Specific gravity: 3.5 to 3.53
Colour: Colourless, yellow, green, pink, blue, black
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Plutonic igneous environments
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
Diamond is formed deep in the mantle at depths greater than 120 km where the temperature is about 950oC and the pressure is about 4.5 GPa. Diamond formed at depths below 240 km may contain metal inclusions and majorite. It is only brought to the surface via kimberlite pipes. Indicator minerals in kimberlite are pyrope, diopside, olivine and ilmenite variety picroilmenite (GAHK 2019 pps28-33).
Diamond may be found in kimberlite, and also occasionally in eclogite. It is also found in alluvial deposits, along with quartz, corundum and zircon.
At the Witwatersrand goldfield, South Africa, diamonds are of placer origin, and they are always green. The colouring is surficial, and due to a thin green outer rind surrounding colourless interiors. The green colour is caused by radiation, presumed to emanate from the uranium present in the conglomerate ore. Since the diamonds are of placer origin they must predate the deposition of the Witwatersrand Mesoarchean (3.2 to 2.8 billion years ago) sediments, and they indicate the onset of modern style plate tectonics as early as 3.6 billion years ago (R&M 96.4.323-324).
Diamond is the high pressure paramorph of graphite. At 800oC diamond is the stable paramorph 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 + coesite → diopside + diamond + oxygen
MgCa(CO3)2 + 2SiO2 → CaMgSi2O6 + 2C + 2O2
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