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Three end-member clinopyroxenes from the NaM13+Si2O6 series (M1 = Al, Fe and Ga) have been examined by single-crystal X-ray diffraction at pressures up to 11 GPa. NaGaSi2O6 was also examined with Raman spectroscopy to 16.5 GPa. NaAlSi2O6 (jadeite) and NaFeSi2O6 (aegirine) are naturally occurring minerals. NaGaSi2O6 is synthetic. Various characteristics of each of the three clinopyroxenes as a function of pressure are reported, including bulk moduli (K0), unit cell volumes, atomic positions, and bond lengths.The compressibilities of a selection of clino- and orthopyroxenes from the literature were examined and considered in terms of M2-O3 bonding and unit cell volumes. As predicted by previous workers, pyroxene compressibilities generally correlate with unit cell volumes at ambient conditions. Compressibilities are also found, however, to be significantly affected by the relationship of M2-O3 bonds with the sense of rotation of silica tetrahedra upon compression. Two such relationships are observed: sympathetic, where the corner of the SiO4 tetrahedron tilts toward M2, and antipathetic, where the corner of the tetrahedron tilts away from M2. All interatomic separations in pyroxenes decrease with pressure, but sympathetic-type separations decrease more than expected based on isotropic scaling of the unit cell. Pyroxene structures may have one of several M2-O3 bond configurations: none, one, two or four bonds, and none, only sympathetic, only antipathetic, or a mixture of both types of bonds. Structures with antipathetic bonds are significantly stiffer than structures without, all else constant. The sympathetic/antipathetic bond hypothesis represents a new, previously unrecognized, first-order control on pyroxene compressibility.M1 size controls ambient unit cell volumes of clinopyroxenes. However, M1 size does not correlate well with pyroxene bulk moduli. Applying the idea of sympathetic and antipathetic M2-O3 bonding, much of the dispersion in a plot of M1 cation size versus bulk modulus can be explained. The three NaM13+Si2O6 clinopyroxenes examined in this study exhibit very similar behavior under compression. All show signs of approaching a C2/c -> C2/c phase transition at ~20 GPa. All exhibit unit strain ellipsoids with similar orientations and dimensions. All have identical bond topologies and bulk moduli that correlate with their ambient unit cell volumes.

Euhedral clinopyroxene mega-crystals have been retrieved from Marion Island, a volcanic island situated on an inactive transform fault near the mid oceanic ridge on Antarctic tectonic plate and part of the Prince Edward Island group. The island is considered to be the product of hotspot- related volcanism. Clinopyroxene megacrysts were sampled from the southern side of the island on a scoria cone named Pyroxene Hill. Several analytical methods including Energy Dispersive Spectrometry (EDS), Electron Probe Microanalysis (EPMA) and EPMA-mapping were utilized in investigating the zoning found within these 5-10 cm large crystals. The zoning found was “patchy”, in that it did not conform to commonly described zoning such as normal, reverse, sector or oscillatory, but rather consisted of chemically distinct areas with either diffuse or sharp boundaries, not orientated parallel to the grain boundaries or crystal lattice. The chemistry of the crystals indicates that they have formed from an evolved basaltic melt, and are likely to have crystallised at a depth of 15- 30 km. A model is postulated for the formation of these crystals in which the megacrysts crystallise rapidly from a supersaturated melt. Pre-existing crystalline material undergoes imperfect diffusion at high temperature to create a patchwork of compositional zones. Supersaturation likely requires a volatile-rich melt, which undergoes rapid degassing owing to an external trigger. The presence of a transform fault directly below Marion Island may provide a seismic trigger for such a degassing event. / Dissertation (MSc)--University of Pretoria, 2013. / Geology / unrestricted