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Reaction rates, crystal growth rates and nucleation rates were examined for wollastonite + calcite + anorthite = grossular +CO(2), which will improve insights into porphyroblastic-rock garnet formation.

Coesite (the dense, high-pressure polymorph of quartz) occurs as inclusions in mechanically strong minerals in deeply subducted, metamorphosed crustal rocks in a number of Eurasian collisional orogens. It is the primary indicator mineral of ultrahigh-pressure (P) metamorphism. Whereas some coesite inclusions are untransformed, most exhibit partial transformation to palisade quartz and a concomitant increase in volume (resulting in rupture and radial fracturing of the host grain). Coesite can be identified by its diagnostic Raman spectrum; the strongest band (at atmospheric pressure, room temperature) is at 521 cm(super -l). Laser Raman microspectroscopic analyses of coesite inclusions within garnet and zircon in ultrahigh-P metamorphic rocks from Kazakhstan, Indonesia, and China reveal consistent differences in the Raman spectra of (1) partially transformed coesite + quartz (main Raman band at 521 cm(super -1)) and (2) untransformed monomineralic coesite grains (main band at 525-526 cm(super -1)). Applying the room-temperature calibration of pressure dependence of the coesite Raman spectrum, we conclude that the latter coesite inclusions are subject to a remarkable pressure differential of 19-23 kbar with the host grains, and are still undergoing pressure-temperature conditions on or close to the quartz-coesite equilibrium boundary.

Garnets of the Alpine Fault mylonites were notably fractured and a detailed study was conducted to explain fracture mechanisms andassociated retrogression. The Alpine Fault mylonites formed during progressive deformation that started at 5 Ma. The garnets grew during the Rangitata orogenyand were older than the Alpine Fault by more the 100 million years. Three fracture types were recognized: garnet fill, narrow retrogressive fill and wide, isolating islands of garnet, retrogressive fill. It was found that types two and three predominate over garnet filled fractures.

Unusual amounts of trace elements were shown by ion probe investigations of Al-Di pyroxenites from the Zabargad peridotite of Italy. This characteristic, not exhibited by pyroxenes in spinel websterite, indicates the presence of a precursor or mantle garnet and emphasizes the importance of metamorphic processes in the redistribution of trace elements through mantle upwelling. Thestudy also shows that the Zabargad peridotite had been formed prior to lithospheric rifting and thinning of the area.

Single crystals of tetragonal [Ca.sub.3][Mn.sub.2][Ge.sub.3][O.sub.12] garnet (space group symmetry [14.sub.1]/a, a = 12.3098(7) [Angstrom], c = 12.3277(9) [Angstrom]) were characterized by X-ray diffraction and transmission electron microscopy. Their structure is topologically isosymmetric to tetragonal high-pressure garnets such as majorite, displaying the same two distinct types of macroscopic twin mechanisms. Twinning occurs as pseudomerohedral ferroelastic twin lamellae with preferred orientation of the twin-domain boundaries parallel to [{101}.sub.tet], whereas merohedral ferrobielastic twin domains occur without any orientational preference. The crystal structure was determined from single-crystal X-ray diffraction data of a crystal fragment completely free of pseudo-merohedral twin domains. It shows two different Jahn-Teller distorted Mn[O.sub.6] octahedra, with a different orientation of the axis of polyhedral elongation. The ordering scheme of these Jahn-Teller distorted octahedra follows in an alternating pattern the densest rod packing, and the cooperative effect of the electronically induced octahedral distortion was found to be responsible for the cubic-to-tetragonal symmetry breaking in [Ca.sub.3][Mn.sub.2][Ge.sub.3][O.sub.12] garnet. The extent of polyhedral distortion indicates a partially dynamic character of the Jahn-Teller effect. The distortion of the tetrahedral T2 sites controls the cooperative effect of the lattice strain induced by the Jahn-Teller distortion of the [Mn.sup.3+] octahedra and thus is responsible for the overall lower symmetry.

The volumes of mixing of six garnet binaries in the four-component system almandine-pyrope-spessartine-grossular have been analyzed and compared. The almandine-pyrope join is thermodynamically ideal and the others show positive deviations from ideality. The magnitude of the excess volume of mixing of a given binary, as described by a symmetric mixing model, is shown to be a function of the volume difference between the two end-member components. The relationship between the magnitude of nonideality and volume difference can be described with a linear or quadratic function, but the latter has a physical basis in strain theory. Positive deviations from ideal thermodynamic mixing are primarily a result of strain resulting from the substitution of dodecahedral X-site cations of different sizes, but second-order chemical effects (e.g., electronic, coulombic) also appear to contribute to thermodynamic properties. XRD and IR spectroscopic measurements show that nonlinear behavior of microscopic properties, such as bond lengths and polyhedral distortions and volumes, across a solid solution is strong for pyrope-grossular garnets and very small to nonexistent for almandine-pyrope solid solutions. The magnitude of the deviations from linearity in the microscopic properties can be related to the excess volume of mixing for a given binary. A good correlation exists between microscopic structural strain and the macroscopic volume of mixing.

Granulite facies metamorphic xenoliths have been found in several Monteregian-age lamprophyric dikes at Ayer's Cliff, Zuebec, Canada. From geothermobarometric calculations, it is shown that middle to lower crustal metapelitic assemblages formed at 700-800 [degrees] C and 8-10 kbar, whereas mafic assemblages formed near 1200 [or + -]100 [degrees] C and 14 [or + -]1 kbar. The metapelitic assemblages are interpreted as Cambrian metasediments similar to those found farther south in Vermont. The mafic assemblages are interpreted as Grenville basement. The occurrence of these lithologies at Ayer's Cliff and in similar dikes in Vermont confirms the seismic interpretation of Grenville basement beneath a large part of the western Appalachians in southern Quebec and northern New England.