Masahide Akasaka

An X-ray Rietveld study of piemontite on the join Ca2Al3Si3O12(OH) Ca2Mn3+3 Si3O12(OH) formed by hydrothermal synthesis

Abstract Mn3+ distribution at octahedral M1, M2, and M3 sites of piemontite and its effect on structural changes were investigated by X-ray Rietveld analysis of synthetic Ca2Al3-pMn3+pSi3O12(OH) piemontites. The material studied was synthesized from starting materials of Ca2Al3-qMn3+qSi3O12.5 + H2O in hydrothermal experiments with Pfluid of 200 and 350 MPa and temperature of 500 °C. Piemontites crystallized as single phases from q = 0.5, 0.75, 1.0, and 1.1 starting materials, whereas minor amounts of bixbyite and parawollastonite were associated with those synthesized from q = 1.5 and 1.75 starting materials. EPMA analyses of synthetic piemontites showed that the maximum Mn3+ content was 1.3(1) apfu. Site preference of Mn3+ at the octahedral sites is M3 > M1 >> M2. Mn3+ occupancies (g) at the M3 and M1 sites are correlated with the p-value in piemontite as gM3 = - 0.20p2 + 1.00p and gM1 = 0.23p2 - 0.06p, respectively, where 0.0 ≤ p ≤ 1.3. With increasing p-value, the mean M3-O and M1-O distances of piemontites increase, but the M1-O1 distance and the O5-Si3-O6 angle change nonlinearly due to the Jahn-Teller effect. The nonlinear variations of the a and c parameters with increasing p are caused by changes in the M1-O1 distance and the O5-Si3-O6 angle, respectively. The M3 octahedra, which are more distorted than the M1 octahedra, become more tetragonally compressed with increasing Mn3+ at the M3 site, due to the substantial increase of the M3-O1 distance.

