Petrologic aspect of pericline twinning in albites of igneous rocks

Abstract

Secondary albites ( ~An0) and primary feldspars replaced by them in volcanic rocks and granitoids have been studied by means of microscopic methods.Particular attention has been given to albite twins and their origin, conditions of origin and differentiation depending on the kind of feldspar replaced by albite. Twinnings of two genetic types have been found to occur in albites, namely: the relicts (i.e. inherited after earlier feldspar) of normal habit of lamellae, and growth twinnings of chessboard habit of twin-bands. The relict twinnings occur an albites developed at the expense of primary plagioclases and rarely of potassium feldspars.A method has been established for the determination of the primary plagioclase composition on the basis of the position of the relict pericline composition plane in secondary albite taking into account the (Al, Si)-ordering state in plagioclase structure of volcanic and plutonic rocks. An extremely rare case has been found of Ala-A twinning in which the rhombic section 0kl is the composition plane (the case theoretically anticipated by Franke, 1920). Chessboard twinning characterizes albites developed at the expense of potassium feldspars, and only occasionally at the expense of plagioclases. They show important differences in their form depending on the kind of primary feldspar. The method of recognition of secondary albites and for the determination of the primary composition of plagioclase on the basis of the position of the pericline composition plane, as used in this paper, implies that the albitization processes in igneous complexes are much more frequent than was hitherto supposed, and the share of primary alkaline rocks – mainly the sodium ones – in the earth's crust is smaller than previously assumed.