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Rock and Mineral Eggs Pisolithic limestone is a sedimentary rock made of pisoids, which are concretionary grains usually made of calcium carbonate. They generally resemble ooids but are larger than 2 mm in diameter and can reach up to 10 mm in diameter. The grains are approximately spherical and have concentric layers. Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava from a volcano cools so rapidly that there is minimal crystal growth. The obsidian samples in the display are regular obsidian, snowflake obsidian, and brecciated obsidian. Some varieties of obsidian have inclusions of small, white, radially clustered crystals of cristobalite in the black glass, called snowflake obsidian. Brecciated obsidian is partially broken down and recrystallized pieces of obsidian in angular patterns. Epidote (Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH)) is easily identified by its characteristic yellowish green or pistachio colour. It is an important constituent of low-­‐ and medium-­‐grade metamorphic rocks and occurs in large amounts in contact-­‐metamorphosed limestones. Azurite (Cu3(CO3)2(OH)2) is a dark to light blue mineral that commonly occurs with malachite (Cu2CO3(OH)2). It forms when carbonated waters and copper minerals undergo a chemical reaction and can be identified by its effervescence with cold dilute HCl. Malachite is a mineral that can be distinguished by its green colour and typical banding. It effervesces in HCl and is a widespread secondary copper mineral. It occurs in the upper oxidized zones of copper deposits where limestone is present. Fluorite (CaF2) is a vitreous and transparent mineral that can vary in colour from clear to yellow, purple, blue, and even green. Fluorite usually has a cubic crystal structure and is commonly fluorescent, meaning it glows under UV light. It occurs most commonly as a vein mineral and can be found in a wide variety of places including the United States, Germany, Mexico, Canada, and Italy. Rhodochrosite (MnCO3) is part of the calcite group and can be various shades of pink, red, and brown while being vitreous and translucent. Some diagnostic features include that rhodochorosite is soluble in warm hydrochloric acid and that it turns black but remains nonmagnetic when heated. Rhodochorosite occurs as a primary gangue mineral in many low-­‐ to moderate-­‐ temperature hydrothermal veins, occurring with silver, lead, zinc, and copper sulfides. It is mainly found in the western United States as well as Europe. Varicite (AlPO4·∙2H2O) is a relatively rare hydrated aluminum phosphate mineral distinguishable by its intense green colour. It is a secondary mineral that forms by direct deposition from phosphate-­‐
bearing water that reacts with aluminum rich rocks. Varicite occurs as fine-­‐grained masses in nodules, cavity fillings, and crusts. Rhyolite is a felsic, extrusive, igneous rock with a mineral assemblage that is usually quartz, sanidine, and plagioclase. It commonly contains the accessory minerals biotite and hornblende. Rhylotie can contain a wide variety of other minerals, including opal (SiO2·∙nH2O). Opal is a hydrated silica mineral that can have a rich iridescence and play of colours. This play of colours is due to the close-­‐packing aggregate of amorphous hydrated silica spheres, each a few tenths of a micron in diameter. When these spheres are uniform in size and are regularly packed a range of colours can be seen called opalescence. The serpentine group ((Mg, Fe)3Si2O5(OH)4) describes a group of common rock-­‐
forming hydrous magnesium iron phyllosilicate minerals. There are over 20 varieties belonging to this group that are difficult to individualize so distinctions are often not made. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock occurring at tectonic plate boundaries. Serpentinite is a rock composed of one or more serpentine group minerals. Lapis lazuli is a rock composed of 25-­‐
40% lazurite ((Na,Ca)8(AlSiO4)6(S,SO4,Cl)1-­‐2). Lazurite is a feldspathoid silicate. Most lapis lazuli also contains calcite, sodalite, and pyrite. It usually occurs in crystalline marble as a result of contact metamorphism and has an intense blue color due to the presence of the trisulfur radical anion in the crystal. Rutilated quartz is a variety of quartz (SiO2) that has needle-­‐like inclusions of rutile (TiO2). Rutile is a common titanium dioxide mineral formed as an alteration product of other titanium minerals like titanite and ilmentite. Amethyst is another variety of quartz with a transparent purple colour. Amethyst is macrocrystallineIt mostly occurs lining cavities in volcanic rocks. The colour of amethyst can result from displacement of transition elements in the crystal structure. Jasper is an aggregate of microgranular quartz. It is an opaque variety of silica that is usually red, but can be yellow or brown. The red colour is due to iron inclusions in the crystal lattice. References Berry, L. G, Brian Harold Mason, and Richard Vincent Dietrich. Mineralogy, Concepts, Descriptions, Determinations. San Francisco: Freeman, 1983. Print. All photos courtesy of the University of Waterloo Earth Sciences Museum.