57Fe Mössbauer study of pumpellyiteokhotskite-julgoldite series minerals

SummaryFe and Mn distribution in the pumpellyite group minerals (W8X4Y8Z12056-n(OH)n) has been studied by using57Fe Mössbauer spectroscopy. The studied Fe-pumpellyites, belonging to the pumpellyite-julgoldite series, were collected from two localities; metabasites in the Tokoro belt, Hokkaido, and gabbroic sills in the Shimane Peninsula, Japan. Okhotskite, an Mn3+-dominant pumpellyite group mineral, was separated from the ores of metamorphosed manganiferous iron ore deposits in the Tokoro belt.57Fe Mössbauer spectrum of Tokoro Fe-pumpellyite is composed of two Fe2+- and two Fe3+-doublets. On the basis of the single crystal structure refinements of Al-pumpellyites published so far, doublets were assigned to FeW2+(IS= 1.01 andQS = 2.73 mm/s), FeX2+ (IS = 0.97 andQS = 3.18 mm/s), FeX3+ (IS = 0.29 andQS =1.37 mm/s) and FeY3+ (IS = 0.36 andQS = 2.09 mm/s), whereIS is isomer shift relative to a metallic iron absorber andQS is quadrupole splitting. The Mössbauer spectrum of the Mitsu Fepumpellyite is composed of three doublets assigned to FeX2+ (IS= 1.14 andQS = 3.20 mm/s), FeX3+ (IS = 0.36 andQS =1.13 mm/s) and FeY3+(IS = 0.37 andQS= 1.93 mm/s). These assignments show strong preference of Fe3+ in the X-site. The Mössbauer spectrum of the okhotskite is composed of two doublets by FeX3+ (IS= 0.37 andQS = 1.13 mm/s) and FeY3+ (IS = 0.42 andQS = 2.18 mm/s). The area ratio shows that FeX3+:FeY3+ ratio is 94:6. On the basis of chemical and Mössbauer analyses, MnX3+:MnY3+ ratio is given as 19:81, indicating stronger Y-site preference of Mn3+ than Fe3+, what is consistent with Jahn-Teller theory. Al, Mn3+ and Fe3+ prefer the Y-site in this order.ZusammenfassungDie Fe- und Mn-Verteilung in Mineralen der Pumpellyit-Gruppe (W8X4Y8Z12O56-n(OH)n) wurde mittels Mössbauer-Spektroskopie studiert. Die untersuchten Fe-Pumpellyite der Pumpellyit-Julgoldit-Serie stammen von zwei verschiedenen japanischen Lokalitäten: von Metabasiten des Tokoro-Gürtels, Hokkaido, und von Gabbro-Sills der Shimane Halbinsel. Okhotskit, ein Mn3+-dominiertes Mineral der Pumpellyit-Gruppe, wurde aus Erzen einer Mn-führenden Eisenerzlagerstätte des Tokoro-Gürtels separiert. Das57Fe Mössbauer-Spektrum der Tokoro Fe-Pumpellyite zeigt zwei Fe2+- und zwei Fe3+-Doubletten. Auf Grund bisher publizierter verfeinerter Einkristall-Strukturuntersuchungen von Al-Pumpellyiten werden diese Doubletten folgendermaßen zugeordnet: FeW2+ (IS = 1.01 undQS = 2.73 mm/s), FeX2+(IS = 0.97 undQS = 3.18 mm/s), FeX3+ (IS = 0.29 undQS =1.37 mm/s) und FeY3+ (IS = 0.36 undQS = 2.09 mm/s).IS bezeichnet dabei die Isomer-Shift relativ zu einem metallischen Eisenabsorbenten,QS das Quadrupole-Splitting. Diese Zuordnungen belegen den bevorzugten Einbau von Fe3+ in die X-Position. Das Mössbauer-Spektrum von Okhotskit zeigt zwei Doubletten bei FeX3+ (IS = 0.37 undQS = 1.13 mm/s) und FeY3+ (IS = 0.42 undQS = 2.18 mm/s). Das Flächenverhältnis zeigt, daß das Verhältnis FeX3+:FeY3+ 94:6 ist. Auf Grund der chemischen und der Mössbauer-Analysen wird das MnX3+:MnY3+ Verhältnis mit 19:81 angegeben. Mn3+ zeigt somit eine stärkere Präferenz für die Y-Position als Fe3+, ein Resultat, das mit der Jahn-Teller-Theorie konsistent ist. Der bevorzugte Einbau in die Y-Position ist, in dieser Reihenfolge, Al>Mn3+>Fe3+.

Piemontite from the manganiferous hematite ore deposits in the Tokoro Belt, Hokkaido, Japan

