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Amblygonite-montebrasite series and lacroixite from Nagatare Li-Cs-Ta (LCT) pegmatite, Fukuoka Prefecture, Japan, were investigated by powder X-ray diffraction (XRD), electron microprobe analyses, and transmission electron microscope (TEM)/scanning transmission electron microscope (STEM) analyses. Scattered patchy or lamellar lacroixite was contained in montebrasite and amblygonite in all observed specimens. TEM/STEM observations revealed that the patchy and lamellar texture comprised lacroixite and low-fluorine montebrasite having same crystal orientations as that of host montebrasite and the boundaries corresponded to well-developed {110} planes. The observed microtexture was newly discovered, and it is an important evidence of the exsolution process. In XRD experiments conducted at high temperature, the unit-cell parameters of amblygonite were closer to that of monoclinic structures such as lacroixite with increasing temperature. Results suggested that scattered patches or lamellae of lacroixite were exsolution textures from a high-temperature phase. Montebrasite and amblygonite specimens from other localities involved varying textures corresponding to their occurrence. The amblygonite-montebrasite series from petalite-bearing pegmatite included low to high lacroixite contents and that from lower-temperature pegmatite with spodumene either did not possess or involved low lacroixite contents. Gem-quality montebrasite from drusy vugs formed at low temperature did not include any exsolution texture or lacroixite. The variety of texture of the amblygonite-montebrasite series indicated in this study generated new possibilities as the indicator of pegmatite-forming process.

DOI： 10.2138/am-2018-6049

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

Montebrasite and amblygonite in an Li-Cs-Ta enriched (LCT) pegmatite from Nagatare, Fukuoka Prefecture, Japan, contain various alteration minerals: fluorapatite, crandallite, goyazite, waylandite, wardite, viitaniemiite, morinite, muscovite, lepidolite, and cookeite. They are associated with lacroixite, quartz, and topaz. Among these minerals, wardite, viitaniemiite, morinite, and lacroixite are newly discovered in Japan. The secondary phosphates and fine-grained mica form fine veins along cleavages and composition planes of polysynthetic twins in the montebrasite and amblygonite. Lacroixite has a different texture from other phosphates, which suggests a possibility of exsolution within montebrasite-amblygonite series. Various secondary phosphates show Ca-, Na- and Sr-metasomatism with leaching of Li, and the formation of low-F montebrasite from montebrasite-amblygonite series indicates an F-OH exchange. However, fluorapatite, morinite, and viitaniemiite crystallized in an F-rich environment. Montebrasite-amblygonite series minerals undergo an acidic alteration to muscovite in the last stage, which is the same process that other Li minerals undergo, such as tourmaline and spodumene.

DOI： 10.2465/jmps.131022d

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN ASSOC MINERALOGICAL SCIENCES  

Namibite [Cu(BiO)(2)VO4(OH)] and hechtsbergite [Bi2O(VO4)(OH)] were found with clinobisvanite, waylandite, eulytite, beyerite and bismutite at the Nagatare mine, Fukuoka Prefecture, Japan. This assemblage occurs as secondary minerals and forms crusts and euhedral crystals associated with lepidolite, quartz, albite and cookeite. Namibite forms a dark green powder with pearly luster. Hechtsbergite is yellow, and is found as fine isolated crystals or aggregates. The compositions of namibite and hechtsbergite, determined by electron microprobe, are Cu0.98Bi1.92Al0.04O2 (V0.96Si0.07P0.02)O-4 (OH) and Bi1.94Al0.01O(V1.00Si0.04)O-4 (OH), respectively. The unit-cell parameters are a = 6.216(4), b = 7.384(6), c = 7.467(6) angstrom, alpha = 90.19(8), beta = 108.65(7), gamma = 107.36(8)degrees, V = 308.1(3) angstrom(3) for namibite, and a = 6.954(5), b = 7.539(8), c = 10.870(9) angstrom, beta = 106.87(5)degrees, V = 545.4(6) angstrom(3) for hechtsbergite.

