Identification of Detrital Feldspars by D., Van Der Plas

By D., Van Der Plas

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In some cases the potassium content can be so high that on cooling the homogeneous phase is too unstable and separates into a potassium-rich phase and a sodium-rich phase, much the same as in perthites. 44). In general, the trace-element content of plagioclases does not interfere with the physical properties to a measurable extent. For soil scientists it might be interesting to know that HOWIE(1955), BARABANOW (1958), YOUNG(1958), HEIER (1960) reported the trace-element content of a number (1960, 1962), and STEFFEN of plagioclases.

It is concluded that the scatter of optical 35 PLAGIOCLASE FELDSPARS properties in his experiments is due to heat treatment. He did not, however, put the clue to an explanation of the irregularities noticed in writing. LARSSON(1940) is the first to point out that plagioclases of intrusive rocks have optical characteristics that differ from those of plagioclases of volcanic rocks. His conclusion is confirmed by the observations of LUNDEGARDH (1941). KOHLER(1942a) finally states plainly that two series of plagioclase feldspar must exist, one of high-temperature origin and the other belonging to a relatively low-temperature genesis.

Plagioclase feldspars: in nature a discontinuous series of mixed crystals between pure sodium feldspar and pure calcium feldspar or peristerites (see Chapter 3). The chemical composition of the end members may be written as: NaAlSisOs, abbreviated Ab, and CaAlzSizOs, abbreviated An (from anorthite). Ternaryfeldspars: FRANCO and SCHAIRER (195 1) synthesized a series of crystalline substances of feldspar structure with a chemical composition of mixtures of Or, Ab, and An at intervals of 10 weight percentages.

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