斜長石ノート : とくに斜長石双晶について

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First, a review of four works on plagioclase by Japanese petrologists after World War II is given. Among them, the articles of KANO (1955) and URUNO (1963) are on temperature form of plagioclase, and the articles of GORAI (1951) and SUWA (1956) are on twinning of plagioclase. And then the problems on the twinning of plagioclase are discussed, especially dealing with GORAI'S and SUWA'S triangle diagrams for twinned plagioclase, with optical ambiguity for determination of twin laws, and with the frequency of composition plane of plagioclase in relation to motamorphism. (1) GORAI (1951) has made a famous U-A-C diagram of plagioclase twinning (Fig. 2). Symbol U designates untwinned plagioclase. Symbol A (A-twin) designates plagioclases twinned after albite-, pericline-, and acline laws, which occur all through igneous and metamorphic rocks. Symbol C (C-twin) designates plagioclases twinned after Carls-bad-, albite-Carlsbad-, Ala-, Manebach-, Baveno-, and albite-Ala B laws, which are common in igneous rocks. Metamorphic region is characte rized by the poverty of C-twin. On the contrary, SUWA (1956) stated that the part of GORAI'S system of grouping, in which Manebach, Baveno, and albite-Ala B twins were included in the C-twin, should be discarded, and he has made another triangle diagram on plagioclase twinning (Fig. 4). During his work, the albite-Ala B twin, which has been reported to occur very rarely and to be limited to occur in igneous rocks, was found to occur all through igneous and metamorphic rocks as commonly as albite and pericline (acline) laws. Although in very small amounts, Manebach- and Baveno- twins occur in both igneous and metamorphic rocks. Fig. 4 illustrates SUWA'S diagram showing the percentages of abundance of respective plagiolase twinnings, where albite-Ala B+Manebach+Baveno, and Carlsbad + albite-Carlsbad + Ala are plotted against albite + pericline (acline). Metamorphic region is characterized by the poverty of Carlsbad+albite-Carlsbad+Ala laws. (2) Without the universal stage, it is hardly possible to distinguish albite-Ala B twin from albite twin because the former usually shows polysynthetic lamellae which are quite similar to those of the latter. Even when the universal stage is employed, a sharp distinction between the albite-Ala B law and the albite law is very difficult, especially when the Kohler angle for the optic elasticity axis X is nearly 180 degrees as in the composition of oligoclase and andesine. Therefore, some of the albite-Ala B twin in this composition around An 30 might well be the albite twin, and vice versa (MIZUTANI, 1959; SUWA, 1961). The composition around An 30 is a range of optical ambiguity for the distinction between the two laws. Owing to this optical ambiguity, albite-Ala B twin is considered to occur abundantly within this narrow range. Judging from above, the difference between GORAI'S and SUWA'S opinions may be due to this optical ambiguity. (3) Similarly, the sodic composition of plagioclase (An 0-3) is a range of optical ambiguity for the distinction between the Carlsbad law and the albite law, and the composition around An 20 is also optical ambiguity for the Carlsbad law and the albite-Carlsbad law. The abundant occurrence of Carlsbad and albite-Carlsbad laws in albite porphyroblast in spotted schist belonging to the greenschist facies (Toei, 1961) may also be due to this optical ambiguity. The systematic and quantitative studies on the optical ambiguity for determination of plagioclase twin laws will be inevitable. (4) Of the twinned plagioclases in granulites from Antarctica (SuwA, 1966), the frequency percentage of the composition-plane (010) is 64%. It is 87% in the Ryoke metamorphic rocks belonging to the amphibolite facies (SUWA, 1956 and 1961) ; while it is about 100% in schists belonging to thegreenschist facies (ToBi, 1961). It thus appears that the frequency percentage of the composition-plane (010) in twinned plagioclases decreases with increase in the temperature of metamorphism. This frequency percentage, however, may also be changeable with the chemical composition of twinned plagioclase in some degree. But the intermediate plagioclase of An 25-74 shows no relation between this frequency percentage and plagioclase composition. Therefore, in the intermediate plagio-clase, the frequency percentage of the composition-plane (010) seems to be an indicator of the physical condition of metamorphism (See Table 4).
地学団体研究会の論文
1968-05-25

斜長石ノート : とくに斜長石双晶について

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