Ron Chapman

Geochemical and structural evolution of micas in the Rožná and Dobrá Voda pegmatites, Czech Republic

SummaryThe chemistry, structural parameters, polytypism, optical properties and Rb-Sr isotopes were examined in 11 to 60 samples of biotite, muscovite and lepidolite from the pegmatites at Rožná (the type locality of lepidolite; 323 ± 4Ma) and Dobrá Voda (306 ± 9Ma) in western Moravia. At both localities, early endocontact biotite is followed inwards by muscovite and lepidolite, which is concentrated in and around the core. At Rožná, a 1M lepidolite follows after 2M1 muscovite but all later generations of lepidolite are 2M2, close to Tri50 Ply50 and in part associated with muscovite 2M1. At Dobrá Voda, all lepidolite types are 1M and free of muscovite, and the late varieties approximate Tri30 Ply70. At both localities, a trend of increasing μHF is indicated during the progress of mica crystallization, culminating in precipitation of topaz. Polytypism of lepidolite is not correlatable with any compositional or growth feature, or their combination. Throughout the mica crystallization, Rb/Cs decreases but K/Rb becomes reversed after an initial decrease. Boron is partitioned preferentially into muscovite (up to 1.10 wt.% B2O3) but Be, Zn, Mn and Sc are enhanced in lepidolite. A slight increase in Fe, Ba and Cl in the last generation of lepidolite might be possibly due to mixing of residual pegmatite fluids with metamorphic pore solutions.ZusammenfassungIn 11 bis 60 Proben von Biotit, Muskovit und Lepidolith aus den Pegmatiten von Rožná (Typlokalität des Lepidoliths; 323 ± 9 Ma) in Westmähren wurden Chemie, Struktur-parameter, Polytypie, optische Eigenschaften und Rb-Sr-Isotopie untersucht. An beiden Lokalitäten wird früher Biotit an Endokontakten nach Innen von Muskovit und Lepidolith gefolgt, letzterer ist in und um den Kern konzentriert. In Rožná folgt 1M-Lepidolith auf 2M1-Muskovit, aber alle späteren Lepidolithgenerationen sind 2M2, nahe Tri50Ply50 und zum Teil mit 2M1-Muskovit vergesellschaftet. In Dobrá Voda sind alle Lepidolithe vom Typ 1M und frei von Muskovit, die späten Varietäten kommen Tri50Ply50 nahe. An beiden Lokalitäten ist während des Fortschreitens der Glimmerkristallisation eine Tendenz von steigendem μHF angezeigt, die in der Ausfällung von Topas ihren Höhepunkt findet. Die Polytypie des Lepidoliths kann nicht mit irgendeiner Eigenheit der Zusammensetzung oder des Wachstums korreliert worden, auch nicht mit einer Kombination von diesen. Während der ganzen Glimmerkristallisation nimmt Rb/Cs ab, aber die Tendenz von K/Rb ändert sich nach anfänglichem Abfall. Das Bor verteilt sich bevorzugt auf den Muskovit (bis zu 1.10 Gew. -% B2O3), aber die Be-, Zn-, Mn- und Sc-Gehalte sind im Lepidolith erhöht. Ein leichter Ansteig von Fe, Ba und Cl in der letzten Lepidolithgeneration könnte vielleicht durch eine Mischung von pegmatitischen Restlösungen mit metamorphen Porenlösungen verursacht sein.

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

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.

The Al (Nb, Ta) Ti(in−2) substitution in titanite: the emergence of a new species?

