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