Petr Cerny

Niobian rutile from the McGuire granitic pegmatite, Park County, Colorado; solid solution, exsolution, and oxidation

Granitic pegmatites potentially record the history of late magmatic fluids, and complex pegmatites can offer insight into the geochemical transport both of “immobile” (Nb, Ta) and volatile (F, Cl, B) components. Tracing the evolution of fluids in pegmatites therefore has been a significant focus of research. The goals of this reconnaissance study of fluid inclusions in a complex, rare-element pegmatite were to determine how much of the fluid history was preserved, to estimate the conditions under which fluids were trapped, and to constrain the minimum conditions of pegmatite formation. Among the pegmatites in central Virginia, Morefield is the only one that is actively mined (Sweet and Penick 1986). This study focused on Morefield because outstanding exposure of the pegmatite offered unique sampling control, and prior work provided a good geological foundation (Brown 1962; Geehan 1953; Jahns et al. 1952; Lemke et al. 1952). At the same time, most work on pegmatites in this area concerned their mineralogy (Glass 1935; Kearns 1992a, 1992b, 1993a, 1993b; Mitchell and McGavock 1960; Sinkankas 1968; Smerekanicz et al. 1991), whereas our emphasis is on petrogenesis. Accordingly, we attempt to describe the evolution of fluids associated with the pegmatite. ABSTRACT

Compositional and structural systematics of the columbite group

The kinetics of homogeneous reactions are important in understanding the cooling history of rocks and in understanding experimental speciation data. We have experimentally studied the kinetics of the interconversion reaction between H20 molecules and OH groups in natural rhyolitic glasses (0.5-2.3% total water) and a synthetic albitic glass (l % total water) at 400-600 0c. The reaction rate increases with temperature and total water content. Equilibrium is not always approached monotonically; the speciation may first depart from equilibrium and then come back to equilibrium. Experimental reaction rates agree with those inferred from previous speciation data of rhyolitic glasses quenched from 850°C. The experimental data are modeled successfully by considering both the reaction and the diffusion of OH that brings OH groups together to react. This study shows that species concentrations in glasses quenched from ::;600 °C reflect those at experimental temperatures unless the water content is higher than that used in the present study. Species concentrations in glasses with total water contents ~0.8 wt% and which were rapidly quenched in water from 850°C do not represent their equilibrium concentrations in the melt at 850 °C, but record a lower apparent equilibrium temperature that depends on water content and quench rate. Natural rhyolitic glasses and glass inclusions do not record preeruptive melt speciation, though total water content may be conserved. The experimental data are used to infer cooling rates for natural obsidian glasses. Pyroclastic glass fragments from the bb site of Mono Craters have cooling rates similar to air-cooled experimental charges (3 °C/s). Different types of glasses from the Mono Craters have different cooling rates, which cover four orders of magnitude. Some natural obsidians appear to have had complex cooling histories. The wide range of cooling rates and thermal histories is consistent with previous inferences that some obsidian clasts at the Mono Craters formed as glass selvages lining volcanic conduits or dikes that were subsequently caught up in the explosive eruption, which led to variable degrees of transient heating followed by rapid cooling and deposition. These experimental data reveal surprisingly rich detail in water speciation in volcanic glasses and show how, at least in principle, quantitative constraints on thermal histories can be extracted by experimentation and application of kinetic models.

Rossmanite, ⃞(LiAl2)Al6(Si6O18)(BO3)3(OH)4, a new alkali-deficient tourmaline: Description and crystal structure

