A New Phase of Natural Feldspar, An_xOr_100-x, X≈50

Abstract

A new phase, An_xOr_100-x was found as inclusions in labradorite from Ylämaa, Finland, during the course of study with the transmission electron microscope (TEM). The chemical composition of this labradorite was determined by X-ray microprobe analysis as Ab₄₀An₅₆Or₄. A thin section parallel to the b*-c* plane of this labradorite was subjected to observations under the TEM equipped with an analytical system. A TEM micrograph obtained showed two kinds of exsolution lamella, A and B and an inclusion, where the compositions of A and B were determined as Ab₄₃An₅₇ and Ab₅₃An₄₇ respectively by TEM analysis. K was confirmed to be localized in the inclusion, and it was also discovered that the inclusion contains Ca but is free from Na. The composition of the inclusion derived by TEM analysis can be expressed as An_xOr_100-x, a phase of natural feldspar having not been reported of its presence up to the present. The value of x in the above formula is tentatively estimated to be close to 50.