A New Occurrence and New Data on Akdalaite, a Retrograde Mineral from UHP Whiteschist, Kokchetav Massif, Northern Kazakhstan

摘要

Akdalaite (5A1_2O_3-H_2O), as well as hogbomite, kyanite, Zn-bearing staurolite, Mg-chlorite, gahnite, hercynite, quartz, apatite, and zircon, are present as submicrometer-size minerals associated with rutile aggregates occurring as inclusions in garnet porphyroblasts of whiteschist from the Kulet Kol region, Kokchetav Massif, northern Kazakhstan. The akdalaite crystals, approx 0.5 to 1 mu m in size, are subhedral to euhedral, and are bounded by (0001), (0001), {ll00}, {l101} facets. EDX analyses show minor Si, Ti, Cr, Fe, Mg, Zn, and Ga, in addition to the major component Al. Electron diffraction data yield the following crystal structural information: space group P6_3mc, and a = 5.58(1) A, c = 8.86(2) A, which are similar to those of synthetic tohdite, but are distinctly different from those reported for the type specimen of akdalaite (P6_12 or P6_1, a = 14.97 A, c = 12.87 A). The akdalaite type specimen, stored in the museum of the Ail-Russian Institute of Mineral Resources (VIMS) in Moscow, had also been re-examined; it has crystallographic parameters similar to tohdite. It is therefore concluded that crystallographic data of the akdalaite type specimen derived from powder X-ray experiments are in error, and that akdalaite is the natural counterpart of tohdite.