Abstract Rossmanite is a new tourmaline species from near Rožná, western Moravia, Czech Republic. It forms pale pink columnar crystals about 25 mm long and 5 mm thick, elongaten along c with striations parallel to c on the prism faces. It is brittle, H= 7, Dmeas = 3.00 g/cm3, Dcalc = 3.06 g/cm3. In plane-polarized light, it is colorless. Rossmanite is uniaxial negative, to = 1.645(1), e = 1.624(1), trigonal, space group R3m, in the hexagonal setting a = 15.770(2), c = 7.085(1) Å, V = 1525.8(4) Å3, Z= 3. The strongest six X-ray diffraction lines in the powder pattern are at d = 3.950 Å with I = 100% for (hkl) = (220); 2.552 Å, 93%, (051); 1.898 Å, 72%, (342); 4.181 Å, 58%, (211); 2.924 Å, 56%, (122); and 3.434 Å, 53%, (012). Analysis by a combination of electron microprobe, SIMS, H-line extraction, and crystal-structure refinement gave SiO2 38.10 wt%, Al2O3 44.60, Na2O 1.43, Li2O = 1.13, B2O3 = 10.88, H2O = 3.70, F = 0.20, O ≡ F 0.08, sum = 99.96 wt%, Fe, Mg, Ca, Mn, Ti, F, K not detected. The formula unit (31 anions) is x(⃞57Na0.43)Y(Li0.71Al2.17)zAl6(Si5.92O18) (B2.92O9)(OH)3.83F0.10O0.07, with the ideal end-member formula ⃞(LiAl2)Al6(Si6O18)(BO3)3(OH)4; thus rossmanite can be derived from elbaite [Na(Al1.5Li1.5)(Si6O18)(BO3)3(OH)4] by the substitution x⃞2 + YAl → xNa2 + YLi, where ⃞ = vacancy. The crystal structure of rossmanite was refined to an R index of 1.7% using 1094 observed (5σ) reflections collected with MoKα X-radiation from a single crystal. The structure refinement confirmed the low occupancy of the X site and the presence of Li at the Y site. There is considerable positional disorder at the O1 and O2 sites induced by the local variations in bond-valence distribution associated with ⃞-Na disorder at X and Li-Al disorder at Y.

Geochemistry of oxide minerals of Nb, Ta, Sn, and Sb in the Varuträsk granitic pegmatite, Sweden: The case of an anomalous columbite-tantalite trend

Abstract The complex, petalite-subtype Varuträsk pegmatite in the Proterozoic rocks of northern Sweden is, as a whole, rather poor in Nb and Ta. The pegmatite consolidated in eight units, but the (Nb,Ta)-oxide minerals attained saturation levels only in a late albite + lepidolite-bearing unit under conditions of high activity of alkali fluorides. Consequently, the compositional trends of columbite-group minerals and cassiterite mimic those typically displayed in pegmatites of the lepidolite subtype: from ferroan manganocolumbite [with Mn/(Mn + Fe)(at.) of 0.35 and Ta/(Ta + Nb) of 0.08] through near-endmember manganocolumbite (0.95 and 0.20, respectively) to Fe-depleted manganotantalite (0.99 and 0.55, respectively), and from (Fe >> Mn, Nb > Ta)-bearing to (Mn > Fe, Ta > Nb)-enriched cassiterite. To date, rare occurrences of cassiterite with Mn > Fe are restricted solely to the lepidolite-enriched granitic pegmatites. The degree of cation order in the Varuträsk columbite-group minerals increases from early to late phases, and with decreasing amounts of heterovalent substitutions. Slower cooling of initially disordered structures in late phases, or their diminished compositional complexity may be responsible for the higher degree of order. Rare primary microinclusions of cassiterite in columbitegroup minerals show consistent and systematic preference for Ta and Fe, suggesting an approach to chemical equilibrium, but columbite-group inclusions in cassiterite show in part a compositional scatter. In contrast, rare inclusions of ferrotapiolite and wodginite closely reflect the (Fe,Mn,Ta,Nb) compositional features of the host cassiterite. Stibiotantalite shows high values of Ta/(Ta + Nb) and mere traces of Bi, reflecting the relative abundance of native antimony and stibarsen in the pegmatite, and the absence of Bi-bearing minerals. Rare primary microlite is Ta- and F-rich, whereas the more widespread pyrochlore-microlite metasomatic after columbite-group minerals reflects the Ta/(Ta + Nb) values of the precursors, as does the stibiomicrolite replacing stibiotantalite. Cesium is elevated in several grains of primary and metasomatic pyrochlore-group phases that also are enriched in Sb, but not in stibiomicrolite. The array of large cations in pyrochlore-microlite metasomatic after columbite-group minerals is quite different from that typical of stibiomicrolite, suggestive of differences in the nature of the parent fluids. The lepidolite-subtype signature of the columbite-group minerals and cassiterite in the petalite-subtype Varuträsk pegmatite emphasizes the importance of specific conditions controlling stabilization of these minerals. The restriction of the columbite-group minerals to a very late lepidoliterich unit imposes a lepidolite-subtype signature on the whole petalite-subtype pegmatite, a signature grossly different from the characteristics typical of petalite-subtype pegmatites elsewhere.