SummaryPiemontites occur in manganiferous hematite ore deposits and radiolarian chest in the Nikoro Group, Tokoro Belt, eastern Hokkaido, Japan. The piemontite-bearing chest and ore bodies have suffered low-grade metamorphism of high pressure intermediate type. In ore bodies, piemontite forms veinlets with quartz and/or pumpellyite-(Mn2+) containing Mn3+ in Y site. In chest, piemontite occurs not only in veinlets but also in radiolarian tests with pumpellyite-(Mn2+). The mineral assemblages characterized by piemontite, pumpellyite-(Mn2+), okhotskite, hematite and bixbyite indicate that chest and ore deposits were metamorphosed under extremely highfO2 condition. Some piemontites in ores contain as much as 1.12 Mn3+, and the sum of Mn3+ and Fe3+ attains 1.46 per formula unit, whereas piemontites in chest contain less (Mn3+ + Fe3+). This difference in compositions may essentially be ascribed to the difference in the host rock compositions. On the other hand, Mn3+ and Fe3+ contents of piemontites in ores vary considerably by Al ⇌ (Mn3+, Fe3+) and Mn3+ ⇌ Fe3+ substitutions. This phenomenon may be interpreted in terms of the local availability of Mn3+ and Fe3+ in the host rocks.The low-temperature stability limit of piemontite is evaluated from the relations between piemontite and pumpellyite and from the estimated P-T conditions of piemontite crystallization in chert and ore deposits.ZusammenfassungPiemontite treten in manganführenden Hämatitlagerstätten und Radiolariten in der Nikoro-Gruppe des Tokoro-Gürtels, Ost-Hokkaido, Japan; auf. Die Piemontit-füh-renden Radiolarite und Erzkörper zeigen eine niedrig temperierte (Low-grade Bereich), Hochdruck (intermediate-type)-Metamorphose. In den Erzkörpern bildet Piemontit Gänge zusammen mit Quarz und/oder Mn3+ (in der Y-Position)-führendem Pumpellyit-(Mn2+). In den Radiolariten tritt Piemontit nicht nur in Gängen, sondern auch zusammen mit Pumpellyit-(Mn2+) in Radiolarien auf. Die Mineralparagenese Piemontit, Pumpellyit-(Mn2+), Okhotskit, Hämatit und Bixbyit deutet darauf hin, daß die Radiolarite und Erzlagerstätten unter hohenfO2-Bedingungen metamorphisiert worden sind.In den Erzkörpern enthalten einige Piemontite bis zu 1.12 Mn3+ und die Summe von Mn3+ und Fe3+ erreicht 1.46 pro Formeleinheit. Die Piemontite in den Radiolariten zeigen geringere Mn3+ + Fe3+ Gehalte. Diese Unterschiede in der Zusammensetzung sind auf die unterschiedlichen Trägergesteine zurückzuführen. Außerdem variieren die Mn3+ und Fe3+-Gehalte der Piemontite in den Erzkörpern deutlich auf Grund der Substitution von Al ⇌ (Mn3+, Fe3+) und Mn3+ ⇌ Fe3+. Dieses Phänomen kann durch die lokale Verfügbarkeit von Mn3+ und Fe3+ im Trägergestein interpretiert werden.Die niedrige Temperatur-Stabilität von Piemontit kann durch die Assoziation Piemontit-Pumpellyit und durch die bestimmten P-T-Bedingungen der Piemontit-Kristallisation in den Radiolariten und Erzlagerstätten abgeschätzt werden.

A crystal-chemical investigation of clinozoisite synthesized along the join Ca2Al3Si3O12(OH)-Ca2Al2CrSi3O12(OH)

Abstract Cr3+-bearing clinozoisite along the join Ca2Al3Si3O12(OH)-Ca2Al2Cr3+Si3O12(OH) was synthesized using cold-seal pressure vessels at PH₂O = 0.35 to 0.40 GPa and T = 500 °C and a piston-cylinder apparatus at PH₂O = 0.8 to 1.5 GPa and T = 500 to 800 °C. Gel-starting materials of Ca2Al3-qCr3+qSi3O12.5 composition with q = 1.00, 0.75, 0.50, and 0.25 were employed to maximize the yields of clinozoisite. Mass fractions of clinozoisite in the experimental products with q = 0.50, 0.75, and 1 were about 70 to 90% along with lesser amounts of eskolaite, garnet, and quartz. Clinozoisite crystallized from the gel with q = 0.25 was associated only with zoisite. The crystal structures of clinozoisite in four runs, containing 0.28, 0.49, 0.50, and 0.62 Cr apfu were refined using X-ray powder diffraction data and the Rietveld method. The amount of Cr3+ at the octahedral M3 and M1 sites ranged from 0.37(1)-0.16(1) to 0.25(1)-0.12(1) apfu, respectively. Corresponding KD = (Cr3+/Al)M1/(Cr3+/Al)M3 values range between 0.57 and 0.73. The M2 site contained only Al. The KD values, and published results for intracrystalline partitioning in epidote and piemontite, show that the preference of Cr3+ for M1 is stronger than that of Fe3+ and Mn3+ in spite of the fact that most Cr3+ is partitioned into M3. Unit-cell parameters of clinozoisite increase with increasing Cr3+. Variations in macroscopic unit-cell parameters can be related to variations in the local M3-Oi and M1-Oi distances.

