Identificazione delle varianti strutturali del K-feldspato in sezioni sottili di rocce. Nota I

Blasi, Achille

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 52 (1972) no. 5, pp. 773-782.

Résumé

To identify the structural variants of K-feldspar, the 2 V angle and the orientation of the optical indicatrix referred to the crystallographic setting are significant. In this paper a method of optical investigation is described, that makes it possible to obtain precise information on the degree of Al/Si-order, the triclinicity and the sub-microscopic twinning, and also gives some indications on the composition of the K-feldspar. This method can be used during normal petrological investigations in that it requires only common thin sections of rocks and the universal stage. By means of it one can study even very small crystals, as well as crystals of different, coexisting generations: it therefore provides useful clues on the physical-chemical factors controlling, at different times, the formation of the rock. This method removes the limitations inherent in that of Marfunin (1961a), which cannot be applied to small crystals since it requires not only a universal stage but a one-circle goniometer and special goniometrically oriented thin sections also, obtained from cleavage fragments previously isolated from the rock. Besides the modifications concerning the investigated material and the apparatus, some new variations are suggested for the instrumental conditions and the elaboration and interpretation of the data. The Nota I of the present paper concerns the universal stage procedures: systematical looking in the rock section for a crystal $\sim \bot [100]$ and suitable for the method itself; analysis of the devices by means of which an accurate determination of $2 V_{X}$, with the direct method, as well as of the orientation of the optical indicatrix in relation with the crystallographic setting is possible. For the same crystal of K-feldspar the following elements are determined, either by a direct or an indirect method: Z, Y, X, A, B, $\bot (010)$, $\bot (001)$, $2 V_{X}$, $\bot (010) \Lambda Z$, $\bot (010) \Lambda Y$, $\bot (010) \Lambda X$, $\bot (001) \Lambda Z$, $\bot (001) \Lambda Y$, $\bot (001) \Lambda X$, $\Delta op$, extinction angle on (010), extinction angle on (001), $\bot (010) \Lambda A$, $\bot (010) \Lambda B$, $A_{san} \Lambda A$, $B_{san} \Lambda B$. The Nota II of the present paper concerns the elaboration of the data: stereographic projection $\bot [100]$ of the optical indicatrix on the basis of Laves's (1951 b) crystallographic setting for microcline; scrutiny of the structural and compositional meanings of the most important direct and indirect parameters; use of these parameters in diagrams which allow to recognize, characterize and classify the different structural variants; and eventually consideration of their relationships with those deduced by X-ray investigation.