Lab 2: Silicate Minerals

GEOLOGY 640: Geology through Global Arts and Artifacts

LAB 2: SILICATE MINERALS

FRAMEWORK SILICATES

The framework silicates quartz and feldspar are the most common minerals in Earth’s crust. Quartz (SiO 2) is one of the few common minerals that is harder than a streak plate. It may display numerous colors (purple= amethyst ; pink= rose quartz ; brown= smoky quartz ; yellow-orange= citrine ). It may form long hexagonal crystals but lacks cleavage, and instead breaks along irregular, curving surfaces (conchoidal fracture). In many cases quartz forms masses of microscopic crystals (e.g., chert, flint, chalcedony ) that still maintain the hardness and conchoidal fracture of quartz. Banded chalcedony is called agate , whereas reddish chalcedony is called carnelian (bloodstone).

Plagioclase is a group of feldspar minerals that have complete solid solution from NaAlSi 3O8 ( albite ) to CaAl 2Si 2O8 ( anorthite ). Na-rich plagioclase tends to white in hands sample, whereas Ca-rich plagioclase tends to be dark grey. Twinning is the intergrowth of two or more crystals in a symmetrical fashion by the sharing of lattice points in adjacent crystals. In plagioclase, the most common twins are planar and repeated (polysynthetic twinning), resulting in the striations that are characteristic of plagioclase in hand-sample. Twinning tends to be better developed in Ca-plagioclase minerals. Ca-rich plagioclase (labradorite and anorthite) may also display iridescent colors (mostly blue). Iridescent albite is rare and is known as the semi-precious gem moonstone .

Microcline (KAlSi 3O8) is the most common alkali feldspar. It is similar to plagioclase in most of its optical properties (hard, blocky, 2 cleavages at 90°). It is most commonly salmon pink, but may also be green ( amazonite ) or white. Microcline lacks striations but commonly displays subparallel, irregular, wispy lines.

Examine samples of plagioclase (albite and labradorite) and microcline. Note the physical characteristics described above. Be sure to be able to recognize the striations of plagioclase and the wispy lines of microcline.

Describe the properties that allow you to differentiate between feldspar, quartz, calcite and dolomite.

SHEET SILICATES Micas are a group of minerals that have perfect basal cleavage and are capable of splitting into thin laminae along their perfect basal cleavage. Although they are monoclinic minerals, they commonly form hexagon-shaped crystals. Micas are hydrated aluminum silicates. Micas are composed of sheets of silicate tetrahedrons that are composed of inter connected six membered rings. These rings are responsible for mica's typical six-sided crystal shape. Muscovite 2+ (KAl 2Si 3AlO 10 [OH,F] 2) and biotite (K[Mg,Fe ]3AlSi 3O10 [OH,F] 2) are the two most common micas.

Clays are a group of hydrous sheet silicates that minerals that are fine-grained (usually microscopic). The mica-like form is usually apparent only at high magnification (see photo below) Most clay is produced from chemical weathering of rocks. For example, kaolinite Al 2Si 2O5(OH) 4, the clay that is used to make porcelain, is formed from the hydrolysis (reaction with acid and water) of feldspar. Bentonite (Na,Ca) 0.33 (Al,Mg) 2Si 4O10 (OH) 2·(H 2O) n is a swelling clay, that is it absorbs water and expands when wet. Its absorptive properties make it useful today in items such as cat litter and absorbants for chemical and oil spills. It is derived from the weathering of volcanic ash. Glauconite 3+ ([K,Na][Fe ,Al,Mg] 2[Si,Al] 4O10 [OH] 2) is a green clay that is found as rounded aggregates of very fine grained scaly particles. It is generally indicative of deposition on a continental shelf with slow rates of sediment accumulation. Talc [Mg 3Si 4O10 (OH) 2], is a very soft, pearly clay with a waxy feel. Rocks composed of massive talc may be referred to as soapstone or steatite .

Scanning Electron Microscope Photo of Kaolinite

Describe the properties that allow you to differentiate between kaolinite, chalk (calcite), and alabaster (gypsum).

Describe the properties that allow you to differentiate between talc and gypsum.

CHAIN SILICATES Amphiboles are a diverse group of generally dark-colored rock-forming minerals composed of double chain SiO 4 tetrahedra. Amphiboles are hydrous minerals and contain Fe and/or Mg. Most amphiboles contain Ca, Na or K as well. Amphiboles have two cleavage planes at around 120 degrees, and crystals tend to be long (prismatic to acicular). Hornblende (Ca 2[Mg,Fe,Al] 5[Al,Si] 8O22 [OH] 2) is the most common amphibole; this black prismatic mineral is a common constituent of both igneous and metamorphic rocks. Jade is a rock composed in large part of the rare amphiboles jadeite and/or nephrite. Jade is a very strong rock due to the intergrowth of these acicular green amphiboles.

The pyroxenes are an important group of single chained silicates, and are common in igneous and high-grade metamorphic rocks. The single-chain structure leads to the prismatic to fibrous character of pyroxenes, and the characteristic cleavage angle of the pyroxenes at nearly 90 o degrees. Pyroxenes are similar to amphiboles in structure, composition and appearance. However, pyroxenes tend to form stubbier crystals and tend to have a greasier luster than amphiboles. The most common pyroxenes is augite (Ca,Mg,Fe,Na)(Mg,Fe,Al)(Si,Al) 2O6, which is dark green. Bronzite (Fe,Mg)SiO 3 is a pyroxene that is brown, and exhibits a reflective, bronze-like appearance on polished surfaces.

Examine samples of amphibole and pyroxene. Note the physical characteristics described above. Note the variations in shape, cleavage, and luster.

Describe the properties that allow you to differentiate between the three most common dark silicate minerals: biotite, hornblende, and augite.

RING SILICATES

Tourmaline Na(Al,Fe,Li,Mg,Mn,Cr) 3(Al,Mg,Fe,Cr) 6[(Si,Al) 6O18 ](BO 3)3(OH,F) 4 is the most common ring silicate. The many possible compositions of tourmaline allow for a wide variety of colors. Cross-sections of tourmaline have a rounded triangular shape.

Describe the properties that allow you to differentiate between the two prismatic minerals tourmaline and hornblende.

ISOLATED TETRAHEDRON SILICATES

Olivine is not an official mineral name itself, but is the name for a solid-solution series between the Fe-rich end members, fayalite (Fe 2SiO 4) and the Mg-rich end member forsterite (Mg 2SiO 4). The two minerals form a series where the iron and magnesium are substituted for each other without much effect on the crystal structure. Olivine is a common component of mafic (e.g., basalt and gabbro) and ultramafic igneous rocks (e.g, dunite).

Garnet is also the name for a group of related minerals that exhibit a wide range of solid solution. Most garnets can be described by the chemical formula (Ca,Fe,Mg,Mn) 3Al 2(SiO 4)3. Garnets tend to be granular and exhibit a complex, many-faced, equant form, and are most common in metamorphic rocks. Garnets lack cleavage. Garnets exhibit many different colors, but the most common color is a deep wine-red.

Examine samples of olivine and garnet. Note the physical characteristics described above.

Describe the properties that allow you to differentiate between the two common pale green minerals olivine and diopside.