Crystal chemistry of macfallite: Relationships to sursassite and pumpellyite

Abstract The crystal chemistry of macfallite from Keweenaw County, Michigan was studied using electron microprobe, thermogravimetry (TG), differential thermal analysis (DTA), powder Fourier transform infrared (FTIR) spectroscopy, and single-crystal X-ray diffraction methods. The chemical formula derived from the electron-microprobe measurements is (Ca2.03Na0.01)Σ2.04(Mn3+2.51Al0.27Mg0.09Cu2+0.03V3+0.01)Σ2.91Si3.05O10.88(OH)3.12 (Z = 2). An analysis using the intensities of the MnLβ and MnLα X-ray lines shows that most Mn is trivalent. The weight loss from TG measurement is 7.7 wt% at 1000 °C, most of which is interpreted to be due to the loss of structural OH groups. The crystal structure of macfallite [a = 8.959(3), b = 6.072(2), c = 10.218(4) Å, β = 110.75(3)°, space group P21/m], which is isostructural with sursassite, was refined using 1717 unique reflections to R = 4.1%. The site populations at the three independent octahedral sites, Mn1, Mn2, and Mn3, are Mn0.82Al0.06Mg0.09Cu0.03, Mn0.75Al0.25, and Mn0.95Al0.05, respectively. In agreement with a bond-valence analysis, three crystal-chemically different OH groups are located at the O6, O10, and O11 positions. The site O7 is mostly occupied by oxygen, but minor amounts of hydroxyl may be located there as well. The powder FTIR spectrum in the region of the OH-stretching vibrations is characterized by three strong bands at 3413, 3376, and 3239 cm-1 and an additional broad absorption band around 2900 cm-1. The latter results from a relatively strong hydrogen bond, O6-H···O11, with a length of ~2.63 Å. Although there are three main hydroxyl groups occurring in macfallite, the exact number depends on the concentration of trivalent and divalent cations at the Mn1 site. If divalent cations occur at Mn1, a fourth OH group is necessary to maintain charge balance.

X-ray Rietveld and 57Fe Mössbauer studies of epidote and piemontite on the join Ca2Al2Fe3+Si3O12(OH)–Ca2Al2Mn3+Si3O12(OH) formed by hydrothermal synthesis

Abstract Fe3+ and Mn3+ distributions on octahedral M1, M2, and M3 sites in synthetic epidote/piemontite from Ca2Al2Fe3+q Mn3+1-qSi3O12.5 starting material and their effects on the crystal structure were investigated using X-ray Rietveld and 57Fe Mössbauer methods. Epidote and piemontite were crystallized as almost single phases from q = 1.0, 0.75, 0.5, and 0.25 starting materials at Pfluid of 200-400 MPa and a temperature of 500 °C, using standard cold-seal pressure vessels. The Mn2O3-MnO2 buffer was used to produce fO₂ adequate to maintain Fe3+ and Mn3+. The Rietveld refinements converged to goodness-of-fit ranges from 1.21 to 1.60. At this temperature, site preferences of Σ(Fe3++Mn3+) for octahedral sites are M3>M1(>>M2). KD values of Σ(Fe3++Mn3+), where KD = [(Fe3++Mn3+)/Al]M1/[(Fe3++Mn3+)/Al]M3, (0.05-0.13) are similar to those of individual Mn3+ and Fe3+ vs. Al3+, respectively. However, the KD values of Fe3+ and Mn3+ for M1 and M3, where KD = (Fe3+/Mn3+)M1/(Fe3+/Mn3+)M3, vary with Fe3+ Total:Mn3+ Total ratios. In epidote with Fe3+ content larger than 0.4 atoms per formula unit (apfu) and Mn3+ < 0.6 apfu, Fe3+ has a stronger preference for M1 than Mn3+. In piemontite with 0.12 Fe3+ and 0.73-0.78 Mn3+ apfu, the preference of Mn3+ for M1 is greater than that of Fe3+. The site occupancies of individual Mn3+ and Fe3+ are governed by the individual KD values and the Mn3+ and Fe3+ concentrations in corresponding epidote and piemontite. Variations of the unit-cell parameters indicate the combined result of linear variation due to Al ↔ Fe3+ substitution and nonlinear variation due to Al ↔ Mn3+ substitution.

