• journal_title：American Mineralogist
• Contributor：Anne M. Hofmeister
• Publisher：Mineralogical Society of America
• Date：2007-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2007.2589
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：92
• issue：11-12
• firstpage：1899
• section：Articles

The phonon component of thermal diffusivity (D) from 12 single crystals in the spinel family was measured at temperatures (T) of up to ~2000 K, using laser-flash analysis. Synthetic disordered spinel, 4 gemstones near MgAl₂O₄, nearly ZnAl₂O₄, 4 “hercynites” [(Mg,Fe²⁺)(Al,Fe³⁺)₂O₄], and 2 magnetites (nearly Fe₃O₄) were characterized using optical spectroscopy and electron microprobe analysis. The magnetic transition in Fe₃O₄ is manifest as a lambda curve in 1/D, but otherwise, D decreases with increasing T and approaches a constant (D_sat) at high T. Part of the decrease in D as T increases results from disordering above ~700 K: these two effects were distinguished by making multiple heating runs. At 298 K, D decreases strongly as either cation substitution or Mg-Al disorder increases, whereas D_sat is moderately perturbed by substitutions. For both ordered and (equilibrium) disordered spinels and hercynites, the temperature dependence of 1/D is best described by low-order polynomial fits. For spinel, combining our data with previous cryogenic studies of thermal conductivity (k) constrains the T dependence of D and k from ~0 K to melting.

The response of D to disorder, impurity content, and cation mass for the aluminates is used to constrain D(T) for γ-Mg₂SiO₄ and ringwoodite. Pressure derivatives are provided by relationships such as ∂ln(k_lat)/∂P = ∂ln(K_T)/∂P. Our results show that the phonon contribution to heat transport in Earth’s transition zone is high, particularly for large proportions of ringwoodite.