Analysis of the modulated structures of plagioclase feldspar by X-ray diffraction and electron microscopy

Abstract

The models of the modulated structure of the plagioclase feldspar based on Xray diffraction and electron microscopy have been reviewed. Important models so far presented can be divided into two categories due to difference in evaluation of the second harmonic of the (Ca, Na) modulation wave. The first type (TF, HTKJ and KHNE models), where the second harmonic is assumed to be negligible, gives the ordered arrangement of Na and Ca among the neighbouring subcells. On the other hand, in the second type (KM model) the second harmonic is strong enough to give the alternating bands of the albite-like and anorthite-like structures. Because of the weak intensities of the second satellite reflections corresponding to the second harmonic, the final answer between the model types has not been obtained using only the X-ray diffraction method. However, the fine textures with the antiphase domain boundaries and crystal-chemical considerations of the plagioclase feldspars support the existence of the second harmonic, or, of the alternating bands of the albite-like and anorthite-like structures. 1.は じ め に 鉱 物 学 に お け る斜 長 石 の重 要 性 に つ い て は こ こで 今 さ らの べ るま で もな く,す で に 出 版 さ れ た 多 数 の解 説 書(た とえ ばBragg and Claringbu11 1965; Smith 1974; Ribbe 1975) に繰 り返 しの べ られ て い る。 ま た,1970年 頃 ま で の斜 長 石 の結 晶構 造 に つ い て の 記 述 も,そ 京都大学理学部地質学鉱物学教室 Kitamura & Morimoto Toman & Frueh Horst, Tagai, Korekawa & Jagodzinski Kumao, Hashimoto, Nissen & Fndo