Carbonate on Mars
The term carbonate either refers to a mineral or to a rock. Examples of carbonate minerals are calcite (CaCO3) and dolomite (MgCa(CO3)2), which are common constituents of limestones and other calcareous sediments; siderite (FeCO3), which also occurs in sedimentary rocks; magnesite (MgCO3), an alteration product of ultramafic rock; and malachite Cu2(CO3)(OH)2, smithsonite (ZnCO3), and cerussite (PbCO3), which result from surficial alteration of metallic ore deposits. The term is also applied to sediments or sedimentary rockssuch as limestones or dolostones that are composed predominantly of carbonate minerals. In the Precambrian, particularly the Proterozoic, carbonate minerals precipitated on microbial mats by their photosynthetic activity and resulting alkalinity of the surface are common. Most Phanerozoic carbonates are composed to a large degree of the shells, tests, and spicules or their fragments of marine organisms, cemented by...