Chloritoid- and taramite-bearing basic schists occur on the streambed at Ikadadu in the garnet zone of the Sanbagawa metamorphic belt, central Shikoku. Their major constituent phases are chloritoid, taramite, chlorite, paragonite, phengite, epidote, ilmenite, titanite, albite and quartz. Chloritoid occurs only as inclusions in garnet. The appearance of chloritoid in the basic schists is strongly dependent on their restricted FeO-rich and high Al2O3/(CaO + Na2O + K2O) bulk-rock compositions. The metamorphic pressure (P) and temperature (T) conditions, 0.9 GPa and 505-515°C, are estimated by using equilibrium relationships among chloritoid, taramite, chlorite, paragonite, garnet and quartz. These P-T conditions are distinctly lower than those estimated for a chloritoid-bearing assemblage at the eclogite facies stage of pelitic schists in the Seba eclogite unit (1.7 GPa/525-555°C), 0.5-1 km north of the Ikadadu outcrop. This implies that there is a significant difference in the peak metamorphic pressure of the chloritoid-forming stages between the Ikadadu and Seba chloritoid-bearing schists, indicating the existence of a tectonic discontinuity between the two localities. Chloritoid and amphibole-bearing assemblages are stable at various metamorphic P-T conditions within suitable bulk rock compositions and their phase equilibria are helpful in discussing P-T conditions for various metamorphic rocks.
Unit-cell parameters, Raman scattering profiles and DTA curves were measured on monoclinic tridymite from fired silica brick. Ten peaks appear in the Raman measurements from 100 to 1250 cm−1 at 25°C, and their positions and widths show no significant change up to 110°C. The monoclinic unit-cell parameters, a, b, c and β, slightly increase or remain nearly constant with increasing temperatures up to 110°C. Both the Raman spectra and the unit-cell parameters undergo significant changes in the range from 110 to 115°C. Only two endothermic peaks, at 117 and 166°C, appear in the DTA curve measured on a powdered sample of nearly pure monoclinic phase in a heating run from 25 to 300°C. In X-ray single-crystal refinements, the structural parameters including anisotropic temperature factors of atoms were fully determined for the monoclinic phase at 25, 80, 90 and 100°C, and the orthorhombic phase at 140°C. The difference between the temperature factors on both sides of the transition point is the most remarkable: 0.0286 Å2 in average for all O atoms at 100°C in the monoclinic phase and 0.0713 Å2 at 140°C, both for Beq. The averaged Si-O distances calculated for the atomic mean positions decrease with increasing temperatures: 1.599 Å at 25°C, 1.595 Å at 100°C and 1.576 Å at 140°C, whereas the values corrected for the thermal displacement parameters remain unchanged at 1.61 Å through all the temperatures studied.
An association of jadeite and omphacite has been newly found in an eclogite-facies metaquartz diorite collected from the Orco Valley area, southern part of the Eclogitic Micaschist Complex of the Sesia Zone, Western Alps, Italy. Both the jadeite and omphacite occur as idiomorphic to subidiomorphic grains in the matrix, and some of them are in contact with each other with a sharp grain boundary. Most of the jadeite shows faint and irregular zoning, with a chemical variation of Xjd (AlVIjd /(Na + Ca)) = 0.75-0.90. Half of the omphacite grains is homogeneous, and the remainder shows various zoning patterns. Some omphacite grains exhibit prograde zoning with an increase in Xjd from 0.31 to 0.55 and a decrease in Xaeg (= Fe3+/(Na + Ca)) from 0.14 to 0.04 from the core to the rim. The rim composition of the zoned omphacite is similar to that of homogeneous omphacite (Xjd = 0.40-0.56). The average rim composition of the jadeite-omphacite pairs in direct contact shows an apparent miscibility gap between Xjd = 0.50 ± 0.06, Xaeg = 0.09 ± 0.03, and Xaug (= Ca/(Na + Ca)) = 0.41 ± 0.05 in omphacite, and Xjd = 0.79 ± 0.04, Xaeg = 0.08 ± 0.03, and Xaug = 0.13 ± 0.03 in jadeite. Application of Powell (1985) garnet-clinopyroxene geothermometer gives T = 470 ± 30°C at P = 12 kbar and application of Waters and Martin (1993) garnet-omphacite-phengite geobarometer gives P = 12.4 kbar at T = 440°C and P = 12.0 kbar at T = 500°C as peak metamorphic conditions. Composition data of the pyroxene P2/n and C2/c compositional fields obtained from both this study and the literature suggest that: (1) the shape of two miscibility gaps between jadeite and omphacite and between omphacite and augite in the jadeite-augite-aegirine phase diagram of Carpenter (1983) is more concordant with the natural data than that of Holland (1990), (2) the miscibility gap between omphacite and augite closes at T ∼ 500°C and P = 15 kbar, but the miscibility gap between jadeite and omphacite may still exist, and (3) the miscibility gap between jadeite and omphacite closes at T = 700-850°C and P =15-45 kbar.