Sursassite: Hydrogen bonding, cation order, and pumpellyite intergrowth

Abstract The crystal chemistry of sursassite, simplified formula Mn2+2 Al3Si3O11(OH)3, from six different localities [(1) Falotta, Switzerland, (2) Woodstock, New Brunswick, Canada, (3) Kamisugai, Japan, (4) Kamogawa, Japan, (5) Molinello, Italy, and (6) Gambatesa, Italy] was studied using electron microprobe analysis (EMPA), Fourier transform infrared spectroscopy (FTIR), and single-crystal X-ray diffraction methods. The structure has two symmetry independent Mn sites. The Mn1 site is seven coordinated by O and hosts, in addition to Mn2+, up to 20% Ca, whereas Mn2 has octahedral coordination and is strongly selective for Mn2+. In the simplified formula, three smaller octahedral M sites are occupied by Al. However, M1 also accepts significant amounts of divalent cations, such as Cu, Mg, Fe, and Mn, whereas M2 is occupied exclusively by Al. The unit-cell parameters of sursassite are a = 8.698-8.728, b = 5.789-5.807, c = 9.778-9.812 Å, β = 108.879-109.060°, V = 465.7-470.0 Å3, the space group is P21/m. Structure refinements converged to R1 values of 2.15-6.62%. In agreement with bond-valence analyses, at least three OH groups, depending on the concentration of divalent cations at M1, are found at the O6, O7, and O11 positions. However, the bond-valence sum at O10 is always low, thus partial hydroxylation is assumed at O10 to maintain charge balance. Owing to the influence of divalent cations at M1 in sursassite the hydrogen-bond systems in sursassite and isostructural macfallite are different. The FTIR spectrum in the region of OH-stretching vibrations is characterized by three strong bands at 3511, 3262, and around 2950 cm-1, the latter being broad. The band at 2950 cm-1 is assigned to strong hydrogen bonds between O6 and O10 (O6···O10 = 2.66 Å). Residual difference-Fourier peaks in the refinement of the Kamogawa and Molinello (specimen 1) crystals indicated less than 5% pumpellyite intergrowth.

Occurrence of Fe3+ and formation process of precipitates within oxidized olivine phenocrysts in basalt lava from Kuroshima volcano, Goto islands, Nagasaki, Japan

Abstract The oxidation state of Fe and precipitates within olivine phenocrysts froman olivine-basalt from Kuroshima volcano, Goto Islands, Nagasaki Prefecture, Japan, were determined using electron microprobe analysis, 57Fe Mössbauer spectroscopy, Raman spectroscopy and transmission electron microscopy, to examine the formation process of the Fe-bearing precipitates. The average Fo content of the olivine phenocrysts is 76.2 mol.%. The olivine phenocrysts occasionally have precipitate minerals at their rims, especially on rims near vesicles. The 57Fe Mössbauer spectrum of olivine separates consists of two doublets assigned to Fe2+ at the octahedral M1 and M2 sites, and a Fe3+ doublet at the M1 and M2 sites. The Fe2+:Fe3+ ratio is 90(5):10(1). The precipitates at the rims of the olivine phenocrysts consist of magnetite and enstatite showing coaxial relations with host olivine, and grow parallel to the olivine c axis. Moreover, clusters consisting of nanoscale domains of a few tens of nm in size occur in the host olivine. Their rounded form and appearance in transmission electron microscope images are similar to those of the magnetite precipitates, but they have an olivine structure and can be regarded as embryos of magnetite within the olivine. The oxidation process of olivine phenocrysts under cooling conditions is: (1) formation of magnetite embryos on the rims of olivine phenocrysts; (2) formation of enstatite-like pyroxene domains by depletion of Fe in olivine due to the generation of magnetite embryos; (3) crystallization of magnetite and enstatite-like pyroxene precipitates.

