Calcite is a calcium carbonate mineral, and it is the most common form of natural calcium carbonate. Therefore, calcite is a widely distributed mineral. Calcite crystals come in a variety of shapes; their aggregates can be clusters of crystals, or they can be granular, massive, fibrous, stalactitic, earthy, etc. Breaking calcite produces many rhombohedral fragments, hence the name "calcite." The color of calcite varies depending on the impurities it contains; for example, when it contains iron and manganese, it is light yellow, light red, brownish-black, etc. However, it is generally white or colorless. Colorless and transparent calcite is also called Iceland spar. This type of calcite has a remarkable characteristic: its birefringence is as high as 0.172, allowing objects to appear as double images. Therefore, Iceland spar is an important optical material. Calcite is a major mineral in limestone and marble, and it has many uses in production and daily life, its most significant use being in the manufacture of cement. Limestone can form caves, and the stalactites, stalagmites, and other formations within these caves are actually composed of calcite. Calcite often contains isomorphous substitutes such as Mn, Fe, Zn, Mg, Pb, Sr, Ba, and Co; when these substances reach a certain amount, they can form varieties such as manganese calcite, iron calcite, zinc calcite, and magnesium calcite. In addition, mechanical impurities such as brucite, dolomite, iron hydroxides and oxides, sulfides, and quartz are commonly found in the crystals. It belongs to the trigonal crystal system; well-formed crystals are common. It exhibits a wide variety of morphologies, with over 600 different aggregates. It mainly occurs as parallel columnar and tabular crystals, and various states of rhombohedral or ditrigonal triangular-faceted crystals. Calcite often forms contact twins, and even more frequently as polysynthetic twins. The orientation of these polysynthetic twin lines on the cleavage planes differs from that of dolomite. In nature, the appearance of such polysynthetic twins can indicate that calcite has undergone geological stress after its formation.
Language
