X-Ray Studies on the Structural State, Orthoclase Content and Al-Distribution in Tetrahedra of Potassium Feldspars from Southern India

Potassium feldspars from the granitic and related rocks occurring in Hyderabad, Salem, Mysore and Waltair areas were subjected to X-ray diffraction studies, both in their natural state and after heating to 1050°C for 8 hrs. The triclinicity (Δ) value in the natural samples varies from 0.98 to 0.74, indicating more or less complete ordered Al/Si distribution, whereas in the heated samples it varies from 0.76 to 0.0, showing increasing disorder with heating. The % Or for these samples as well as for the K-feldspars of Chipurupalle-Razam area was estimated using d-_̅201 following different methods. The % Or obtained from the equations of Wright (1968) very closely agrees with the value obtained from reliable chemical analysis.

Quantitative estimation of the Al contents in the four tetrahedral sites (T₁O, T₁m, T₂O and T₂m), following the method of Stewart and Ribbe (1969), shows that in orthoclase the Al content is equally distributed between T₁O and T₁m sites, whereas in intermediate and maximum microcline A1 is concentrated more in T₁O site than in T₁m site. Based on the triclinicity values, the K-feldspars from the Chipurupalle-Razam area belong to the high structural state followed by those from Salem, Waltair, Hyderabad, and Mysore areas with low structural states, and the differences in the structural state may be related to the differences in the P-T conditions prevaling at the time of K-feldspar formation in these areas.