Structural and chemical variations in pyroxenes

Within the last 15 years, approximately 80 high-quality, three-dimensiona l structure re­ finements of terrestrial, lunar, and meteoritic pyroxenes were published. The majority of the refinements involve Cl/c clinopyroxenes and Pbca orthopyroxenes, but P2,/c structures are also well represented, Few data are available on the Pbcn pyroxenes that constitute the fourth major structure type. The topology of the four space groups can be described with idealized models composed of tetrahedral-octa hedral-tetrahedr al I-beam units that lie parallel to [001]. The different symmetries are a result of different stacking sequences of the octahedral layers and/or of symmetrically-di stinct tetrahedral chains in adjacent layers. In all of the py­ roxenes refined, the Ml cation site is coordinated by six oxygens arranged in a regular pseudo-octahedra l configuration. The M2 site is irregularly coordinated by six, seven, or eight oxygens. The M2 coordination depends upon the size of the cation occupying the site: higher coordination numbers are usually associated with larger cations. maximum devia­ tion of the tetrahedral chains from an extended configuration (03-03-03 = 180°) occurs in the B chains of several Pbca structures where 03-03-03 � 136 o. The A tetrahedral chains in most P21/c structures are S-rotated, but all other chains are 0-rotated. Studies of pyroxene structures at elevated temperatures and pressures revealed that the cation polyhedra expand and compress differentially. The high temperature studies documented a P21/c !:::+ Cl/c tran­ sition in the Fe-Mg pyroxenes, and showed that the temperature of the transition decreases with increasing ferrosilite content. In addition, these studies provided further insight into the miscibility between the high-calcium and low-calcium pyroxenes and produced cell parame­ ter data that are basic to geothermometry studies involving exsolution lamellae. Site occu­ pancy refinements confirmed the preference of the Fe2+ for the larger, more distorted M2 site in the Fe-Mg pyroxenes. Cations in synthetic pyroxenes show a preference of Mn > Zn > Fe2+ >Co> Mg for the M2 site. T-0 distances in the Pbca orthopyroxenes indicate that Al concentrates in the TB tetrahedron. Examination of the chemistry of 175 naturally-occurr ing pyroxenes from a variety of lith­ ologies confirms complete solid solution between diopside and hedenbergite and extensive solid solution between enstatite and ferrosilite under crustal P-T conditions. In this limited set of samples the number of Ca atoms per formula unit does not exceed 1.0, which is consis­ tent with its occurrence only in the M2 site. The range in total AI between 0 and 1.0 is smaller than expected, and the maximum amount of Iv Al substitution is 55% of the T site occupancy. Na, when present as a jadeite or acmite component, is responsible for the highest non-quadri­ lateral contents of the pyroxenes examined. The most important substitutional couples in ter­ restrial Fe-Mg pyroxenes and augites are viFe3+ _,v AI and IvAI- v'Al. Detailed statistical analysis of 1200 high-quality pyroxene analyses from 11 planetary basalt suites revealed that the v'Ti-'v AI couple is one of the two most important couples for essentially all suites consid­ ered. Fe3+ is important in all of the terrestrial suites, but is virtually absent in the lunar and

Ҳали таржима қилинмаган