A variety of anatectic migmatites occur in high-grade zones of a Mesozoic high-T/P typemetamorphic belt — the Higo metamorphic belt, Kyushu, Japan. This paper deals with two major lithotypes of the migmatites — metatexite and diatexite — and attempts to characterize them in terms of field occurrence, texture, mineralogy and whole-rock chemistry. The metatexite is a layered migmatite that is composed of leucosome and melanosome with metamorphic fabrics, while diatexite is a massive and more homogeneous type and has plutonic igneous textures. Both types of migmatite occur in a high grade garnet-cordierite zone (D zone) and an even higher grade garnet-hypersthene zone (E zone) of the Higo metamorphic belt, and they are both considered to represent the products of partial melting of pelitic and psammitic gneisses. The diatexite occurs in masses, sporadically, in the matrices of metatexite both in the D zone and E zone, and it does not appear to increase in abundance with the increase of metamorphic grade. Another important finding is that no systematic compositional difference has been detected between the metatexite and the diatexite despite their textural and structural differences, if compared within the same D zone. Treating the metatexite and diatexite together, it was shown that the E-zone migmatites are chemically and mineralogically more depleted than the D-zone migmatites, which may imply that the former represents the refractory residues of partial melting. Some leucogranites, which form veins, dikes and pods in metamorphic rocks, have compositions of anatectic melts and are considered to be compositionally complementary to the residual E-zone migmatites. Although the D-zone rocks must also have been partially molten at the peak of metamorphism, melt segregation probably did not take place effectively there, and, as a whole, they largely retain their original metasedimentary composition. The factor controlling the structural types of the migmatite is also discussed.

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Manganese carbonate-silicate rocks in the Sausar Group, India are often traversed by pegmatite veins. Nambulite, a lithium- and sodium-bearing hydrous manganosilicate, is formed by the reaction of a hydrothermal solution with the host mineral, rhodonite. Despite the similarity of natronambulite and nambulite, the chemical composition shows a variation in the concentration of Na⁺ in the empirical formula: (Li_0.94-0.60Na_0.06-0.40)_Σ1.00(Mn_3.57-3.82Ca_0.13-0.07Mg_0.36-0.07Fe_0.03-0.01)_Σ3.96-3.86Si_5.01-4.98O₁₄(OH)_1.00. This is the first report of the sodium-rich variety from India. Powder X-ray diffraction studies reveal the unit cell parameters of nambulite in the triclinic system as a =7.55 ± 0.06 Å, b = 11.77 ± 0.07 Å, c = 6.68 ± 0.04 Å, α = 91.3° ± 0.8°, β = 95.4° ± 0.5°, γ =107.2° ± 0.7° and V= 563 ± 6 ų.

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Single-crystal X-ray experiments have been performed for calcite, calcite II, and calcite III at 300 K and pressures up to 4.1 GPa. The volume decrease from calcite to calcite II is estimated to be 0.54 cm³/mol (1.5 %) at 1.5 GPa, which compares well with the value 0.48 cm³/mol in the calcite to calcite II transition at 1.4 GPa reported previously based on piston-cylinder volume-compression experiments. The crystal data of calcite III have been determined using single-crystal X-ray analyses at 300 K and pressures between 2.3 and 4.1 GPa. The calcite III crystal is found to be monoclinic with the space group either C2, Cm, or C2/m from the systematic absence of reflections, and with the refined cell parameters of a = 8.707(7), b = 9.315(6), c = 6.154(4) Å, and β = 115.18(2)° at 2.3 GPa. These crystal data of calcite III at 2.3 GPa yield a density of 2.94 g/cm³ for Z = 8, which is much less than the previously estimated densities, ranging from 3.11 to 3.28 g/cm³ at pressures from 1.9 to 2.9 GPa, using powder X-ray analyses based on orthorhombic structural models for calcite III, but is generally consistent with the reported data by the piston-cylinder experiments. Thus the present study reveals the density of calcite III is substantially less than that of aragonite (3.02 g/cm³ at 2.3 GPa), indicating that calcite III is metastable with respect to aragonite at high pressures.

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