Mihoko Hoshino

The chemistry of allanite from the Daibosatsu Pass, Yamanashi, Japan

Abstract The crystal structure of allanite from granitic pegmatite, the Daibosatsu Pass, Yamanashi, Japan, has been refined under the constraint of chemical composition determined by electron microprobe analysis of rare earth elements. Back-scattered-electron images and X-ray element maps of the allanites show that each of their crystal grains has chemically homogeneous distribution of major elements. A typical formula for the chemistry is: (Ca0.920 ⃞0.080)∑1.000(La0.238Ce0.443Pr0.048Nd0.100Sm0.019Th0.042Mn0.008 ⃞0.102)∑1.000(Al0.607Fe3+0.317Ti0.076)∑1.000(Al1.000)(Fe2+0.543Fe3+0.365Mn0.055Mg0.037)∑1.000(SiO4)(Si2O7)O(OH). The crystal structure of allanite, monoclinic, a 8.905 (1), b 5.7606 (5), c 10.123 (1) Å, β 114.78°(1), space group P21/m, Z = 2, has been refined to an unweighted R factor of 3.46% for 1459 observed reflections. Although the H atom position was not determined on the Difference-Fourier map, inspection of the bond valence sums demonstrates that the H atom is uniquely located at the O10 atom and involved in a hydrogen bond to O4. A systematic examination as to crystal chemistry of allanites suggests that the isolated SiO4 tetrahedron has the largest distortion of three kinds of the tetrahedron containing Si2O7 groups in the allanite structure. This observation is common to the epidote group minerals, while the larger distortion of A2 sites caused by occupancy by REE in allanites contrasts with the smaller one of A sites in other epidote group minerals. In the allanite groups the bond angles between the O10−H bond and hydrogen bond H…O4 are found to range from 170 to 180°. Compilation of the chemical compositions of the title allanite and the others from granitic rocks, Japan, which reveals Th-incorporation as the coupled substitution of 3Th4+ + ⃞ (vacancy) ⇌ 4REE3+, provides an explanation for the observation that higher Th concentrations characterize allanites from the island arcs. The ternary Al2O3-Fe2O3-∑REE diagram illustrates that allanites are grouped, according to their origins, into three classes suggestive of tectonic backgrounds for the crystallization localities; (1) intracontinental, (2) island arc and (3) continental margin.

Crystal chemistry of zircon from granitic rocks, Japan: genetic implications of HREE, U and Th enrichment

Zircon from granitic rocks, Japan is classified into two major types; based on the minor element content determined by electron microprobe, a heavy rare earth element (HREE)-U-Thpoor (Type I) and a HREE-U-Th-rich (Type 2). Zircons characteristically occurring in common granites of type-1 are relatively poor in HREE including Y and Sc, up to 2.72 wt% HREE2O3; up to 0.44 wt% ThO2; up to 2.10 wt% UO2, while zircons characteristically occurring in granitic pegmatites of type-2 are much richer in HREE including Y and Sc, up to 18.99 wt% HREE2O3; up to 11.09 wt% UO2; and up to 6.58 wt% ThO2 However, analyses showing a significantly high amount of REE+Y almost certainly represent altered zircon and/or accidental analysis of inclusions (Hoskin & Schaltegger 2003). Only crystal structure of zircon free of inclusions and zoning (type-2; Takenouchi granitic pegmatite) was refined to R = 4.3 % and for the first time has been confirmed that zircons of type-2 exist as a single-crystal phase in nature. .

Waimirite-(Y), orthorhombic YF3, a new mineral from the Pitinga mine, Presidente Figueiredo, Amazonas, Brazil and from Jabal Tawlah, Saudi Arabia: description and crystal structure

