The thermal expansion of the clinopyroxene and garnet polymorphs of Na2MgSi5O12 determined by X-ray powder diffraction

Abstract The temperature dependence of the unit-cell parameters of synthetic Na(Mg 0.5 Si 0.5 )Si 2 O 6 clinopyroxene and Na 2 MgSi 5 O 12 garnet were measured, at atmospheric pressure, using X-ray powder diffraction, from 40 K up to their decomposition temperatures (673 K and 793 K respectively). At 300 K the cell parameters were found to be: (i) a = 9.4073(3) Å, b = 8.6487(3) Å, c = 5.2685(2) Å, β = 108.113(2)°, V = 407.41(3) Å 3 and ρ = 3.286 g/cm 3 , for Na(Mg 0.5 Si 0.5 )Si 2 O 6 pyroxene in space-group P2/n with Z = 4 and (ii) a = 11.4155(7) Å, V = 1487.61(5) Å 3 and ρ = 3.561 g/cm 3 for Na 2 MgSi 5 O 12 garnet in space-group Ia $$\overline{3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mn>3</mml:mn> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> d with Z = 8. The thermal expansion tensors have been calculated and analysis of the lattice parameters in terms of a Grüneisen-Debye model is used to estimate the Debye temperature and the incompressibility of the two materials at ambient pressure.