Physicochemical factors of
diamond and graphite formation in carbonatite melts
on experimental grounds
Litvin Yu.A.*, Spivak A.V.*, Solopova
N.A.**, Litvin V.Yu.*, Bobrov A.V.**
*Institute of Experimental
Mineralogy, Chernogolovka, Russia; **Moscow State University, Moscow, Russia.
ššššššššššššššš Here the purpose is experimental kinetic study of
diamond and graphite nucleation and growth in multi-component K-Na-Mg-Ca-carbonatite
melt with dissolved carbon under conditions of diamond PT stability. Phenomenal
peculiarity of the carbonatite-carbon melt-solution, while oversaturated by
elemental carbon in respect to diamond, is nucleation and growth of
thermodynamically unstable graphite phase jointly with diamond (similarly to
other diamond-forming «melt-carbon solution» systems including metallic [1],
silicate [2], silicate-carbonate [3], etc.). Hence the elucidation of the physico-chemical
stimulation and mechanism of joint formation of the carbon polymorphs in mantle
carbonatite melts has to be understood not only for making better control over
diamond crystal growth in carbonatite-carbon growth medium but for revealing
the reasons for co-existence of diamond and graphite under the Earth’s mantle
conditions as well as syngenetic formation of mantle-derived diamond with
primarily included graphite therein. Mechanism of graphite formation in
carbonatite melts may be extended over the conditions of graphite PT stability
as well but for as long as pressure-induced congruent melting of carbonate
solvent of carbon is attained and the couple of solid carbon and
carbonate-carbon melt-solution works as a strong self-buffering system [4].
Earlier
experimental study of multi-component carbonatite-carbon systems [5] revealed
that diamond formation in their melts is very efficient under high pressure. The
following investigations made it evident that carbonatite melts are effective
solvents for graphite and diamond [6], kinetic of diamond growth is sensitive
to change of PT parameters [7], and carbon dissolved in carbonatite melt is of
elemental atomic and/or cluster form [8]. š
Present experimental
investigation of diamond nucleation and growth is carried out with the use of
starting carbonatite compositions, wt. %: K2CO3 35.0, Na2CO3
10.0, MgCO3 25.0, CaCO3 30.0, which is a Fe-free version
of carbonatite composition studied earlier [7]: K2CO3 27.21,
Na2CO3 2.89, MgCO3 17.36, CaCO3 26.91,
FeCO3 25.63. To a large degree both the carbonatite compositions are
chemical replicas of the carbonatite end member for multi-component
carbonate-silicate compositions of primary carbonatite inclusions in Botswanian
diamonds [10]; graphite is used as a starting carbon material. The experiments
were carried out at fixed temperature of 1800oC and variable
pressure within the 7.0 – 8.5 GPa range for time duration of 5 – 30 min. Quantity
of spontaneously formed diamond crystals in the volume unit of the sample after
quenching and solidification of the growth melt was taken as the conventional
indicator of nucleation density for diamond phase («survived nucleation centers»).
It was found that the nucleation density is distinctly lowered from 1.8ž103
nucleižcm-3 to 1.1ž103 nucleižcm-3 for 30 min.
duration while pressure decreases from 8.5 to 7.25 GPa. At the same time the linear
size of diamond crystals ranges from 40 µm to 160 µm; the maximal size is
achievable at the lowest pressure. The normal growth rate for octahedral (111)
face of diamond crystal at 7.25 GPa is measured as changeable from 10 µm/min
after 5 min to 6 µm/min after 10 min and to 2.3 µm/min after 30 min of
duration. The effect of lowering of normal rate of diamond growth arises from
difference in density of starting graphite and diamond. This is due to time-sensitive
local pressure depression in the experimental samples while the less dense
starting graphite re-crystallizes into more dense diamond product (effect of «volume
loss» in high-pressure experiment). In case if diamond is used as a starting
material the time-dependent diamond growth rate is close to the linear one. š
It is symptomatic
that re-crystallized graphite crystals came into being at lowest pressures when
the density of diamond nucleation is minimal. This is a specific «signal» of
some carbon over-saturation heterogeneity under experimental conditions and pointed
out that the field of diamond spontaneous nucleation and crystallization is
close to termination with further pressure decrease. The PT points of
termination of diamond nucleation establish a boundary line for the field of
diamond nucleation inside of the PT-region of diamond stability. Effect of
diamond nucleation demonstrates that carbon solutions in carbonatite melts are
highly over-saturated in respect to diamond phase (by physico-chemical term, a «labile»
over-saturated melt-solution). The resulting data of this study as well as
relevant experimental data [6] testify that šdegree of labile carbon over-saturation to
diamond that is responsible for diamond nucleation has regularly decreased with
pressure lowering until the critical limit is attained in a boundary line.
