Vimsite was found as a mass or a veinlet in crystalline limestone associated with gehlenite–spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Vimsite occurs as colorless to white aggregates of anhedral or prismatic transparent crystals up to 1 mm in length in association with shimazakiite, sibirskite, priceite, uralborite, calciborite, kurchatovite, and calcite. It is also formed as a 0.5 mm wide veinlet. An electron microprobe analysis of vimsite gave an empirical formula (Ca0.993Mg0.009Fe0.003Mn0.001)Σ1.006B1.996O2(OH)4 based on O = 6. The unit cell parameters are a = 10.021(3), b = 9.566(4), c = 4.447(2) Å, β = 91.231 (9)°. The calculated density is 2.523 g cm−3. The Vickers microhardness is 186–206 kg mm−2 (25 g load). It is likely that vimsite from the Fuka mine was formed by subsequent hydrothermal alteration, as with uralborite and priceite, of sibirskite produced by the hydrothermal alteration of shimazakiite.
The local structure around antimony (Sb) atoms in Cretaceous–Tertiary (K–T) boundary sediments from Stevns Klint in Denmark was studied using Sb K–edge X–ray absorption fine structure (XAFS) spectroscopy to obtain information about the chemical state and coordination environment. We also performed arsenic (As) K–edge XAFS measurements. The Sb K–edge X–ray absorption near edge structure (XANES) spectrum of K–T boundary sediments was compared with those of various kinds of reference Sb minerals, such as Sb sulfides and Sb5+ complex oxides, and soils containing ferrihydrite (schwertmannite), Sb2O3, and Sb2O5. The XANES pattern and threshold energy of K–T boundary sediments are similar to those of ferrihydrite (schwertmannite) soil samples. There is no chemical shift in the threshold energies among K–T boundary sediments, Sb5+ oxide complex minerals, and Sb2O5. In As XAFS analyses, the threshold energy of K–T boundary sediments is approximately similar to those of As5+ minerals, and the XANES pattern of K–T boundary sediments is almost similar to those of ferrihydrite (schwertmannite) soil samples. The oxidation states of Sb and As of K–T boundary sediments are estimated to be Sb5+ and As5+, respectively. Sb and As in K–T boundary sediments are coordinated with oxide ions, and Sb and As exist in the same local structure positions as Sb and As in ferrihydrite (schwertmannite). The XANES spectra and radial structure function for Sb atoms also showed that Sb in K–T boundary sediments is stored in a SbO6 octahedral coordination environment. The Sb–O interatomic distance in the K–T boundary sediments sample is 1.99(1) Å. Abundant ferric hydroxides occur in K–T boundary sediments. Sb is considered to be coprecipitated with As and Fe ions, and Sb and As in K–T boundary sediments are incorporated in low crystalline ferrihydrite (schwertmannite) throughout precipitation and sedimentation. The environment at K–T boundary sediments resembles that of soil contaminated by Sb and As in local areas at the present age. However, in an unusual environment, such as widely distributed K–T boundary sediments in the world, unusually high concentrations of Sb5+ and As5+ could become an index of the soiling of the global environment with dust and ashes derived from asteroid impact ejecta falls.
We examined the importance of tremolites as reservoirs of LILE (large–ion lithophile elements) in the mantle wedge. The Ochiai–Hokubo complex in the Suo metamorphic rocks of greenschist to blueschist facies, Japan, is mainly composed of depleted lherzolites, which commonly contain tremolite. The complex is concentrically zoned in terms of degree of hydration and metamorphic mineral assemblage, showing two mineral zones, tremolite + olivine and clinopyroxene + antigorite from the center outward. The inner tremolite–bearing lherzolite is enriched with some LILE such as Na and Cs, which are stored mainly in tremolites (with up to 4 wt% Na2O) and subordinately in chlorites. Relic primary clinopyroxenes are intact from the metasomatism. The fluids liberated from the high–P schists performed this modification on the lherzolites on progressive metamorphism, thus closely mimicking the mantle metasomatism by slab–derived fluids.
FeAlO3a) was synthesized under ultrahigh–temperature metamorphic conditions. The assemblage of FeAlO3, SiO2–rich melt and vapor was obtained from a mixture of Rundvågshetta sillimanite and reagent–grade Fe2O3 (weight ratio of 95:5) in Pt capsules at pressures of 5 and 9 kbar and a temperature of 1050 °C under hydrous conditions. No hematite was found in these runs. Corundum was a rare crystallization product in the 9 kbar experiment. We also produced FeAlO3, SiO2–rich melt and vapor in the sillimanite–Fe3O4 (weight ratio of 86:14) system in a AuPd capsule at 9 kbar and 1050 °C under hydrous condition. A crystal domain comprising FeAlO3, corundum, magnetite–hercynite spinel and ulvöspinel formed at the bottom part of the charge due to contamination by Ti from titanium oxide, probably rutile, included within the sillimanite. This domain was in contact with melt among sillimanite crystals. Our results suggest that FeAlO3 would be an index mineral for ultrahigh–temperature metamorphism of partially melted Fe– and Al–rich granulites generated under hydrous and oxidizing conditions.
a) FeAlO3 is used for both composition and phase in this paper.
Minakawaite, a new mineral with a RhSb composition, in association with a platinum–group mineral (PGM) placer is found from a small stream crossing the clinopyroxenite mass in serpentinite mélange of the Kurosegawa belt on the northeast side of Hikawa Dam, Haraigawa, Misato machi, Kumamoto Prefecture, Japan. Almost all PGM placer grains are based on isoferroplatinum, of which the rims are often covered by tulameenite and tetraferroplatinum. This isoferroplatinum–based grain contains small inclusions and accessories consisting mainly of osmium, erlichmanite, laurite, bowieite, cuprorhodsite, and ferhodsite–like mineral. Minakawaite occurs as the outmost surface layer with a rose gray metallic luster on the nub consisting of cuprorhodsite, ferhodsite–like mineral and/or Rh(Ge,Cu,Fe) mineral in association with an isoferroplatinum–based grain. The density of minakawaite is 10.04 g/cm3, calculated using the empirical formula and powder X–ray diffraction (XRD) data. Minakawaite has a pale gray color under the microscope in reflected light, and pleochroism is weak as a variation from pinkish pale gray to bluish pale gray. Anisotropy is moderate as reddish gray to bluish gray. Average results of ten energy dispersive X–ray spectroscopy (EDS) analyses give Rh 46.83, Sb 48.97, As 4.08 and total 99.88 wt%. The empirical formula is Rh0.998(Sb0.882As0.120)Σ1.002, based on 2 atoms per formula unit. Minakawaite is orthorhombic (Pnma) with a = 5.934(7) Å, b = 3.848(3) Å, c = 6.305(4) Å, and V = 144.0(2) Å3 (Z = 4). The seven strongest lines of minakawaite in the powder XRD pattern [d in Å(I /I0) (hkl )] are 2.860(63) (111), 2.774(35) (102), 2.250(47) (112), 2.199(100) (211), 2.162(38) (202), 1.923(49) (020), and 1.843(51) (013). Minakawaite is identical to the synthetic RhSb phase with MnP–type structure. PGM including minakawaite may occur with chromite in the magma chamber of the clinopyroxenite. Minakawaite was named in honor of Japanese mineralogist, Prof. Tetsuo Minakawa (b. 1950) of Ehime University for his outstanding contribution to descriptive mineralogy from Kyushu and Shikoku, Japan.
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