Azurite is one of the two basic copper(II) carbonate minerals, the other being bright green malachite. Simple copper carbonate (CuCO 3) is not known to exist in nature. Azurite has the formula Cu 3 (CO 3) 2 (OH) 2, with the copper(II) cations linked to two different anions, carbonate and hydroxide.
Calcium carbonate crystals are referred to as calcite. The calcite crystal generally is considered a rhombohedron because of its cleavage properties. Cleavage is what causes crystals to angle where the bonding forces are weak and are apt to break into planes.
The most common magnesium carbonate forms are the anhydrous salt called magnesite (MgCO 3) and the di, tri, and pentahydrates known as barringtonite (MgCO 3 ·2 H 2 O), nesquehonite (MgCO 3 ·3 H 2 O), and lansfordite (MgCO 3 ·5 H 2 O), respectively.
Smithsonite is a variably colored trigonal mineral which only rarely is found in well formed crystals. The typical habit is as earthy botryoidal masses. It has a Mohs hardness of 4.5 and a specific gravity of 4.4 to 4.5. Smithsonite occurs as a secondary mineral in the weathering or oxidation zone of zinc-bearing ore deposits.
The spar is precipitated from fresh or marine water percolating through the sediment after deposition, but before final cementation. This oosparite shows well the spar cement. Classification of Carbonates The classification of carbonates using the allochem/interstitial material system (the Folk System) is very systematic and straight forward.