An X-ray study of exsolution phenomena in the Skaergaard pyroxenes

The mineralogy of metamorphosed basic rocks from the Willyama Complex, Broken Hill district, New South Wales. Part II. Pyroxenes, garnets, plagioclases, and opaque oxides

Chemical and optical data are presented for the orthopyroxenes, clinopyroxenes, garnets, plagioclases, and opaque oxides in high-grade regionally metamorphosed basic rocks and their garnetiferous associates from the Willyama Complex. Exsolution phenomena in the pyroxenes have been studied by X-ray diffraction methods and with the electron probe micro-analyser. The (001) lamellae in one lime-rich clinopyroxene are clinohypersthene. Comparison of the Willyama and other granulite facies clinopyroxenes with those of slowly cooled basic igneous rocks suggests that their chemical compositions may be used as guides to temperature conditions during formation. Willyama plagioclases are complexly twinned and possess intermediate structural states.

Diffraction and Mössbauer Studies of Minerals from Lunar Soils and Rocks

Constituents of lunar soils and rocks were studied by powder and singlecrystal x-ray diffraction. In addition to identification of minerals, including rare amphibole, mica, and aragonite, a detailed study of the important rock-forming minerals of the plagioclase, pyroxene, and olivine groups has begun. Mössbauer spectra were recorded from lunar soils, ground rock samples, and separates of iron-bearing minerals. The proportions of iron-bearing minerals were estimated from computer-fitted areas for the bulk samples. The Fe(2+) in the lower-density fraction of pyroxene was ordered, whereas that of the higher-density fraction was disordered.

Exsolution in Clinoamphiboles

Ten amphibole specimens from a variety of metamorphic rocks such as talc schists, eclogites, and metamorphosed iron formations contain lamellae of a second amphibole oriented parallel to ([unknown]101) or (100), or both, of the host. Tremolites, actinolites, and hornblendes commonly have lamellae of a calcium-poor clinoamphibole with P2(1)/m space-group symmetry, or lamellae of cummingtonite with C2/m space-group symmetry. Likewise cummingtonites and P2(1)/m clinoamphiboles commonly contain lamellae of calcium-rich C2/m amphiboles such as tremolite. Results of x-ray diffraction, electron-probe, and microscope studies indicate that most lamellae result from unmixing of a homogeneous amphibole. The P2(1)/m clinoamphibole is analogous to the clinopyroxene pigeonite in agreement with the results of M. G. Bown.