DOI： 10.2465/jmps.121022d

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Tourmaline in a complex type granitic pegmatite of an Li-Cs-Ta enriched (LCT) family from Nagatare pegmatite, Fukuoka Prefecture, Japan, were classified into five types: type-A (black), type-B (indigo), type-C (light blue-pink), type-D (pink), and type-E (pink-light blue), on the basis of localities, associated minerals, color, and texture. Type-C shows color zoning textures of a light blue core and a pink rim. In contrast, type-E has a pink core and a light blue rim. A total of 29 samples were analyzed by an electron microprobe analyzer (EPMA) and an X-ray diffractometer (XRD). Significant substitution of (Li + Al) for Fe is observed at the Y sites. The minor cations at the Y site, such as Mg, Mn, Zn and Ti, were also characteristic on each type. Large amounts of Na were detected at the X site in type-B
the Na contents decreased with increasing (Li + Al) in types-C, D, and E. F contents were well correlated with X-site charge except in type-A. The mineral species from the Nagatare pegmatite were schorl, fluor-schorl, elbaite, fluor-elbaite, and rossmanite. The unit-cell parameters were in the range of rossmanite, elbaite, and schorl. The tourmaline from the Nagatare pegmatite has similar major chemical components but different minor elements such as Zn and Mn as compared to the tourmaline from other LCT pegmatites. The intermediate fractionated tourmaline containing Zn (0.00-0.20 apfu) and Mn (0.11-0.38 apfu) is characteristic of the Nagatare pegmatite. In type-E, the rim showed greater Fe and Mn contents than the core, differing from the trend of the melt development on types-B and C. Furthermore, the parts rich in Al, Li, and OH with considerable site vacancy were selectively replaced by muscovite. These features indicate that several stages of alterations occurred during the late stages of the pegmatite formation.

DOI： 10.2465/jmps.121022e

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A field-emission scanning electron microscope (FE-SEM) with energy-dispersive X-ray spectrometer (EDS) detector of a superconducting transition-edge sensor (TES) microcalorimeter is a new system for electron-microprobe chemical analyses. FE-SEM with TES was used for qualitative and semi-quantitative analyses of rare-earth elements (REE) at a low accelerating voltage of 5 kV. Four characteristic M-lines were detected in the LaB6 spectrum: LaM zeta at 640, LaM alpha beta at 841, LaM gamma at 1021, and a weak line (M2N4 transition) at 1100 eV. The spectra of other rare-earth borides, rare-earth phosphates, and monazite were assigned in the same way as the La M-lines were. For quantitative analyses, we used a calibration curve method, using standard specimens of known chemical compositions. Linear calibration curves for plots of P, Ca, La, Ce, Pr, and Nd intensities versus each weight percentage were obtained. Semi-quantitative analyses of rare-earth minerals should be carried out at low accelerating voltages using a calibration curve method. In a TES-EDS system, a low accelerating voltage can be used to improve the spatial resolution, without the sensitivity disadvantages of low-energy X-ray emissions. Moreover, a strong increase in the M alpha beta intensity with increasing atomic number Z was seen, so the detection limits of heavy REE was much lower than those of light REEs. These results suggest that the TES-EDS system could be a useful analytical tool in rare-earth mineralogy.

DOI： 10.2465/jmps.111020d

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Philipsburgite, (Cu,Zn)(5)Zn(AsO4,PO4)(2)(OH)(6)center dot H2O, an As analogue of kipushite, was discovered in the Yamato mine, Yamaguchi Prefecture, Japan. Philipsburgite occurs in the form of spherical aggregates, 0.5 mm in diameter, with bright green in color. It is associated with malachite and cornwallite in cavities of the oxidized rocks, which consist of quartz and goethite. Spherical philipsburgite consists of platy crystals, 100 mu m in length and 1 mu m in thickness, with sharp termination. The empirical formula is (Cu-4.91,Zn-0.09)(Sigma 5.00)Zn-1.00[(AsO4)(1.63)(PO4)(0.37)] (Sigma 2.00)(OH)(6)center dot 1.60H(2)O on the basis of O = 11 in the anhydrous part per formula unit and the water content was measured by thermogravimetric analysis. The composition is closest to the As end member thus far reported. The unit cell parameters from the X-ray diffraction data are a = 12.359(8), b = 9.247(7), c = 10.734(5) angstrom, beta = 97.22(5)degrees and V = 1216.9(9) angstrom(3).

DOI： 10.2465/jmps.101021e

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