Abstract Waimirite-(Y) (IMA 2013-108), orthorhombic YF3, occurs associated with halloysite, in hydrothermal veins (up to 30 mm thick) cross-cutting the albite-enriched facies of the A-type Madeira granite (~1820 Ma), at the Pitinga mine, Presidente Figueiredo Co., Amazonas State, Brazil. Minerals in the granite are ‘K-feldspar’, albite, quartz, riebeckite, ‘biotite’, muscovite, cryolite, zircon, polylithionite, cassiterite, pyrochlore-group minerals, ‘columbite’, thorite, native lead, hematite, galena, fluorite, xenotime-(Y), gagarinite-(Y), fluocerite-(Ce), genthelvite–helvite, topaz, ‘illite’, kaolinite and ‘chlorite’. The mineral occurs as massive aggregates of platy crystals up to ~1 μm in size. Forms are not determined, but synthetic YF3 displays pinacoids, prisms and bipyramids. Colour: pale pink. Streak: white. Lustre: non-metallic. Transparent to translucent. Density (calc.) = 5.586 g/cm3 using the empirical formula. Waimirite-(Y) is biaxial, mean n = 1.54-1.56. The chemical composition is (average of 24 wavelength dispersive spectroscopy mode electron microprobe analyses, O calculated for charge balance): F 29.27, Ca 0.83, Y 37.25, La 0.19, Ce 0.30, Pr 0.15, Nd 0.65, Sm 0.74, Gd 1.86, Tb 0.78, Dy 8.06, Ho 1.85, Er 6.38, Tm 1.00, Yb 5.52, Lu 0.65, O (2.05), total (97.53) wt.%. The empirical formula (based on 1 cation) is (Y0.69Dy0.08Er0.06Yb0.05Ca0.03Gd0.02Ho0.02Nd0.01Sm0.01Tb0.01Tm0.01Lu0.01)∑1.00[F2.54⃞0.25O0.21]∑3.00. Orthorhombic, Pnma, a = 6.386(1), b = 6.877(1), c = 4.401(1) Å, V = 193.28(7) Å3, Z = 4 (powder data). Powder X-ray diffraction (XRD) data [d in Å (I) (hkl)]: 3.707 (26) (011), 3.623 (78) (101), 3.438 (99) (020), 3.205 (100) (111), 2.894 (59) (210), 1.937 (33) (131), 1.916 (24) (301), 1.862 (27) (230). The name is for the Waimiri-Atroari Indian people of Roraima and Amazonas. A second occurrence of waimirite-(Y) is described from the hydrothermally altered quartz-rich microgranite at Jabal Tawlah, Saudi Arabia. Electron microprobe analyses gave the empirical formula (Y0.79Dy0.08Er0.05Gd0.03Ho0.02Tb0.01 Tm0.01Yb0.01)∑1.00[F2.85O0.08⃞0.07]∑3.00. The crystal structure was determined with a single crystal from Saudi Arabia. Unit-cell parameters refined from single-crystal XRD data are a = 6.38270(12), b = 6.86727(12), c = 4.39168(8) Å, V = 192.495(6) Å3, Z = 4. The refinement converged to R1 = 0.0173 and wR2 = 0.0388 for 193 independent reflections. Waimirite-(Y) is isomorphous with synthetic SmF3, HoF3 and YbF3. The Y atom forms a 9-coordinated YF9 tricapped trigonal prism in the crystal structure. The substitution of Y for Dy, as well as for other lanthanoids, causes no notable deviations in the crystallographic values, such as unit-cell parameters and interatomic distances, from those of pure YF3.

Single crystal X-ray and electron microprobe study of Al/Si-disordered anorthite with low content of albite

