Thermal diffusivity of orthopyroxenes and protoenstatite as a function of temperature and chemical composition

The phonon component of thermal diffusivity (D) was measured up to temperatures (T) of ∼1000-1500 K from laser-flash analysis from four chemically distinct, gem quality single crystals and six cleavage flakes or mats of orthopyroxenes, plus synthetic protoenstatite. From electron microprobe analysis, Mg/(Mg + Fe) varies from 0.65 to 1.0 and Al, Mn, Cr, Ti, and Na occur as minor impurities. From visible spectroscopy, Fe³⁺ content is small to negligible. From near-IR spectroscopy, hydroxyl contents range from ∼0 to 600 ppm. Heating large crystals redistributes H⁺ among various sites: thus, thermal history influences speciation. For Mg/(Mg + Fe) ∼ 0.9, at 298 K, D is 1.8, 1.9, and 3.2 mm²s^-1 along [010], [100], and [001]. For all samples, D decreases with increasing T. Roughly, anisotropy is independent of T, and unaffected by transition to the protoenstatite structure, which causes D to drop by ∼10 %. Low-order polynomials describe D^-1(T) for orthopyroxenes. A second-order polynomial describes the dependence of D₂₉₈ on Mg content. Only 300 ppm OH suffices to lowers D by ∼10 % whereas Fe³⁺ makes D decrease more rapidly with T but coupled Al substitution has the opposite effect. Available heat capacity and volumetric data were used to calculate orthopyroxene and protoenstatite thermal conductivity.