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