Plagioclase determination with the aid of the extinction angles in sections normal to (010); a critical comparison of current albite-Carlsbad charts

Geology and petrology of the Campbell—Aviator Divide, Northern Victoria Land, Antarctica

“…Plagioclase composition, determined from the high temperature curves of Tobi (1963), ranges between An 4 7 and An 60 in different specimens although most values lie in the range An 5 0 -5 4 • It should be noted that previous writers have used low temperature determinative curves which give more An-rich values than those given above. Gunn (1962) analysed the order-disorder relations of two plagioclases, finding one to be partly disordered and the other in the disordered state typical of volcanic rocks.…”

“…Gunn (1962) analysed the order-disorder relations of two plagioclases, finding one to be partly disordered and the other in the disordered state typical of volcanic rocks. For this reason it is preferable to use the high temperature curves although it is realised that the values obtained may be slightly in error (see Tobi, 1963).…”

Recent Thermal and Volcanic activity on Mount Melbourne, Northern Victoria Land, Antarctica

“…Most crystals show marked normal zoning, and the core or a narrow inner zone is commonly slightly altered. The composition, measured from the high temperature curves of Tobi (1963), varies between An30 and An 43 in different specimens, whereas phenocryst composition in P 34918 is An w4o ' Many Df the plagioclase crystals in P 34914 (which is the most highly crystalline of all the specimens) are rimmed by a narrow border of alkali feldspar.…”

Volcanic Rocks of Naturaliste Plateau, Eastern Indian Ocean, Site 264, DSDP Leg 28

“…Plagioclase compositions were determined optically by the universal-stage method of Uruno (1963), in which Kohler's angles XAX and Y Y are measured across (010) on stereogram plots of optical directions of adjoining albite twins. After determining compositions and average ordering degree from several such measurements ( Figure 5), additional determinations were made by the faster method of rotating the crystal axis a parallel to the microscope axis and using, or interpolating between, the appropriate high-or low-T extinction curves of Tobi (1963). Optical plagioclase compositions listed in Table 2 are average determinations of the major parts of crystals and do not include zoned rims or groundmass plagioclases, which are usually much more sodic.…”