Carbonates. Borates

Abstract

Carbonates, main components of limestones, dolomites and marbles, are scarcer in igneous rocks, but they can be abundant in carbonatites as well as in certain metasomatic rocks, hydrothermal veins and mineral deposits (skarn, epithermal systems, etc.). Summing up, carbonates may be common gangue minerals, but also important ores (Cu, Zn, Fe, Mn, REE). The distinction between ore and gangue carbonates is not strictly mineralogical but economic. Moreover, the classification of the mineral in one or another of the three cabonate structural groups (those of calcite and dolomite, trigonal, that of aragonite, orthorhombic) is irrelevant from the point of view of its recognition by reflected light, so the description must refer to individual species. Since the ores are described in Part II (e.g., azurite, bastnäsite, malachite), this part will only deal with carbonate gangue, although some of them are in an ambiguous position, as rhodochrosite, siderite or cerussite, which can be either ore or gangue depending on the market and on grade/tonnage of the orebody. Only carbonates considered as gangue are the subject of the chapter. And they are characterized both as class, considering the characteristics common to all carbonates, and individually. As class, carbonates share some easily observable, distinctive properties, such as: a remarkable bireflectance, distinct anisotropism, visible twins, and abundant internal reflections. Individually, the species calcite, rhodocrosite, magnesite, smithsonite, siderite, dolomite, ankerite, cerussite have been characterised, as well as the borate vonsenite.