Åkermanite and nepheline found within melilite-olivine nephelinite in Nagahama Plateau, Hamada city, Shimane Prefecture, Southwest Japan, were investigated to characterize their occurrence and chemical compositions. Samples of massive and dense blue-gray melilite-olivine nephelinite were collected from a Naka-Uchida tunnel construction site. Their bulk composition is characterized by an extremely low SiO₂ content of 35.75 wt% and high Sr and Ba contents, reaching 2091 ppm and 958 ppm, respectively. Forsterite, åkermanite, augite, ulvöspinel and apatite occur as microphenocrysts. Nepheline occurs as an interstitial mineral between these microphenocrysts. Euhedral nepheline, zeolite-group minerals, diopside and calcite occur in druses.  
Åkermanite occurs as prismatic to anhedral microphenocrysts. The rims of the åkermanite microphenocrysts are dissolved and overgrown by augite. Åkermanite has an average composition of (K_0.011Na_0.390Ba_0.004Sr_0.031Ca_1.566)_Σ2.002(Mn_0.007Fe²⁺_0.088Mg_0.457Ni_0.001Fe³⁺_0.134Cr³⁺_0.001Ti_0.003Al_0.295)_Σ0.986(Al_0.074Si_1.926)_Σ2.000O₇, and contains up to 4.62 wt% Na₂O, 1.68 wt% SrO, and 0.64 wt% BaO. Calculated Fe₂O₃ and FeO based on charge balance are 5.36 wt% and 3.11 wt%, respectively. Fe²⁺/(Fe²⁺ + Fe³⁺) ratios estimated from the FeL_β/FeL_α intensity ratios range from 0.47 to 0.79, indicating that a considerable amount of Fe³⁺ is contained within åkermanite.  
The chemical composition of nepheline in the groundmass is (Na_0.655K_0.304)_Σ0.959(Fe³⁺_0.025Al_0.978)_Σ1.003(Al_0.012Si_0.988)_Σ1.000O₄ on average. No excess SiO₂ nor anorthite components were detected.  
The bulk composition of the Hamada melilite-olivine nephelinite and the compositions of åkermanite and nepheline are similar to those of equivalents from the carbonatite-nephelinite-phonolite Oldoinyo Lengai volcano in Tanzania. This similarity may support the proposed genetic model that the Hamada nephelinite magma was derived from a source material formed by carbonate metasomatism of a lherzolite upper mantle.

View full abstract

Corundum in direct contact with quartz and associated with andalusite has been found as an inclusion in alkali feldspar obtained from a leucosome, which forms a part of migmatized pelitic granulite from the Kerala Khondalite Belt, southern India. Thermodynamically, corundum + quartz is metastable relative to andalusite. Textural features suggest that it is a rare example of an arrested reaction corundum + quartz → andalusite which may have been triggered off by the introduction of H₂O-rich fluids along the cracks of the host alkali feldspar during the uplift and cooling of the leucosome. The metastable corundum + quartz association most likely originated during the crystallization of the Al₂O₃-rich leucosome melt when early corundum came into contact with later quartz under conditions with a relatively low H₂O activity.

View full abstract

Horomanite and samaniite are found in the interstices among grains of silicate minerals in lherzolite from the Horoman peridotite massif, Samani-cho, Hokkaido, Japan. The mean analytical data for the horomanite, as determined by electron-probe micro-analysis (EPMA), are Cu: 0.43, Fe: 41.82, Ni: 23.76, Co: 0.52 and S: 33.29 for a total of 99.82 wt%. The empirical formula is (Fe_5.77Ni_3.12Co_0.07Cu_0.05)_Σ9.01S_8.00. Crystallographic data for the horomanite acquired by X-ray single-crystal (precession) and powder (Gandolfi) diffraction methods. They revealed a tetragonal symmetry, space group P4/mmm, a = 8.707 Å, c = 10.439 Å, V = 791.4 ų, and Z = 4.
The mean chemical composition of samaniite obtained by EPMA is Cu: 16.90, Fe: 34.60, Ni: 15.48, Co: 0.16, and S: 32.87 for a total of 100.01 wt%. The empirical formula is Cu_2.08(Fe_4.84Ni_2.06Co_0.02)_Σ6.92S_8.00. The crystallographic data for samaniite, which were obtained by X-ray single-crystal and powder-diffraction methods, are tetragonal symmetry, space group P4₂/mnm, a = 10.089, c = 10.402Å, V = 1058.9Å3, and Z = 4.

View full abstract

Several hydrous silicates with similar chemical formulae and related crystal structures form under low-grade metamorphism and hydrothermal activity. Among them, epidote and pumpellyite are the most important minerals because of their common occurrence. Moreover, sursassite and iso-structural macfallite were also categorized as structurally related to pumpellyite. Their cation distributions and structural features are similar to those in epidote and pumpellyite. The following subjects are reviewed: 1) the topological relation of crystal structures among these minerals; 2) The cation distributions among the octahedral sites in epidote, pumpellyite, sursassite and macfallite and their effect on the structural variations; 3) the relationship between the oxidation state of transition elements and the hydrogen bonding systems in these hydrous silicates. Special attention was paid to the variety of hydrogen-bond systems with the oxidation states of transition elements, Me²⁺ + OH^- ↔ Me³⁺ + O^2-, commonly occurring in the pumpellyite, sursassite and macfallite structures. A model of the structural relationship among pumpellyite, sursassite, macfallite and epidote is proposed from a new stand point.

View full abstract

Edited by the Abstracting Committee, Japan Association of Mineralogical Sciences

Rock-Forming Minerals and Petrology
Mantle and Lower Crust 106060—106067;
Igneous Rocks 106068—106081

View full abstract