Two
opposing theories were proposed on the origin of the diamond: (1) the
diamond crystals had formed in the original magma before eruption, i.e.
before the Kimberlite pipes were forced up, and (2) the crystals were
formed after eruption (the carbon being derived from the Karoo shales)
—the latter theory is considered the more probable due to the
inclusions.
Sutton
discusses the genesis of diamonds comprehensively. After dealing with
all the prevailing views, he comes to the following conclusions:—
1. Diamonds formed as a late constituent on eclogite
margins.
2. It separated from the magma first as a plastic
crystal, becoming solid later on.
3. It grew by the successive superimposition, mostly
continuous though sometimes intermittent, of plastic shells either on the plastic or on the solid core.
4. Its form and habit were determined by: (i) the
octahedral
spacings of the atoms, (ii) the tetrahedral spacing of the same, (iii)
shrinkage on solidification, (iv) its environment, i.e. the chemical
composition of the mother liquor or whether it grew on or beneath the
eclogite surface of the matrix.
5. There is no evidence to prove that it has suffered
resorption.
The internal structure is governed by the process indicated in the
third clause. By the first clause the temperature of crystallization
must have been relatively low though covering probably a considerable
range dependent on the richness of its solution.
The
genesis of the diamond is intimately associated with the origin of the
Ejmberlite, which has been discussed at length by Alpheus F. Williams.
According to him,
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