SummaryThe highest (Nb, Ta) content ever encountered in titanite is reported from the Maršíkov 11 pegmatite in northern Moravia, Czech Republic. This dike is a member of a pegmatite swarm of the beryl-columbite subtype, metamorphosed under conditions of the amphibolite facies. The pegmatite carries, i.a., rare tantalian rutile intergrown with titanian ixiolite, titanian columbite-tantalite, fersmite and microlite. Fissures generated in the Nb, Ta oxide minerals during deformation are filled with titanite, formed by reaction of the oxide minerals with metamorphic pore fluids. The titanite displays limited degrees of substitutions Na(Ta > Nb)(CaTi)−1, (Ta > Nb)4□Ti−4Si−1 and AI(OH, F)(TiO)−1, but an extensive (and occasionally the sole significant) substitution (Al > Fe3+)(Ta > Nb)Ti−2, responsible for widespread oscillatory zoning. This substitution reduces the proportion of the titanite componentsensu stricto, CaTiSiO4,O, to less than 50 mole % in many analyzed spots. The extreme composition corresponds to (Ca0.994Na0.011)(Ti0.436Sn0.007Al0.280Fe3+0.006Ta0.199Nb0.079)Si0.988O4(O0.974F0.026). However, so far this substitution fails to generate compositions that would define a new species.ZusammenfassungDie AI(Nb, Ta)Ti−2 Substitution im Titanit: Auftauchen einer neuen Mineralspecies? Die höchsten (Nb, Ta) Gehalte, die jemals für Titanit gefunden wurden, werden für den Maršíkov II Pegmatit in Nordmähren, Tschechei, berichtet. Der Intrusivgang ist Teil eines Amphibolit-faziell überprägten Pegmatitschwarms vom Beryll-Columbit Subtypus Der Pegmatit führt u.a. seltene tantalbetonte Rutile verwachsen mit titanbetontem Ixiolith, titanbetontem Columbit-Tantalit, Fersmit and Mikrolith. Deformationsbedingte Frakturen in den (Nb, Ta) Oxiden sind mit Titanit, als Folge der Reaktion der metamorphen Porenlösungen mit den Oxidmineralen, verkittet. Titanit zeigt begrenzte Substitutionen Na(Ta > Nb)(CaTi)−1,(Ta > Nb)4□Ti−4Si−1 and Al(OH, F)(TiO)−1, aber extensive (und gelegentlich einzig bedeutsame) Substitution (Al >> Fe3+)(Ta > Nb)Ti−2, die eine weitverbreitete, oszillierende Zonierung hervorruft. Diese Substitution verringert den Anteil der Titanit-Komponentesensu stricto, CaTiSiO,O, auf weniger als 50 Mol% in vielen Analysen. Die Extremzusammensetzung entspricht Ca0.994Na0.11) (T10.436Sn0.007Al0.280Fe3+0.006Ta0.199Nb0.079)Si0.988O4(O0.974F0.026). Das AusmaB dieser Substitution ist unzureichend, um eine neue Mineralspecies zu definieren.

Exsolution intergrowths of titanian ferrocolumbite and niobian rutile from the Weinebene Spodumene Pegmatites, Carinthia, Austria

SummaryTitanian ferrocolumbite is a rare accessory mineral in the spodumene-bearing pegmatites at Weinebene, Carinthia, Austria. It contains abundant exsolved niobian rutile and scarce inclusions of cassiterite that may be primary. The titanian ferrocolumbite is relatively homogeneous with Mn/(Mn + Fe) 0.24–0.33, Ta/(Ta + Nb) 0.09–0.13 (atomic ratios) and 0.47–0.88 Ti per 12 cations (2.7–5.0 wt.% TiO2). Natural specimens are considerably disordered but become more ordered on heating. Niobian rutile has Mn/(Mn + Fe) 0.00–0.04 and Ta/(Ta + Nb) 0.26–0.38; it concentrates Fe, Ta, Ti and Sn relative to the Mn- and Nb-enriched ferrocolumbite. The overall scarcity of Nb, Ta-oxide minerals in the spodumene-bearing pegmatites of southern Ostalpen conforms to their general features ranking them with the albite-spodumene type of rare-element pegmatites.ZusammenfassungTitan-Ferrocolumbit ist ein seltenes Akzessorium in den Spodumenpegmatiten der Weinebene, Kärnten, Österreich. Er enthält entmischten niobhaltigen Rutil und selten Einschlüsse von möglicherweise primärem Zinnstein. Die Zusammensetzung des TitanFerrocolumbits ist relativ homogen und weist die folgenden Elementverhältnisse auf. Mn/(Mn + Fe) 0,24 – 0,33, Ta/(Ta + Nb) 0,09 – 0,13 (Atomverhältnisse) und 0,47 – 0,88 Ti pro 12 Kationen (2,7 - 5,0Gew.%TiO2). Natürlich auftretende Kristalle sind strukturell merklich ungeordnet und erlangen durch Erhitzen einen höheren Ordnungsgrad. Die Mn/(Mn + Fe) sowie Ta/(Ta + Nb) Verhältnisse des niobhaltigen Rutil betragen 0,00 – 0,04 bzw. 0,26 – 0,38. Der niobhaitige Rutil ist im Vergleich zu Mn- und Nb-führendem Ferrocolumbit an Fe, Ta, Ti und Sn angereichert.—Die Seltenheit von Nb, Ta-Oxiden in den spodumenführenden Pegmatiten der südlichen Ostalpen steht in Übereinstimmung mit ihrer Zugehörigkeit zum Albit-Spodumen Typ der selten-Element-führenden Pegmatite (rare-element pegmatites).

