沼貝隕石に関する鉱物学的研究

抄録

The Numakai meteorite fell near Numakai, Bibai City, Hokkaido (43°17.4'N, 141°51.3'E) at 16:00 hrs, 5th September 1925. Although two stones were observed to fall, only one of them was collected by M. Tanaka. The stone was a flat block, triangular in shape, weighing about 356 gr. The surface is covered with a well-developed fusion crust and has many characteristic “thumb prints”, while numerous chondrules (18 vol. %) are present in the pale bluish grey matrix (82 vol. %) on the broken surface. Constitutent minerals are olivine, orthopyroxene, clinopyroxene, plagioclase, glass and opaque minerals such as troilite, kamacite, taenite and chromite, associated with minor amounts of apatite, spinel, quartz, cristobalite, and glass.
Chondrules form circular or ellipsoidal aggregates, 0.5-2.0mm in diameter, consisting of the same minerals as the matrix, with varying crystallinity. Most common are barred olivine chondrules, radiating pyroxene or olivine chondrules, and porphyritic chondrules. Olivine (Fo_75-81) is most common, both in chondrules and matrix, and forms euhedral or subhedral crystals, sometimes surrounded by a reaction rim of clino- and orthopyroxenes. Shadow extinction due to strain is occasionally observed. Orthopyroxene (En_79-84) usually forms homogeneous, single grains, but sometimes has exsolution lamellae or patches of clinopyroxene, or is surrounded by a reaction rim of clinopyroxene. Clinopyroxene is rare, and is a subcalcic augite. It usually forms intimate mixtures with very fine feldspar crystals, resulting in a bulk composition rather rich in Al₂O₃, Na₂O and K₂O. The distribution of Cr₂O₃ in the Fe-Mg silicates is highest in clinopyroxene, and lowest in olivine, and is probably explained by the substitution of Cr_??_Al in the (Si, A1)O₄ tetrahedra in these minerals.
Plagioclase (An₂₄) is usually not twinned, and not zoned, and belongs to the hightemperature type. Apatite forms subhedral or anhedral crystals, occupying the interstices between olivine and orthopyroxene. A small amount of glass is present, and is partially devitrified. Its composition indicates a mixture of alkali feldspar or plgiaoclase and pyroxene. Kamacite and troilite are the most common opaques. Intimate intergrowth of kamacite and taenite is frequently observed, and can be distinguished by the difference in reflectivity and color. Compared with the terrestrial chromites, the meteorite chromite is rather low in A1₂O₃. In the Mossbauer spectrum of the meteorite two pairs of doublet absorption lines and a Zeeman-splitted six-line pattern are assigned to pyroxene, olivine and troilite, respectively.
The bulk chemical composition of the Numakai meteorite indicates that it belongs to the H-group of chondrites, and petrographically to the H-4 type of Van Schmus and Wood (1967).