Aplicação do método analítico a plagioclásios em 4 posições de extinção

Abstract

Chomard's analytical method was applied to 13 plagioclases belonging to 7 twins. Twelve operations (φ, θ, ψ) were conducted for each individual specimen in each of their four extinction positions. These operations resulted from the combination of three values θ (15º, 30º and 45º) and four values for φ (45º, 135º, 225º and 315º). The values of ψ were the average of 8 determinations. A summarized version of the method is presented. A computer program was used to determine the values of 2V and the directions of the optical axes in 12 different ways for each specimen. The total number of operations resulted from the combination of each θ with all the C³4 possible combinations of φ. The resulting D was determined for each value of θ. The salient conclusion of this study is that previous works reported by the author on the subject are confirmed, i.e., the analytical method yields reliable results for. plagioclases; the results are at least as precise as those obtained by means of the conventional methods using, θ = 30º and φ as a multiple of 45º. In addition no advantage was found in using four extinction positions over using one chosen at random.