Cornelis Klein

Exsolution Lamellae and Optic Orientation of Clinoamphiboles

Exsolution lamellae are abundant in coexisting hornblende and cummingtonite, and in hornblende coexisting with anthophyllite in Ordovician volcanics metamorphosed in the kyanite and sillimanite zones in central Massachusetts and adjacent New Hampshire. The lamellae have the same orientation relative to the internal structure as the (100) and (001) exsolution lamellae in clinopyroxenes, but are indexed (100) and (ī01) with the C2/m space group commonly chosen for amphiboles. Specimens from the kyanite zone contain very thin (100) and (ī01) lamellae. In the sillimanite zone, both (100) and (ī01) lamellae are thicker and more abundant in iron-rich specimens than they are in magnesian specimens, as might be expected by analogy with pyroxenes from layered mafic intrusions. The (ī01 lamellae allow correct determination of the relations between the optic vibration directions and the crystallographic axes for two alternatively selected space group C2/m and I2/m. This evidence shows that there has been much confusion concerning these relations.

Ureyite, NaCrSi2O6: A New Meteoritic Pyroxene.

The new mineral NaCrSi206 (ureyite) has been found as rareemeraldgreen grains in the iron meteorites Coahuila, Toluca, and Hex River Mountains. X-ray studies of the natural and synthetic material have established that the mineral is isostructural with jadeite, NaAlSi206. Indexed data for powder patterns obtained by x-ray diffraction and precise cell dimensions are given for the Cr, Fe, and Al members of the jadeite group. Unlike jadeite, a high-pressure phase, ureyite can be synthesized from melts at 1-atmosphere pressure.

Equilibrium coexistence of three amphiboles

Electron probe and wet chemical analyses of amphibole pairs from the sillimanite zone of central Massachusetts and adjacent New Hampshire indicated that for a particular metamorphic grade there should be a restricted composition range in which three amphiboles can coexist stably. An unequivocal example of such an equilibrium three amphibole rock has been found in the sillimanite-orthoclase zone. It contains a colorless primitive clinoamphibole, space group P21/m, optically and chemically like cummingtonite with blue-green hornblende exsolution lamellae on (100) and (¯101) of the host; blue-green hornblende, space group C2/m, with primitive cummingtonite exsolution lamellae on (100) and (¯101) of the host; and pale pinkish tan anthophyllite, space group Pnma, that is free of visible exsolution lamellae but is a submicroscopic intergrowth of two orthorhombic amphiboles. Mutual contacts and coarse, oriented intergrowths of two and three host amphiboles indicate the three grew as an equilibrium assemblage prior to exsolution. Electron probe analyses at mutual three-amphibole contacts showed little variation in the composition of each amphibole. Analyses believed to represent most closely the primary amphibole compositions gave atomic proportions on the basis of 23 oxygens per formula unit as follows: for primitive cummingtonite (Na0.02Ca0.21− Mn0.06Fe2+2.28Mg4.12Al0.28) (Al0.17Si7.83), for hornblende (Na0.35Ca1.56Mn0.02Fe1.71Mg2.85Al0.92) (Al1.37Si6.63), and for anthophyllite (Na0.10Ca0.06Mn0.06Fe2.25Mg4.11Al0.47) (Al0.47Si7.53). The reflections violating C-symmetry, on X-ray single crystal photographs of the primitive cummingtonite, are weak and diffuse, and suggest a partial inversion from a C-centered to a primitive clinoamphibole. Single crystal photographs of the anthophyllite show split reflections indicating it is an intergrowth of about 80% anthophyllite and about 20% gedrite which differ in their b crystallographic dimensions. Split reflections are characteristic of all analyzed orthorhombic amphiboles so far examined from Massachusetts and New Hampshire except the most aluminous gedrites, and the relative intensity of the gedrite reflections is roughly proportional to the degree of Na and Al substitution. Thin sections of a few of these anthophyllite specimens show lamellae parallel to (010) that are just resolved with a high power objective.

Mineralogy and Composition of Lunar Fines and Selected Rocks

Mineralogical descriptions and both wet chemical analyses and microprobe analyses are given of the glasses and crystalline components of the lunar fines and of the minerals in microgabbros (samples 10050 and 10047). The principal minerals described are various clinopyroxenes, plagioclase, olivine, low cristobalite, low tridymite, ilmenite, iron-nickel, iron, schreibersite, cohenite, troilite, and a new CaFe pyroxenoid. Descriptions are given of small craters produced by hypervelocity particle impact on glass and iron-nickel fragments in the fines. The rounding of grains in the fines and of surface rocks is attributed to mechanical ahrasion and not to cratering.