NATURAL ANALOGS OF THE δ-AL2O3 AND θ-AL2O3 ALUMINA MODIFICATIONS IN SEDIMENTS OF THE PERMAFROST REGION

Abstract

In investigating Quaternary permafrost rocks in various regions of Yakutia, we come across neogenic alumina in the form of friable white and yellowish spheres about 0.1 mm in diameter. The spherical alumina concretions that we found occur in strata with pronounced indications of cryogenic soil formation. The mineralogical samples were collected from drill cores and from natural outcrops. As we proved previously, the white spheres are supergene corundum. Yellow spheres of various hues and lustre are present among the white spheres in samples in which the total of such spheres constitutes 2 to 3 percent of the heavy mineral fraction. The white and yellow spheres are practically identical in terms of morphology and optical properties (n ranges from 1.69 to 1.74; birefringence is less than 0.006). However, the x-ray diffraction patterns of the yellow spheres differ substantially from those of the white spheres (corundum). A detailed x-ray powder pattern and microprobe analysis was therefore undertaken in order to compare the yellow and white spheres. We have thus discovered natural analogs of tetragonal (δ-Al2O3) and monoclinal (θ-Al2O3) modifications of alumina. Their presence in syngenetically frozen Quaternary sediments indicates that these phases undoubtedly formed outside their stability ranges, either at below-zero or at just above zero temperatures. By taking into account the effect of the small particle size on phase equilibria (dimensional phase effects, we can satisfactorily explain the above-described formation of the metastable modifications of alumina in syngenetically freezing sediments.