DIAMONDITE
Carbonado diamondites (small specimen at top: 4.05 carats and 9x9x6 mm in size;
medium-sized specimen at left: 4.07 carats and 10x13x7 mm in size; large
specimen at right: 5.09 carats and 12x6x6 mm in size).
Diamondites are rare, polycrystalline, nearly monomineralic diamond rocks.
The term “diamond” refers to a mineral, while the term “diamondite”
refers to a diamond rock. The three diamondites depicted here are
a variety referred to as “carbonados” in the diamond trade.
Carbonados are typically a dark brown to blackish color. The shiny luster
seen in the specimens below is typical for carbonados (adamantine
luster). Individual diamond crystals in carbonado diamondites are
exceedingly small, typically <20 m.
Carbonados are also characterized by having obvious porosity in all portions of
the rock (see pits in specimens shown above).
Despite a relative abundance of published research,
the origin of carbonado diamondites is still unresolved. Some workers
interpret them as being crustal in origin. Some consider them to have a
mantle origin. Some suggest that they are impact-generated. Recent
research has indicated that carbonado diamondites are extraterrestrial -
extrasolar, in fact (apparently supernova-generated; see story produced by
the American Institute of Physics).
True carbonado diamondites have only been recorded
from Brazil and the Central African Republic (CAR). Considering South
America & Africa were physically connected for a long time while a part of
the ancient Gondwana continent, these two carbonado occurrences may ultimately
represent one deposit. The specimens shown above are from the CAR.
In the CAR, carbonado diamondites are recovered from
modern to subrecent alluvial gravels (placer deposits) below the topsoil.
They are only found in & immediately adjacent to the outcrop belt of the
Carnot Formation (Cretaceous). The Carnot Formation is an ancient fluvial
& lacustrine siliciclastic unit that hosts scattered diamonds &
carbonados. Upon weathering & erosion, any included diamonds in the
Carnot Formation enter modern river systems’ sediment loads.
Age:
carbonado diamondites are exceedingly old. Published research indicates
they are 3.2 billion years old (mid-Mesoarchean).
Locality:
unrecorded locality in the “Bangui region” (area between the cities
of Bangui and Berberati), southwestern Central African Republic, central
Africa.