99
However, the quality of impact diamonds is very low, because of the small
size of crystals and friable texture of joints, which makes them suitable only for
the abrasive industry. At the crushing and abrasion of impact rocks the loss of
diamonds reaches more than 25%.
There are two opposite models of the genesis of ring impact structures.
Some researchers consider that they were formed due to external space forces by
falling and impacts of super large meteorites or astroblemes
on the surface of the
Earth (Masaitis et al. 1998); others - due to internal terrestrial forces at brisant
explosions of mantle gases of huge capacity in the places of crossing abyssal
breaks (Polyakov and Truchalev, 1980; Marakushev, 1995).
A.A.Marakushev (1995) suggests that the so-called astroblemes or ring-
impact-structures are generated by the brisant explosions
of hydrogen-rich gases,
which are rising from the earth’s core. The diamonds were formed in the impact
melts owing to a reactions of Cr, Ni, Fe and others metals (M) in
the form of metal-organic combinations: M (CN)
2
and M CN
2
; M (CH)
2
and
MCH
2
. There are determine, hat the gas inclusions in the glass rocks of Popigay
structure are contain in unique amounts of H
2
(up to 20 %) and CH
4
(8-22 %).
The presence of hydrogen and methane is interpreted by rapid uplift of
hydrogen-rich and methane fluids from the earth’s core,
where the juvenile gases
are held at the greatest pressure.
5. NATURAL SOURCES OF DIAMONDS IN THE EARTH
Kimberlites and lamproites are the main and principal natural sources of
commercial diamonds on the Earth. They make a significant quota in gross crop
of diamonds, and also are an unique source of jewelry and high-grade technical
stones. The finds of diamonds in others magmatites, impactites, metamorphic and
fluiogenic rocks definitely represent the scientific interest, but have no practical
value (Table 6).
The primary ultrabasic and alkaline-ultramafic melts that generated the
diamond-bearing kimberlites and lamproites appeared in the non-depleted upper
mantle at the thermodynamic condition appropriate to the stable crystallization of
diamond and pyrope. Then, during the periods of tectono-magmatic activity, these
melts rather quickly reached to the superficial terrestrial horizons, where the
preservation of barophilic minerals and diamonds in kimberlites and lamproites
had emanated. V.A.Milashev (1963) has offered to parcel out under the name
“kimberlite facies” of magmatism, which the crucial minerals are barophilic
diamond, pyrope and titanium magnochromite. The rocks of similar chemical and
mineralogical composition, however, without diamond and its barophilic minerals
- satellites, concerning to low bariphilic “pikrite facies”, have to kept the habitual
petrographic names.
Increasing the alkaline higher than the limits in the phlogopite kimberlites at
the common basicity of alkaline-ultramafic melts resulted to appearance in these
rocks at first K-richterite and then leucite. For difference the usual “olivine
100
kimberlite” I type and “phlogopite-olivine kimberlite” II type from potassium
enriched lamprophyres (lamproites), which are some of variety rocks of
kimberlite facies V.A.Milashev (1994) has offered the name "K-richterite-olivine
kimberlite” III type and “K-richterite-leucite kimberlite” IV type. The borderline
between the four listed basic types of kimberlites carry a statistical property and
correspond approximately between olivine and phlogopite-olivine kimberlites - 1
% К
2
О, then among phlogopite-olivine and K-richterite-olivine kimberlites
(lamproites) - 3,3 % К
2
О, and between K-richterite-leucite kimbrlites
(lamproites) - 7 % К
2
О ( Table 7).
Table 6.Classification of the natural sources of diamonds. (Milashev, 1994).
Genetic type
Type
of rocks
Geological
structura
Ledge forms
Diamond
types and
riches
Diamonds
habit and
morphology
Explosive
Kimberlite,
lamproite
Cratons:
platforms,
shields
Diatrems,
dikes,sills
P-type, E-
type,
high rich
Monocrystals
(O,D,C,T ),
twins, joints
Intrusive
Dunite-
harzburgite,
peridotite-
pyroxenite,
ophiolite
Obduction
oceanic
plates. Eugeo-
synclines
Allochtho-
nous plate,
lens,stocks,
massifs
P-type,
E-type, low
rich, seldom
high rich
Monocrystals
(O,D,C,T)
Effusive
Alkaline
basalt
Zones
tectono-
magmatic
activity
Diatrems
and dikes
E-type,
low rich
Polycrystals
aggregates
Fluidogenic
Quartz,
carbonate-
peach veins
in graphite
contents
rocks
Crystalline
fundament
platforms,
median
massifs
Veins, linear
strike-slips
shear zones
E-type,
high rich
Monocrystals
(C, C-O ),
sceletal
forms, joints
Dynamic
metamorphism
Graphite
contents
gneisses,
eclogites
Crystalline
fundament
platforms,
median
massifs
Lens and
plates ledges
E-type,
high rich
Monocrystals
(C, C-O,
T-H)
Impactite
Cripto-
volcanite,
impactite rocks
Astroble-mes
and
Ring impact
structures on
platforms
Circular
depressions,
volcanic calder,
meteorite
craters
L-type,
high rich
Monocrystals
(L) and
polycrystal
aggregates of
C and L
Notes: Crystals form of diamod:
O- octahedron, C- cube, C-O- cube-octahedron,
D- dodecahedron, T- tetrahedron, T-H – tetrahexahedron, L- lonsdaleite.