Luster: vitreous : Fracture & Tenacity: conchoidal, brittle : Color: In hand sample: colorless, white, or gray in most cases; can be pink, green, yellow or other colors . Clear cleaved rhombohedra are referred to as "Iceland Spar" (see picture). Difficult and imperfect as the basal cleavage in beryl. The surface will show a pearly luster because of its laminated nature. Calcite resembles dolomite (to which it is chemically related) but is softer and effervesces in hydrochloric acid while dolomite does not. Courtesy of Prof. Dexter Perkins via license CC4.0-BY . Hardness: 3 (down to 2.5 on some surfaces) – easily scratched by a metal nail, but too hard to be scratched by a fingernail. Laptop/Desktop: click model below. Cleavage and Fracture. Cleavage: Calcite has perfect cleavage in three directions to produce rhombohedra. Aragonite is slightly harder, from 3.5 to 4. cleavage fract. Identification Tips. Calcite possesses perfect rhombohedral cleavage. Cleavage / fracture. 1. Cleavage is perfect in three directions, forming rhombohedrons. Perfect but difficult as the octahedral cleavage in diamond. Specific Gravity is approximately 2.7 (average) Streak is white. 3 on cleavage, 2.5 on {001} Cleavage: perfect rhombohedral : rhombohedral cleavage is a distinguishing characteristic in thin section. Explore 3D Model below. 3. Uncredited images and diagrams: Scott Brande. cleavage geometry. Difficult as in peridot. image credit: Scott Brande. Cleavage - The tendency of a mineral to break along flat planar surfaces as determined by the structure of its crystal lattice. Definitions. These two-dimensional surfaces are known as cleavage planes and are caused by the alignment of weaker bonds between atoms in the crystal lattice. Other: Effervesces vigorously in dilute hydrochloric acid. Explore/observe features. Cleavage is a crystal’s ability to break along weak bonds within the crystal structure. This is termed fracture, eg. In thin section, calcite can be distinguished by its parallel extinction. Fracture: None. Cellphone: best to display in new cellphone browser tab. Aragonite is a polymorph of calcite. For identifying cleavage: On the surface observe some minutely stepped flat planes, or a wavy pattern. Calcite exhibits rhombic cleavage, which means that it breaks along three planes of weakness that create a rhombic shape for the crystal. Cleavage and fracture describe different ways in which a mineral can break. the conchoidal fracture of obsidian. Hardness is 3 (only on the basal pinacoidal faces, calcite has a hardness of less than 2.5 and can be scratched by a fingernail). calcite. Easy and perfect as the rhombohedral cleavage in calcite. Image credit: Scott Brande. (Cleavage in aragonite is generally less well developed.) Interactive 3D Model. Fracture is conchoidal. The break results in a smooth surface. 2. calcite. calcite. In hand sample, calcite can be distinguished by its rhombohedral cleavage and, if the sample is translucent enough, its double refraction.

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