 〔Dartmoor Torsの中の『Geology』の『Minerals』から〕 三角ダイアグラムにおいて、上端はKAlSi3O8（代表的な鉱物はオルソクレース〔正長石〕）、左端はNaAlSi3O8（アルバイト〔曹長石〕）、右端はCaAl2Si2O8（アノーサイト〔灰長石〕）である。代表的な長石の組成は、比較的低温での生成の場合は青色の範囲、比較的高温での生成の場合は青色＋赤色の範囲となる。KAｌSi3O8－NaAlSi3O8系列をアルカリ長石〔alkali feldspar：とくにKAｌSi3O8を主体とするときはカリ長石（K-feldspar）〕とよび、NaAlSi3O8－CaAl2Si2O8系列を斜長石（plagioclase）とよぶ。 |
 Fig 1.2 The maximum extent of solid solution in feldspars, at >1000℃. No feldspars crystallize in the unshaded field. The boundary between plagioclase and alkali feldspar is placed, for convenience only, on the line where An = Or. The named subdivisions of PL at 10, 30, 50, 70 and 90% An are based solely on composition. Anorthoclase and high albite are triclinic at room T but become monoclinic when observed at elevated T because of the shearing transformation. Sanidine is monoclinic at all T.  Fig 1.3 Feldspars stable below ～300℃. Solid solution is limited to the black shaded areas. The stable phases are triclinic low microcline, low albite and anorthite, all with Si-Al order. Orthoclase is a metastable monoclinic Or-rich feldspar with a ‘tweed’ microstructure composed of alternating triclinic ordered domains at the scale of a few nm. In PL, ordering (giving low plagioclase), leads to changes of lattice type and optical properties but not crystal class (Fig 1.7). PL in the range ~An25.An75 has a structure known as e-plagioclase in which thin slabs with albite-like and anorthite-like ordering patterns alternate with a periodicity that is out of step (‘incommensurate’) with the overall lattice.  Fig 1.5 Stable equilibrium ‘strain-free’ phase diagram for An-free alkali feldspars with equilibrium Si-Al ordering, at atmospheric P.  Fig 1.7 Phase diagram for Or-free plagioclase feldspars, showing liquidus and solidus curves at atmospheric P and probable subsolidus relationships in feldspars with equilibrium order. The Mineralogical Societyによる『FELDSPARS 1 Nomenclature, Structure and Phase Relationships』から |