Subsolidus rubidium-dominant feldspar from the Morrua Pegmatite, Mozambique; paragenesis and composition

Abstract At the Morrua pegmatite, Mozambique, alkali feldspar has replaced pollucite under low-temperature (250−150°C) hydrothermal conditions. Fluids invading a fracture system in pollucite formed round granular aggregates of (K-Rb)-feldspar in three stages: (1) a compositionally heterogeneous core of the feldspar cluster (+cookeite±apatite) with 7−20 mol.% RbAlSi3O8, grading outward into a Rb- dominant feldspar with 66 mol.% RbAlSi3O8 (20 wt.% Rb2O); (2) an intermediate layer of non-porous, inclusion-free, end-member K-feldspar; (3) an outer layer of porous end-member K-feldspar. Feldspars of all three stages seem to be monoclinic and disordered, with metastable sanidine structure. Zoning in K/Rb, preserved on a fine scale, was formed during growth at a temperature too low for subsequent alkali-cation diffusion or (Al,Si)-ordering.

Ferrotitanowodginite, Fe (super 2+) TiTa 2 O 8 , a new mineral of the wodginite group from the San Elias pegmatite, San Luis, Argentina

Simmonsite, Na2LiA1F6, a new mineral of pegmatitic-hydrothermal origin, occurs in a late-stage breccia pipe structure that cuts the Zapot amazonite-topaz-zinnwaldite pegmatite located in the Gillis Range, Mineral Co., Nevada, U.S.A. The mineral is intimately intergrown with cryolite, cryolithionite and trace elpasolite. A secondary assemblage of other alumino-fluoride minerals and a second generation of cryolithionite has formed from the primary assemblage. The mineral is monoclinic, P21 or P21/m, a = 7.5006(6) Å, b = 7.474(1) Å, c = 7.503(1) Å, β = 90.847(9)o, V = 420.6(1) Å, Z = 4. The four strongest diffraction maxima [d (Å), hkl, I/I100] are (4.33, 111 and 111 _ , 100); (1.877, 400 and 004, 90); (2.25, 131 _ , 113, 131 and 311, 70); and (2.65, 220, 202, 022, 60). Simmonsite is pale buff cream with white streak, somewhat greasy, translucent to transparent, Mohs hardness of 2.5–3, no distinct cleavage, subconchoidal fracture, no parting, not extremely brittle, Dm is 3.05(2) g/cm, and Dc is 3.06(1) g/cm. The mineral is biaxial, very nearly isotropic, N is 1.359(1) for λ = 589 nm, and birefringence is 0.0009. Electron microprobe analyses gave (wt%) Na = 23.4, Al = 13.9, F = 58.6, Li = 3.56 (calculated), with a total of 99.46. The empirical formula (based on 6 F atoms) is Na1.98Li1.00Al1.00F6. The crystal structure was not solved, presumably because of unit-cell scale twinning, but similarities to the perovskite-type structure exist. The mineral is named for William B. Simmons, Professor of Mineralogy and Petrology, University of New Orleans, New Orleans.

Bederite, a new pegmatite phosphate mineral from Nevados de Palermo, Argentina; description and crystal structure