The boundary line is a
demarcation one between the higher pressure regions of «labile» and the lower
pressure region of «metastable» carbon over-saturation in respect to diamond.
Metastable carbon over-saturation in respect to diamond is responsible for seeded
growth of diamond only (possibility of diamond nucleation is suppressed here). But,
it is also responsible for spontaneous nucleation and growth of single
crystalline graphite (the process accompanies the seeded growth of diamond from
the growth medium of metastably over-saturated carbonatite melt-carbon
solution). The PT region of the metastably over-saturated solutions is limited
by the graphite-diamond equilibrium line for which diamond seed growth and
unstable graphite crystallization are suppressed because carbonate melts
reaches the state of carbon saturation in respect to both carbon phases –
diamond and graphite (solubility value for diamond and graphite becomes equal
as for thermodynamically stable phases). šBut, graphite can crystallize at lower
pressures as thermodynamically stable phase. Crystallization of unstable
graphite phase under PT conditions of diamond stability may be estimated as the
example of realization of Ostwald’s rule [10]. The facts of nucleation and
growth of unstable single-crystalline graphite under mantle conditions are
illustrated by syngenetic inclusions of graphite in diamond [11]. On this basis
the assumption that graphite associated with diamond is indicator of «metastable»
formation of diamond may be estimated as mistaken.
Moreover,
thermodynamically unstable graphite may be a primordial mineral of the Earth’s
mantle. The thermodynamics-based carbon phase diagram includes equilibrium graphite-diamond
boundary [12] but it is not capable to ascertain that both graphite and diamond
phases are kinetically capable to exist firmly as thermodynamically unstable
phases far off the equilibrium boundary;š
these facts have been determined experimentally [13]. This means that
the direct transition of graphite to diamond and vice versa is not practically
realizable in the one-component carbon system at the most depths of the Earth’s
mantle. But the equilibrium graphite-diamond line controls re-crystallization of
graphite to diamond and vice versa in the system «carbon solvent – carbon» [14].
For the re-crystallization, it is important that solubility of
thermodynamically unstable solid phase exceeds solubility of the stable one.
Hence formation of
thermodynamically stable diamond and unstable graphite in carbonatite
melt-carbon solution is under control of the next physico-chemical factors: (1)
pressure-induced high carbon solubility in congruent carbonatite melts and
formation of carbonatite-carbon melt-solution, (2) initiation of over-saturated
carbon solution in respect to diamond because of carbon solubility difference
of unstable graphite and diamond if graphite is carbon source or šif temperature gradient exists over
experimental sample in the case when diamond is a carbon source, (3) formation
of labile carbon over-saturationš for
diamond spontaneous nucleation, (4) formation of metastable carbon over-saturation
for seeded diamond growth and accompanying unstable graphite nucleation
(following the Ostwald’s rule), (5) carbon over-saturation degree is
progressively dependent on pressure increase (at fixed temperature), (6) carbon
over-saturation degree is regressively dependent on temperature increase (at
fixed pressure), (7) kinetics of diamond and graphite nucleation and crystallization
is progressively dependent on the degree of carbon over-saturation in
carbonatite melt.š šš
ššššššššššššššš This study is supported by the INTAS project 05-1000008-7938,
RFBR grant 08-05-00110-a and the RF President grant 4122007.5.
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