Diamond

native element

placer

Formula: C native element
Specific gravity: 3.5 to 3.53
Hardness: 10
Streak: none
Colour: Colourless, yellow, green, pink, blue, black
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Placers

Most commonly diamond is found as placers in alluvial deposits where it accumulates because of its inert chemical nature, its great hardness (it is the hardest natural substance known) and its fairly high specific gravity. The rock in which it is found in situ is kimberlite. It is formed deep in the mantle, and is only brought to the surface via kimberlite pipes. It is also found in alluvial deposits, along with quartz, corundum and zircon. With decreasing pressure the diamonds dissolve back into the rock. To occur and survive in a metastable state at the surface they must arrive from depth quickly.
Diamond may be found in kimberlite, and also occasionally in eclogite.

Alteration

Diamond is the high pressure polymorph of graphite. At 800oC diamond is the stable polymorph at pressures above about 35 kbar (SERC).

Coexistence of diamond and carbonate minerals:
The coexistence of diamond and carbonate minerals in mantle eclogite is explained by the reaction:
dolomite + coesitediopside + diamond + oxygen
MgCa(CO3)2 + 2SiO2 → CaMgSi2O6 + 2C + 2O2

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