Mibei (2014) classified rocks into three major types based on their formation process: the sedimentaries; the igneous; and the metamorphics. Sedimentary rocks, which comprised the majority (up to 66 percent) of Earth’s crust, formed through depositions of materials (e.g. minerals and organic particles) at the surface of or within water bodies on Earth. These rocks include limestones, sandstones, conglomerates, shales, and gypsum. Most of these rocks can be found in ocean floors and basins, which account to 70 percent of the Earth’s total land area.
Igneous rocks form from magma, which solidified and cooled, including the partial melting of pre-existing rocks in the Earth’s mantle or crust (Mibei, 2014). Thus, this type of rock is associated with volcanic activity and their distribution through plate tectonics, except for melting rocks, which transformed from their exposure to a surge in temperature, a decline in pressure, or a change in its chemical composition. It comprised the majority of the remaining 34 percent of Earth’s rock. These rocks include granite, pumice and basalt.
Metamorphic rocks formed from protoliths through exposures to changes in high temperature and pressure levels below the zone of diagenesis (Mibei, 2014). Metamorphics comprised the minority of the remaining 34 percent of the Earth’s rocks. These rocks may be classified according to its mineralogical composition, its general chemical constituents, and its texture, the most metamorphic of which involves foliation, which generally resulted from the orientation of sheet silicate within (Nelson, 2015). Thus, slates are so named because these metamorphics have a slatey cleavage for its foliation.
Conversely, phyllites have a phyllitic foliation and gneiss has a banded foliation. Non-foliated rocks are either called hornfels (with small grain size) or granulite (with large grain size and wherein large distinct minerals can be easily identified using hand lens (Nelson, 2015).
Mineral ores, such galena, pyrite, sphalerite, and chalcopyrite, are metamorphic rocks, usually granulites, with sulfide chemical components of various proportion to other chemicals such as lead (Pb), iron (Fe), zinc (Zn), and copper (Cu) among minor chemical components.
Galena [lead sulfide]
The lead ore galena chemically consists of lead sulfide (PbS) with opaque, metallic lead-gray color, which looks white at its polished section. It is composed of at least 85.9 percent lead and 12.6 percent sulfide. Its streaks are lead-gray in color. Although it occurs in association with Sphalerite, pyrite, and chalcopyrite (among others) and forms a series with clausthalite, it often shows no polymorphism. It is a principal mineral component of the lead-zinc deposit ores (Bonev, 2007). The study observed a range of small and elongated cubic crystals with white exposed surfaces possessing metallic luster on large octahedral metallic crystals (IMG_8862). Apart from this, nothing has been observed close to the standard description of galena, particularly its distinct metallic luster.
Pyrite [Ferrous Sulfide]
As a Sulfide ore, pyrite is opaque, metallic or splendent, pale brass-yellow colored, usually with greenish black to brownish black streak. Its polished side appears creamy white. Chemically, it is ferrous sulfide (FeS2) with a minimum known content of 29 percent iron and 53 percent sulfur. It is the most abundant and widely distributed among the sulfide ores and often in association with Sphalerite and chalcopyrite. The study observed various forms and formations of pyrite, such as single cubic grain alone (IMG_8809; IMG_8861) or among scattered smaller grains (IMG_8812; IMG_8817; IMG_8818), flattened t-shaped grains or in various (e.g. triangular) shapes (IMG_8814; IMG_8819; IMG_8820) or flattened dodecahedral grains (IMG_8815; IMG_8816). There were also aggregations of cubic and elongated crystals (IMG_8779) or long schistositic aggregates of octahedral or dodecahedral nugget-like crystals (IMG_8811). Observed also were rounded and flattened grains with octahedral, dodecahedral, rectangular cubic shapes (IMG_8810; IMG_8812). Some pyrites appeared in tarnished grains of various sizes among non-tarnished grains (IMG_8821; IMG_8823; IMG_8812). Pyrites of less defined shapes due to magnification constrains were also observed as schistosity (IMG_8780).
