creates a charge imbalance that must be made up elsewhere in the silicate structure. The other common elements in the Earth's crust that enter the silicates do so in other types of coordination. O is oxygen, and W is a hyrdoxyl OH -1 site into which can substitute large anions like F -1 or Cl The subscripts m, n, and r depend on the ratio of p to q and are chosen to maintain charge balance.
This is summarized in the table shown here. In this table note that there is very little substitution that takes place between ions that enter the X, Y, and Z sites. Nesosilicates Island Silicates. We now turn our discussion to a systematic look at the most common rock forming minerals, starting with the common nesosilicates.
Among these are the olivines, garnets, Al 2 SiO 5 minerals, staurolite, and sphene the latter two will be discussed in the last lecture on accessory minerals.
As discussed above, the nesosilicates or island silicates are based on the isolated SiO 4 -4 tetrahedral groups. There is limited substitution of the following end members:. The phase diagram for the common end members of the olivine solid solution series shows that pure forsterite melts at o C and pure fayalite melts at o C. Occurrence Pure forsterite is limited to metamorphosed Mg-rich limestones and dolomitic metamorphic rocks.
Fo 60 - 90 is found in basic igneous rocks likes basalts and gabbros, and sometimes in andesites, where it occurs with plagioclase and pyroxene. Fa - 40 is found in Fe-rich siliceous igneous rocks like rhyolites and granites.
Mg-rich olivines rarely occur in quartz bearing rocks and quartz rarely occurs with Mg-rich olivine because the reaction shown below runs to the right for most pressures and temperatures. Garnets are isometric minerals and thus isotropic in thin section, although sometimes they are seen to be weakly birefringent slightly anisotropic. They are also nesosilicates, and therefore based on the SiO 4 structural unit. Garnets with no Ca in the A site and Al in the B site are called the pyralspite series.
Occurrence The garnets occur mostly in metamorphic rocks where they are often seen to form euhedral well-formed crystals. The Mg-rich garnet, pyrope, is found in metamorphic rocks formed at high pressure and in eclogites basalts metamorphosed at high pressure and peridotites ultrabasic rocks containing olivine, Opx, Cpx, and garnet. The Fe-rich garnet, almandine, is the most common garnet and is found in metamorphic aluminous schists. The Mn-rich variety, spessartine, is limited to Mn-rich metamorphic rocks like meta-cherts.
The Al 2 SiO 5 minerals are common in aluminous metamorphic rocks meta-shales and meta-mudstones and sometimes found in aluminous igneous rocks. In metamorphic rocks the Al 2 SiO 5 polymorphs provide rather general estimates of the pressure and temperature of metamorphism, with Kyanite indicating relatively high pressure, andalusite indicating low temperature and pressure, and sillimanite indicating high temperature.
Better estimates of pressure and temperature are provided if two of the minerals are present in the same rock. Maximum birefringence is generally seen to be between 2 o yellow to 2 o red.
Sillimanite is biaxial positive with a 2V of 21 - 31 o. Staurolite is a common mineral in medium grade metamorphic rocks, usually metamorphosed shales. In thin section Staurolite is commonly seen to contain tiny inclusions of other minerals, usually quartz.
There are very few minerals which can be confused with Staurolite. Zircon ZrSiO 4. Zircon is a common accessory mineral in nearly all kinds of rocks, particularly the more siliceous igneous rocks, like granites, granodiorites, and syenites.
Still, it is not often found in thin section because it is so hard that it gets plucked out during the grinding of the section. Zircon usually contains high amounts of radioactive elements like U and Th. Thus, when it is found as inclusions in minerals like biotite, it produces pleochroic haloes in the biotite as seen in thin section.
Because it contains high concentrations of U and Th, it is very useful in obtaining U-Pb and Th-Pb radiometric dates on old rocks. It is very resistant to weathering and may also survives during metamorphism, allowing for dates to be obtained on the original rock prior to metamorphism often called the protolith.
In hand specimen Zircon usually occurs as tiny reddish colored crystals. Zircon has high birefringence, with interference colors in the higher orders lots of reds, pinks and light greens. It is commonly colorless to pale brown or pinkish brown in polarized light without the analyzer.
Generally it occurs as small crystals with relief higher than almost anything else in the thin section. This latter property should tip you off to its presence. Sphene is another common accessory mineral in plutonic igneous rocks like granites, granodiorites, and syenites. It is also found as larger crystals in metamorphic gneisses and chlorite bearing schists. In hand specimen as an accessory mineral, it is usually seen as small wedge-shaped crystals with a resinous to adamantine luster and brown to yellow brown color.
In thin section, Sphene, has a relief similar to that of zircon, and is usually found in small crystals with an elongated diamond shape. Sorosilicates are the double island silicates. Only one important mineral group, the epidote group, has this structure. The important minerals in the epidote group are epidote, clinozoisite, and zoisite. Zoisite has the same chemical formula as clinozoisite, but is orthorhombic.
It is optically negative with a 2V of 64 - 90 o.
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