LAB INTRO MINERALS, ROCKS, and GRAINS Definitions: Crystal: a homogeneous solid with an orderly internal atomic arrangement. e.g. Quartz, SiO2, made up of a network of corner linked SiO4 tetrahedra: Minerals: Are naturally occurring, inorganic, crystalline solids that have a definite chemical composition and possess characteristic physical properties. (e.g. Quartz is a naturally occurring, inorganic crystalline solid composed of SiO2 with trigonal symmetry. It has a density of 2.65 gm/cm3, a hardness of 7 and no cleavage. It exists as colorless or white but can be purple, black, green etc.) Grains: Are mineral particles of diameter less than a few millimeters. Usually these are small single crystal particles of a single mineral. Thus, sand is predominantly made up of grains of quartz, as are sandstones. A granite is composed of grains of quartz, feldspar and mica. Rocks: Are naturally formed, consolidated material composed of grains of one or more minerals. Mineral Properties Color: color can be used for mineral identification only if it is always typical of that mineral. Many minerals such as quartz have multiple colors depending on the presence of impurities (e.g., addition of iron to quartz can make it purple, giving amethyst or yellow, giving citrine; addition of aluminum will make it smoky). Color does, to some extent, provide a way to narrow down the possible mineral species though; mantle-derived minerals are often black or green, reflecting the higher iron contents (e.g., olivine, biotite, hornblende etc.) Streak: is the color of pulverized substance, used in mineral identification. Streak is more reliable than color in identification of mineral species. Streak is usually obtained by rubbing a piece of the mineral on a plate of unglazed porcelain. Luster: the way light is reflected from a mineral surface. The major subdivisions of luster are metallic luster and nonmetallic luster. Metallic luster is shiny like gold or pyrite. Nonmetallic luster is most commonly glassy luster (or vitreous luster) with a glazed appearance like porcelain e.g. quartz, feldspars. Nonmetallic luster can also be earthy luster like the surface of unglazed porcelain e.g., clay. Less common are resinous, silky or pearly luster. Hardness: or scratchability is one of the most characteristic mineral properties. It is usually quantified using the Moh's Hardness Scale (see Table 2.2 in Skinner and Porter). If a mineral will scratch with a fingernail, it has a hardness of <2.5, with a clean copper coin, hardness <3-4, with a knife blade, hardness <about 5, depending on the steel of the blade, a file of tempered steel, hardness <6-7. The softest mineral is talc with a hardness of 1; the hardest mineral is diamond with a hardness of 10. Crystal Form: When crystals grow in an environment where they do not compete with each other for space, they may develop a characteristic crystal form, e.g. quartz growing in an open, water-filled fracture, or garnet growing in a rock mass. Cleavage: Many minerals tend to break or cleave along certain crystal planes because the bonding between the atoms is weaker there. When a mineral cleaves, you are left with flat surfaces, e.g. salt cleaves into cube shapes as it has 3 excellent cleavage directions at 90o to each other; mica cleaves to form planar sheets as it has a single perfect cleavage along planes linked by weak hydrogen bonds. Diamond has very good cleavage in for directions, which allows diamonds to be faceted. Fracture: occurs when a mineral breaks other than by cleavage, e.g., quartz, olivine, garnet have no cleavage and will form irregular fractures with no simple shape or crystal alignment to the fracture surfaces. Some minerals form conchoidal fractures, which are curved smooth fracture surfaces. Primitive man made crude tools and weapons using the fact that glass and flint fracture this way (also sometimes quartz). Other minerals develop splintery fracture. Other properties useful in mineral identification include density, microstructural features such as striations or lamellae, magnetic properties such as in magnetite, unusual optical properties such as double refraction in calcite, taste (but beware, potassium ferrocyanide is a mineral too). Rock Properties Grain Size: Absolute grain size is the measured diameter of grains in a rock (usually quoted as the average of many grains) in dimensions of millimeters or inches. Relative grain size is a qualitative statement about the range of grain sizes in a rock. An equigranular rock has grain sizes that are all about the same. An inequigranular rock has a range of grain sizes (one specific example is bimodal grain size where there are 2 distinct grain sizes). Grain Origin: grains may be clastic which means that the grains were formed by mechanical fragmentation or crushing, or they may be nonclastic which means that they were formed by precipitation from a fluid (e.g. NaCl in tidal lagoons or salt lakes) or crystallization from a melt (e.g., grains in a basalt cooled from lava). It is very difficult to distinguish between the textures for the different origins. Grain Shape: grains within rocks may vary a lot in shape. Grains without strong cleavage or strong preferred growth direction will be approximately equant in shape (e.g., quartz, feldspar, olivine). By contrast, grains of mica, biotite etc. will often form long tabular grain shapes. Orientation and fissility: Grains that have non-equant shapes may be preferentially aligned in a rock, such as mica grains may all have the long dimension of the grains parallel to one orientation. If there is a lot of aligned minerals, such as mica, in a rock, the rock may break into well-defined plates or flakes parallel to the alignment orientation. This ability of a rock to flake is called fissility and the rock is called fissile.