Thermal Expansion of Garnets Included in Diamond

The lattice parameters of two garnet inclusions in diamonds have been measured by X rays as functions of temperature, with the inclusions first in situ and then freed from their hosts. The included garnets have smaller lattice parameters than the free garnets and must therefore be under pressure. From an equation of state for garnet calculated from thermoelastic data, the pressures in situ are found to vary from 2 to 15 kbar for one garnet (D1) and from 2 to 7 kbar for the other (D2) as the temperature is increased. A model is proposed which assumes that, under uniform isotropic stress and temperature, diamond forms around garnets and behaves purely elastically thereafter. The model is used to calculate theoretical values for the difference in lattice parameters of free and included garnet; it gives the external pressure that removes shear stress in the host at each temperature, a curve that should pass through the temperature and the pressure of formation of the host. For diamond D1, whose behavior agreed with the model, this curve crosses the thermodynamic diamond-graphite equilibrium line at about 500°C; because some nonelastic processes may have occurred, the curve is proposed merely as a lower bound to pressures of formation of the diamond. The method cannot be applied to D2: below 400°C the thermal expansion of the included garnet was greater than that predicted by the model, and the curve obtained from the high-temperature data lies wholly in the region where graphite is stable. It is concluded that in D2 pressure has probably been released by a nonelastic mechanism during some stage of the diamond's history.

[1]  I. Kushiro,et al.  Origin of some eclogite inclusions in kimberlite , 1968 .

[2]  H. Meyer.  Chrome Pyrope: An Inclusion in Natural Diamond , 1968, Science.

[3]  Naohiro Soga,et al.  Elastic constants of garnet under pressure and temperature , 1967 .

[4]  R. J. Davis.  The use of an off‐centred crystal in double oscillation photographs , 1967 .

[5]  F. R. Boyd,et al.  The join Mg2Si2O6-CaMgSi2O6 at 30 kilobars pressure and its application to pyroxenes from kimberlites , 1966 .

[6]  T. Evans,et al.  Plastic bending of diamond plates , 1965 .

[7]  H. Meyer.  NOTES ON EXPERIMENTAL TECHNIQUE AND APPARATUS: X-ray diffraction photographs of criented single crystals at temperatures up to 1800°C , 1965 .

[8]  R. Roy,et al.  Residual Stresses in Two‐Phase Systems as Determined by Changes in Transition Temperature , 1963 .

[9]  John L. Rosenfeld,et al.  Pressure and temperature of crystallization from elastic effects around solid inclusions in minerals , 1961 .

[10]  F. Bundy,et al.  Diamond‐Graphite Equilibrium Line from Growth and Graphitization of Diamond , 1961 .

[11]  R. Verma,et al.  Elasticity of some high-density crystals , 1960 .

[12]  B. Skinner.  Physical properties of end-members of the garnet group , 1956 .

[13]  A. Davey,et al.  XL. Thermal expansion of diamond , 1956 .

[14]  J. Welch.  A simple microscope attachment for observing high-temperature phenomena , 1954 .

[15]  E. Prince,et al.  Determination of elastic constants of crystals from diffuse reflexions of X‐rays. III. Diamond , 1953 .

[16]  B. Rao.  Elastic constants of garnets , 1945 .

[17]  J. Bhimasenachar,et al.  Elastic Constants of Diamond , 1944, Nature.

[18]  B. Saksena.  The thermal expansion of Diamond , 1944 .

[19]  W. J. Henwood.  LXXX. Remarks on Mr. J. Taylor's paper on the accidents incident to steam-boilers , 1827 .

[20]  S. Clark,et al.  Handbook of physical constants , 1966 .

[21]  T. Evans,et al.  Plastic deformation of diamond at temperatures below 1800°c , 1966 .

[22]  Harry G. Drickamer,et al.  Effect of High Pressure on the Lattice Parameters of Diamond, Graphite, and Hexagonal Boron Nitride , 1966 .

[23]  P. J. Reddy,et al.  Temperature dependence of elastic compliances of garnet , 1964 .

[24]  W. L. Bond,et al.  Elastic Moduli of Diamond , 1957 .

[25]  B. Skinner.  The Thermal Expansions of Thoria, Periclase and Diamond , 1957 .

[26]  J. R. Sutton.  KIMBERLEY DIAMONDS: ESPECIALLY CLEAVAGE DIAMONDS , 1918 .

[27]  D. Brewster.  XXIV.—Observations relative to the Structure and Origin of the Diamond. , 1835, Transactions of the Geological Society.