dolomite, magnesium is typically between 44 and 50 percent of total magnesium plus calcium, indicating some substitution of calcium into the magnesium layers. A small amount of ferrous iron typically substitutes for magnesium, particularly in more ancient dolomites. Carbonate rock tends to be either almost all calcite or almost all dolomite, with intermediate compositions being quite uncommon. Dolomite outcrops are recognized in the field by their softness (mineral dolomite has a Mohs hardness of 4 or less, well below common silicate minerals) and because dolomite bubbles feebly when a drop of dilute hydrochloric acid is dropped on it. This distinguishes dolomite from limestone, which is also soft but reacts vigorously with dilute hydrochloric acid. Dolomite usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. This is released and oxidized as the dolomite weathers. Dolomite is usually granular in appearance, with a texture resembling grains of sugar. Under the microscope, thin sections of dolomite usually show individual grains that are well-shaped rhombs, with considerable pore space. As a result, subsurface dolomite is generally more porous than subsurface limestone and makes up 80% of carbonate rock petroleum reservoirs. This texture contrasts with limestone, which is usually a mixture of grains, micrite (very fine-grained carbonate mud) and sparry cement. The optical properties of calcite and mineral dolomite are difficult to distinguish, but calcite almost never crystallizes as regular rhombs, and calcite is stained by Alizarin Red S while dolomite grains are not. Dolomite rock consisting of well-formed grains with planar surfaces is described as planar or idiotopic dolomite, while dolomite consisting of poorly-formed grains with irregular surfaces is described as nonplanar or xenotopic dolomite. The latter likely forms by recrystallization of existing dolomite at elevated temperature (over ). The texture of dolomite often shows that it is secondary, formed by replacement of calcium by magnesium in limestone. The preservation of the original limestone texture can range from almost perfectly preserved to completely destroyed. Under a microscope, dolomite rhombs are sometimes seen to replace oolites or skeletal particles of the original limestone. There is sometimes selective replacement of fossils, with the fossil remaining mostly calcite and the surrounding matrix composed of dolomite grains. Sometimes dolomite rhombs are seen cut across the fossil outline. However, some dolomite shows no textural indications that it was formed by replacement of limestone. Occurrence and origin Dolomite is widespread in its occurrences, though