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Hydroxyl-rich topaz was recognized in thick kyanite quartzites from both high-pressure (HP) and ultrahigh-pressure (UHP) belts of the Sulu terrane, China. These quartzites contain variable proportions of quartz, kyanite, and topaz, with minor phengite, pyrite, and futile. Some topaz grains from the UHP belt contain abundant inclusions of oriented kyanite, whereas those from the HP belt are partially replaced by woodhouseite [Ca[Al.sub.3](P[O.sub.4])(S[O.sub.4])[(OH).sub.6]]. Most topaz crystals contain 9.5 to 13.5 wt% F [0.92-1.30 atoms per formula unit (apfu)], indicating 35-55% substitution of F by OH. Such naturally occurring, hydroxyl-rich topaz has not been previously reported. Some topaz grains from the HP belt show distinct zoning: (1) decreasing F content from narrow cores (13.3-16.5 wt%) to thick rims (9.5 wt%) or (2) oscillatory zoning (9.44-12.77 wt%). Unit-cell parameters of topaz show a positive linear correlation between the OH content and a and b as well as volume. Based on our petrologic data, the experimentally determined curve of Ky + [H.sub.2]O = topaz-OH at very high pressures, and calculated OH/(OH + F) isopleths of topaz at low pressures, the topaz with [X.sub.OH] ~0.35 from the UHP belt may have formed at P-T conditions within the coesite stability field. The P-T condition of HP topaz is less well constrained; its high [X.sub.OH] (0.40-0.55) may have been caused by lower metamorphic temperatures and higher initial [X.sub.OH] in comparison with UHP topaz. Hydroxylrich topaz together with other hydrous minerals in UHP rocks may be important carriers of [H.sub.2]O to mantle depths of 100-200 km during continental subduction.

Ultrahigh-pressure (UHP) minerals of the Maowu mafic-ultramafic complex in the Dabieshan, east-central China exhibit many exsolution textures. Magnetite lamellae are common in olivines and Ti clinohumites from harzburgite and garnet pyroxenite. Monazite-(Ce) lamellae occur in apatites from the garnet pyroxenite. Independent P-T estimates suggest that these ultramafic rocks formed at P > 5 GPa and T = 700 [+ or -] 50 [degrees] C. The lamellae-bearing minerals are believed to preserve an earlier, higher P-T record prior to exsolution. Compositions and unit-cell parameters of the magnetite and host olivine and intergrowth relations were determined using a newly developed X-ray diffraction microprobe technique employing synchrotron radiation. The host olivine and magnetite lamellae bear a topotaxial relation with [[220].sub.Mag] || [[200].sub.Ol], [[111].sub.Mag] || [Mathematical Expression Omitted], [Mathematical Expression Omitted] || [[331].sub.Ol], [[242].sub.Mag] || [Mathematical Expression Omitted]. The recalculated composition of primary olivine may contain up to 1.5 wt% [Fe.sub.2][O.sub.3]. Four hypotheses may explain the observed intergrowths of oriented magnetite lamellae in olivine: (1) oxidation of olivine; (2) decomposition of [Fe.sup.3+]-bearing olivine formed at >6 GPa; (3) exsolution of a spinel (wadsleyite) solid-solution [Fe.sub.3][O.sub.4]-[(Fe,Mg).sub.2]Si[O.sub.4] during decompression; and (4) breakdown of phase A [[Mg.sub.7][Si.sub.2][(OH).sub.6]] + enstatite. The third hypothesis appears to be the most likely inasmuch as no additional silicate phase occurs as associated inclusions in the olivine host. However, the actual mechanism for exsolution of magnetite from olivine remains to be studied experimentally.