Crystal chemistry of chromian pumpellyite from Osayama, Okayama Prefecture, Japan

Abstract The crystal structure and crystal chemistry of chromian pumpellyite from basic schist in the Osayama ultramafic body, Okayama, Japan, were investigated using electron probe microanalysis (EPMA), Fourier transform infrared spectroscopy (FTIR), and single-crystal X-ray diffraction to determine the distribution of chromium between two independent octahedral sites and structural changes caused by ionic substitutions in pumpellyite, VIIW8VIX4VIY8 IVZ12O56-n(OH)n (Z = 1). Five chromian pumpellyite crystals (ocp1211, ocp0604, ocp1028, ocp1013, and ocp1016) with 0.52, 1.65, 1.26, 1.94, and 1.43 Cr atoms per formula unit (apfu) (EPMA data), respectively, were picked from a hand specimen for X-ray diffraction analysis. The crystal structures (space group C2/m) of ocp1211 [a = 19.1129(6), b = 5.8963(5), c = 8.818(1) Å, β = 97.449(2)°], ocp0604 [a = 19.0935(4), b = 5.900(1), c = 8.810(2) Å, β = 97.540(7)°], ocp1028 [a = 19.105(2), b = 5.9021(6), c = 8.8143(7) Å, β = 97.513(3)°], ocp1013 [a = 19.1558(6), b = 5.9125(9), c = 8.844(1) Å, β = 97.448(4)°], and ocp1016 [a = 19.174(3), b = 5.9194(8), c = 8.830(1) Å, β = 97.497(4)°] were refined using 1058, 829, 1070, 1105, and 1095 unique reflections, and calculations converged at R factors of 4.08, 5.02, 6.32, 6.92, and 7.88%, respectively. The resulting site populations at the X and Y sites are (Mg1.88Al1.51Fe2+0.38 Cr0.16Mn2+0.05Ni0.02)X(Al7.90Ti0.07V0.03)Y for ocp1211, (Mg1.81Al1.53Cr0.42Fe2+0.18Mn2+0.04Ni0.01)X(Al7.34Cr0.65V0.01)Y for ocp0604, (Al1.62Mg1.60Cr0.61Fe2+ 0.13 Mn2+0.03Ni0.01)X(Al7.36Cr0.61V0.03)Y for ocp1028, (Mg1.79Al1.33Cr0.47 Fe2+0.33Mn2+0.08)X(Al6.66Cr1.31V0.03)Y for ocp1013, and (Mg1.94Al1.23Cr0.38Fe2+0.37Mn2+0.08)X(Al6.72Cr1.25V0.03)Y for ocp1016. Cr3+ ions in ocp1211 are distributed only in the X site. The distribution coefficient of Cr and Al between the X and Y sites [(Cr/Al)X/(Cr/Al)Y] is 1.66, 1.79, 3.09, and 4.54 for ocp1016, ocp1013, ocp0604, and ocp1028, respectively, indicating a stronger preference of Cr for the X site than the Y site. The a and b axes increase with increasing Cr content, whereas the c axis is almost constant. The mean Y-O distances increase linearly with increased Cr3+ content in the Y site. However, the mean X-O distances do not depend on the substitution of Cr3+ for Al3+ at the X site. The bond valence sums and the difference-Fourier synthesis indicate that hydroxyl groups are located at the O5, O7, O10, and O11 positions. FTIR spectrum shows main absorption bands at ca. 2911, 3220, 3397, and 3512 cm-1 of OH-stretching vibrations, indicating the presence of OH⋅⋅⋅O hydrogen bonds.

Clinopyroxene on the join CaMgSi2O6-CaFe3+AlSiO6-CaTiAl2O6 at low oxygen fugacity

AbstractSubsolidus phase relations on the join CaMgSi2O6-CaFe3+ AlSiO6-CaTiAl2O6 were studied by the ordinary quenching method at