Abstract Bederite, ideally ⃞Ca2Mn2+2Fe3+2Mn2+2(PO4)6(H2O)2, orthorhombic, a = 12.559(2), b = 12.834(1), c = 11.714(2) Å, V= 1887.8(4) Å3, Z = 4, space group Pcab, is a new mineral from the El Peñón pegmatite, Nevados de Palermo, Salta Province, República Argentina. The mineral occurs as rare ellipsoidal nodules (~5 cm in diameter) enclosed in potassium feldspar or quartz at the core-margin zone of a beryl-type rare-element pegmatite. Associated minerals are quartz, potassium feldspar, muscovite, beryl, columbite, possibly heterosite, and powdery coatings of Mn- and Fe-oxides; in the dumps of the pegmatite, there are numerous other phosphates including altered triphylite-lithiophyllite, arrojadite, eosphorite, laueite, brazilianite, and fairfieldite. Bederite is very dark brown to black with a dark olive-green streak and a vitreous luster. It is brittle with an irregular fracture and a good cleavage parallel to {100}, Mohs hardness is 5, and the observed and calculated densities are 3.48(1) and 3.50 g/cm3, respectively. In transmitted plane-polarized light, bederite is pleochroic X = Y = olive green, Z = brown with X = Y > Z and X = a, Y = c. Z = b. In cross-polarized light, it is biaxial negative with strong dispersion, v > r, 2V(obs) = 54° and 2V(calc) = 60°. Refractive indices are as follows: α = 1.729(3), β = 1.738(3), γ= 1.741(3). Chemical analysis by electron microprobe plus the Penfield method and thermogravimetry gave P2O5 41.76, Al2O3 0.82, Fe2O3 12.00, FeO 2.25, MnO 20.59, MgO 3.45, ZnO 0.40, CaO 10.91, SrO 0.43, Na2O 0.63, H2O 3.52, sum 96.76 wt% where the Fe2O3 and FeO contents were derived from the refined crystal structure. The five strongest lines in the X-ray powder diffraction pattern are as follows: d Å), I, (h k l): 2.768,100, (4 0 2); 2.927, 78, (0 0 4); 3.006, 67, (1 4 1); 2.814, 35, (0 4 2); 2.110, 33, (1 6 0). The crystal structure of bederite was refined to an R index of 2.8% based on 2530 observed (>5σF) reflections measured with MoΚα X-radiation. Bederite is isostructral with wicksite, grischunite, and an unnamed wicksite-like phase; it is related to wicksite by the substitutions Na⃞ + M2Fe3+ → NaNa + M2Mg, M1Mn2+ → M1Fe2+ and M3Mn2+ → M3Fe2+.

Foordite-thoreaulite, Sn2+Nb2O6–Sn2+Ta2O6: compositional variations and alteration products

The foordite-thoreaulite series of Sn 2+ -bearing Nb, Ta-oxide phases is a rare constituent of complex, rare-element, Li-Cs-Ta-rich (LCT-family) granitic pegmatites with local low f O 2 environment. In this study, detailed electron-microprobe analyses (EMPA) reveal a broad range of nearly continuous foordite-thoreaulite solid solution: at. Ta/(Ta+Nb) =0.23–0.92. Valence equilibration of the formulae suggests up to 6 at.% of total Sn in Sn 4+ state occupying the octahedral Nb, Ta-populated B -site. Three substitution mechanisms dominate the foordite-thoreaulite chemistry: NbTa −1 , Pb 2+ Sn −1 2+ and Sb 3+ Sn 4+ Sn −1 2+ (Nb, Ta) −1 5+ ; Pb 2+ and Sb 3+ occupy ⩽ 21.5 at. % and ⩽ 7.6 at. % of the A -site position, respectively. Sb shows positive correlation with Ta/(Ta+Nb). Primary large foordite-thoreaulite crystals are compositionaly homogeneous, only areas of secondary alteration show small zones of recrystallized foordite-thoreaulite and grains with diffuse or patchy zoning, variable Nb/Ta ratio and locally increased Pb content. Influx of late-magmatic to hydrothermal fluids and/or alkali elements under higher f O 2 causes breakdown of foordite-thoreaulite and production of cassiterite and numerous Nb, Ta-oxide minerals. At Lutsiro pegmatite, foordite is replaced by mosaic fine-grained aggregate of secondary foordite-thoreaulite + columbite-tantalite + Ta-rich cassiterite. Simpsonite is present in Manono and Maniema pegmatites. Local replacement of foordite-thoreaulite by alkali-bearing Nb-Ta phases is widespread; irregular veinlets and zones of lithiotantite, calciotantite, irtyshite/natrotantite, cesplumtantite, a mineral with composition (Na, Cs) 2 (Pb, Sb 3+ ) 3 Ta 8 O 24 , rankamaite, fersmite, and pyrochlore-group minerals occur.

Contact relationships between the Phuket pegmatites and host rocks Thailand

Abstract The diagnostic minerals at the primary contact aureoles of the Phuket pegmatites indicate that there were dispersions of B, Mn and alkalis to their host rocks, both the metasedimentary rocks of the Phuket Group and the biotite-hornblende-muscovite granites. The contact aureoles of the pegmatitic segregations and their parental granites provide evidence that besides B, Mn, K and Na, specific metals such as Sn, Nb, Ta and W were also contributed in the late-stage fluids of the Phuket granite-pegmatite system.