Optical Properties Figure 1: Thin Section of Augite in PPL and XPL Plane Polarized LightCross Polarized Light My Pet Mineral: Augite Jacob Wall ‘16 Ge225: Mineralogy, Geology Program Colby College, Waterville, ME Introduction Augite was identified through physical property observations and physical tests, using the classification tables in the back of text book 1. Optical mineralogy properties were identified using a petrographic microscope. A review of literature on Augite’s history, occurrence in the environment, uses and chemical and structural data was also conducted. Both physical and optical properties are useful in mineral identification and history determination. Physical Properties Augite mostly appears to be dark green to black color but can also exist as a brown green color. The streak color is determined by rubbing the sample on the a porcelain streak plate. Augite streaks greenish white. Luster is the overall sheen of a mineral’s surface. Augite has a similar sheen to that of glass and hence a vitreous luster. Hardness of augite was based on Moh’s hardness scale, which is the relative scratch resistance of the mineral. Augite could just scratch glass with a hardness of 5.5, and therefore has a similar hardness. Specific gravity is a measurement that determines density of a mineral relative to that of water. Specific gravity was determined using minerals with known specific gravities and measured on a relative scale. Cleavage is the tendency of a mineral to break along smooth planes parallel to zones of weak bonding. Augite has 2 planes of cleavage, which intersect at 90°. Augite has a relatively rough cleavage plane compared to that of micas and feldspars. Habit is the general shape that a mineral is likely to form. The sample of augite had no apparent shape and therefore is considered to be massive. Augite can have fibrous, granular and columnar habit as well. History Abraham Gottlob Werner, a German geologist and the so called “father of German geology,” discovered and named augite in Werner named the mineral after the Greek word “auge,” which means shine or luster, due to augite’s vitreous luster. Occurrence Augite is an important rock-forming silicate mineral and is prominent in the formation of a wide variety of basic igneous rocks, including gabbros, dolerites and basalts. Augite is the most abundant pyroxen mineral. Augite can also occur in ultrabasic and intermediate rocks as well as a limited number of high-grade metamorphic rocks such as granulites and charnockites. Structural and Chemical Data Augite’s chemical formula is: (Ca,Na,Mg,Fe 2+,Fe 3+ Al,Ti) 2 [(Si,Al) 2 ]O 6 ]. Augites range widely in composition and show a continuous variation in chemical composition. Augite is a complex silicate consisting of a series of calcium-magnesium or calcium- iron silicates with aluminum as a substitution. Infinite single and double chains of silicon-oxygen tetrahedrons are prominent in Augite’s crystal structure. The single chains are linked through the sharing of oxygen tetrahedrons and the double chains share edges to form continuous sheets. The direction of the chains is parallel to the crystal c-axes, which leads to its elongation and augite’s fibrous habit. Uses Augite does not have any “real world” uses but it has been used extensively in geological research and studies. For example, Skoby (2006) used augite to study water concentrations in upper mantle minerals and found that pyroxenes including augite play an important role both in providing a repository for water in the upper mantle and in mantle water recycling processes.. Literature Cited Katchpole, Sarah. “Augite.” Smith College Geology. Web 1 Nesse, W. D. (2012). Introduction to optical mineralogy (2nd ed., p. 335). New York: Oxford University Press. Ralph, Jolyon, and Ida Chau. (2013). "Augite." : Mineral Information and Data. N.p., Web.. Skogby, H. (2006). Water in natural mantle minerals I: pyroxenes. Reviews in mineralogy and geochemistry, 62(1), Wilshire, H. G., & Shervais, J. W. (1975). Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. Physics and Chemistry of the Earth, 9, Physical Property Characteristic ColorDark Green, Black StreakGreenish White LusterVitreous Moh’s Hardness5.5 Specific Gravity3.2 – 3.6 Cleavage2 Planes HabitMassive Table 1. Physical Properties Source: Katchpole Optical Properties Characteristics Birefringence:0.026 Interference Colors:1 st and 2 nd order Pleochroism:Weakly pleochroism. X = pale green, Y = pale green, Z = pale brown/green Extinction Angle:Inclined extinction, 45° Twinning:Simple twinning Cleavage:Two planes of cleavage with a 90° cleavage angle Sign of Elongation:Positive Optic Sign:Positive 2V angle:60° Index of Refraction:nα = nβ = nγ = Characteristics:Augite is a monoclinic, biaxial mineral. Crystallographic and Indicatrix Axes 2 : Source: Nesse Table 2: Optical Properties Zoom: x4 Zoom: x5