Sphalerite [Zinc, Ferrous Sulfide]
The tetrahedral or dodecahedral mineral ore called sphalerite is a sulfide ore with high concentrations of zinc (e.g. at least 44 percent zinc and 33 percent sulfide), which at times dominated by high levels of iron. Thus, although zinc is bluish silver in color, many samples of sphalerite ranges in color from yellow (due to high sulfide content) to red, dark brown, gray, and black (due to variable iron content; the darker the color the higher the iron content). Green may indicate high zinc sulfide content while brown, ferrous sulfide (Barton & Bethke, 1987). Streaks also vary from pale brown to pale yellow and even white. Thus, chemically, a sphalerite can be a zinc sulfide, a ferrous sulfide, or a mixture of both. Low iron sphalerite is transparent or translucent, which turn opaque when iron content increases. Its luster may appear resinous or adamantine. It may naturally occur in association with galena, chalcopyrite, and pyrite.
Multicolored sphalerites of low to moderate in iron content were also found along with high iron content crystals. There were light to dark brown, light to dark gray crystals as well as bluish silver-colored copper sulfide crystals and larger high iron-content black tetrahedral crystals (IMG_8834; IMG_8843; IMG_8844; and IMG_8846). Most hornfels were colored light gray to black while granulites largely brown, light gray, dark gray, and black with rare presence of bluish silvers. Larger aggregates of light to dark brown tetrahedral crystals can be found interspersed in certain areas with bluish silver, light gray and gray crystals, partly surrounded with bluish silver, gray, and brown hornfels as well as diffuse black aggregates (IMG_8847; IMG_8848). Light gray to black sphalerites were also found as granulites (IMG_8849; IMG_8826; IMG_8827) and as hornfels (IMG_8837; IMG_8835). Sphalerite high in iron, usually in black color, can be found invaded by white or opaque brass-yellow chalcopyrite crystals (IMG_8824; IMG_8831; IMG_8850; IMG_8852; and IMG_8840). Dominantly high iron sphalerites were also seen (IMG_8855; IMG_8856; IMG_8859).
Chemically, copper ferrous sulfide (CuFeS2) with equally at least 30 percent content for copper, iron, and sulfide, chalcopyrite is an opaque, metallic, brass-yellow, tetragonal crystal with greenish black streak. It occurs naturally in association with sphalerite, which it often invades, galena, pyrite, and other copper sulfides (Barton & Bethke, 1987). In the samples obtained, most chalcopyrites were found sparsely invading iron-dominated sphalerites (IMG_8824; IMG_8830; IMG_8831; IMG_8840; IMG_8845; IMG_8850; IMG_8851) as well as prominently so (IMG_8853). Cataclastically deformed pyrites with polished portions exposed can be seen in IMG_8877, IMG_8785, and IMG_8786 (Kuscu & Erler, 2002).
There were two optical limitations observed in the study. First, inadequate magnification power of the microscope cannot obtain clearly defined crystals that can be easily studied and identified outside its characteristic colors and shapes, which constitute the primary bases for identifying the individual mineral ores in this study. Second, optical metallicity has also been difficult to faithfully extract from the images obtained. At certain points, the unclear metallic lust of chalcopyrites made them hard to differentiate from pale pyrites (e.g. IMG_8852).
Barton, P.B., Jr., & Bethke, P.M. (1987). Chalcopyrite disease in sphalerite: Pathology and
epidemiology. American Mineralogist, 72(1): 451-467.
Bonev, I.K. (2007) Crystal habit of Ag-, Sb-, and bi-bearing galena from the Pb-Zn ore deposits
in the Rhodope Mountains. Geochemistry, Mineralogy, and Petrology, 45(1): 1-18.
Kuscu, I. & Erler, A. (2002). Pyrite deformation textures in the deposits of the Kure Mining
District (Kastamonu-Turkey). Turkish Journal of Earth Sciences, 11(1): 205-215.
Nelson, S.A. (2015, September 17). Physical geology: Metamorphisms and metamorphic rocks.
New Orleans, LA: Tulane University.