Experimentally, at 66 kbar, biotite and lepidomelane are converted into associations of several minerals, at 1300-1500 0 and 900-1450°C, respectively, with the accompanying increase in the density; garnet is the densest product of both minerals; all of their original potassium passes into the low-density minerals during the conversion. The biotite and lepidomelane associations with garnet and other minerals are converted into biotite and lepidomelane above 1500° and 1450°C, respectively. Thus the range of instability of the micas, with its high pressures and relatively low temperatures, appears to be conducive to migration of potassium into upper horizons of the earth, e. g. by the gravitational differentation or by a fractionation in melting of the mantle substance. --V. P. Sokoloff.
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