Abstract The crystal chemistry of anorthite with low content of albite (An92.0 Ab3.4), part of a rapidly cooled anorthite megacryst occurring in 1940 ejecta from Miyake-jima volcano, Japan, has been investigated using a single-crystal X-ray diffractometer and an electron microprobe analyzer with wavelength dispersive X-ray spectroscopy (EMPA-WDS). The structure was refined in space group P1̅ and cell parameters, a=8.182(6) Å, b=12.883(4) Å, c=7.092(4) Å, α=93.19(4)°, β=115.91°(4), γ=91.18°(4). The final weighted R-factor is 3.77% for 1549 reflections. Averaged T-O distances are 1.681 Å for T1(0), 1.674 Å for T1(m), 1.677 Å for T2(0) and 1.680 Å for T2(m), indicating Al occupancies of 0.501, 0.453, 0.472, and 0.496, respectively. These results suggest that the Al/Si-distribution in the tetrahedral framework is highly disordered (QOD=0.06), which results in bisection of the c-period in Al/Si ordered anorthites (c∼14 Å). Chemical composition of the refined crystal obtained by EMPA-WDS is (Ca0.93 Na0.03 Fe0.02□0.01)(Mg0.01 Al1.94 Si2.05)O8. The extra-framework site-populations consist of the following two: (1) A(000) site occupied by Ca (86%), Na (8%), Fe2+ (4%) and □(2%), and (2) A(zi0) site by Ca (100%). The Al/Si tetrahedral framework is hence pseudo-face-centered in symmetry (C1ʩ so that both extra-framework cations and their defects appear to reduce the symmetry of the overall structure to P1ʮ Although the Al/Si disordered anorthite can be interpreted as a metastable phase, the observed chemical non-stoichiometry may stabilize such a metastable structure by introducing minor -Si4+-O-Si4+- bonds into the tetrahedral framework and anti-phase-boundary.

First report of natural oxyallanite: oxidation and dehydration during welding of volcanic tuff

The oxidation of Feto Fe, the release of H2, and the concomitant replacement of OH by O would produce an oxy-equivalent of allanite (CaREEAl2FeSi3O11O(OH)) (Dollase 1973). The reaction Fe + OH↔Fe+ O+ 1/2H2 is formally equivalent to the oxy-reaction observed in other hydrous Fe -bearing silicate minerals, such as mica and amphiboles (e.g., Hogg & Meads 1975; Ferrow 1987). However, occurrence of oxyallanite (CaREEAl2FeSi3O11O(O)) in natural environment has never been reported. The purpose of the preset study is to discuss occurrence of natural oxyallanite. Chemical compositions and crystal structures of the allanites from two rhyolitic rocks--(1) Youngest welded tuff from Toba (YTT), Sumatra, Indonesia and (2) volcanic ash from SK100 (SK100-VAB), Niigata, Japan--were determined by electron microprobe analysis and single crystal diffractometer, respectively. Despite the close similarity, in chemical composition, between YTT allalnites and SK100 ones, their unit-cell parameters are distinct from each other. The former have shorter b axis and longer c axis and larger β value in comparison with cell parameters of the latter. FT-IR analysis shows that YTT allanites have both the smaller OH-absorption band area and the shift of its bands to higher wavenumbers as compared to SK100 ones. Welding of the ash flow tuff including the allanites preformed in Youngest Toba welded tuff would cause them to undergo oxidations, dehydration and replacement. The sequential reaction would result in producing the present YTT allanite, namely oxyallanite. Although oxyallanite was only obtained by heating the natural allanite (e.g., Armbruster et al. 2006), this study first reports that oxyallanite may commonly occur in welded rocks at high temperatures.

Fe-kaolinite in granite saprolite beneath sedimentary kaolin deposits: A mode of Fe substitution for Al in kaolinite

Abstract Fe-kaolinite has been detected in granite saprolite beneath sedimentary kaolin deposits in the Seto district of central Japan. Granite saprolite, which was found underneath sedimentary kaolin deposits formed in fluvial and lacustrine environments, had been subjected to kaolinization. The clay fractions of granite saprolite consist mostly of kaolinite with subordinate micaceous clay, quartz, and feldspars. Electron probe microanalysis (EPMA) showed that the kaolinite in clay fractions contained an average 3.30–3.72 wt% of Fe2O3, indicative of Fe-kaolinite. Fe+Si was inversely proportional to Al in Fe-kaolinite, indicating coupled substitution between Fe+Si and Al. The K2O contents of Fe-kaolinite increased with increasing Fe2O3 up to 0.77 wt%, whereas K did not correlate with other elements, suggesting that K was not contained with the structure of kaolinite but was present in its interlayers. X-ray absorption near-edge structure (XANES) spectroscopy showed that about 60 to 70% of Fe in the clay fractions is ferric iron, and extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that Fe is situated in octahedral sites replacing Al. Fe-kaolinite was likely precipitated by the infiltration of acidic groundwater with higher Fe and alkali contents into granite saprolite, accompanied by the intense kaolinization of sedimentary kaolin deposits.