Rubidium- and cesium-dominant micas in granitic pegmatites

Abstract The mode of occurrence and chemical composition of five types of micas with Rb- or Cs-dominant populations of interlayer cations, collected from the Red Cross Lake rare-element pegmatites in north-central Manitoba, are described here. All five micas are candidates for new mineral species but crystal-structural data and Li contents could not be determined to date because of extremely small particle size, restricted to the margins of strongly zoned microcrystals. Based on electron-microprobe analyses, on Li contents estimated from Li/F (at.) = 1.0, and on bulk analysis of ferromagnesian micas for FeO and Fe2O3, the micas correspond to Rb- and Cs-dominant polylithionite (with representative interlayer populations of Rb0.82K0.12Cs0.07 and Cs0.74Rb0.12K0.08 apfu, respectively), Rb and Cs-dominant magnesian annite (Rb45K0.37Cs0.20 and Cs0.67Rb0.20K0.12 apfu, respectively), and Cs-dominant ferroan phlogopite (Cs0.92Rb0.04K0.02 apfu).

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.

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.

A ferrotantalite-ferrotapiolite intergrowth from Spittal a.d. Drau, Carinthia, Austria

SummaryIntimate intergrowths of ferrotantalite and ferrotapiolite occur in a pegmatite in Spittal a.d. Drau, Carinthia. They are associated with muscovite, albite, smoky quartz, cassiterite, and microscopic uranmicrolite, zircon and uraninite. An assemblage of secondary uranium minerals is also present, generated by extensive alteration and leaching of the uranmicrolite and zircon. Textures of the ferrotantalite-ferrotapiolite intergrowths suggest considerable recrystallization that obliterated most of their primary features; neither coprecipitation nor exsolution can be recognized with certainty. Despite intersecting tielines indicating disequilibrium, the ferrotantalite and ferrotapiolite compositions show very restricted ranges (Mn/(Mn + Fe) 0.08–0.11, Ta/(Ta + Nb) 0.53–0.57 for ferrotantalite, and 0.01–0.04, 0.84–0.89 for ferrotapiolite, respectively), particularly in comparison with compositions from other localities featuring primary textures. A degree of compositional equilibration could have been attained during recrystallization. This process may also explain the high level of structural order characterizing both minerals; they are considerably disordered in other localities. Extensive deformation typical of pegmatites in the southern Ostalpen in general, and specifically of the Spittal pegmatite, is probably responsible for the recrystallization phenomena in the Ta, Nb, Sn-bearing mineral assemblage.ZusammenfassungIn einem Pegmatit von Spittal a.d. Drau, Kärnten, treten enge Verwachsungen von Ferrotantalit und Ferrotapiolit auf. Diese werden von Muskovit, Albit, Rauchquarz, Zinnstein sowie-in mikroskopischem Masstab-von Uranmikrolith und Zirkon begleitet. Die Textur der Verwachsungen lässt Rekristallisation erheblichen Ausmasses erkennen, die die primären Merkmale weitgehend auslöscht. Weder eine gemeinsame Auskristallisation der beiden Mineralphasen noch eine Bildung durch Entmischung kann mit Sicherheit erkannt werden. Trotz einander kreuzender Verbindungslinien, die einen Hinweis auf Ungleichgewicht darstellen, zeigen die Zusammensetzungen des Ferrotantalits und des Ferrotapiolits lediglich geringe Schwankungsbreiten: Mn/(Mn + Fe) 0,08–0,11, Ta/(Ta + Nb) 0,53–0,57 für den Ferrotantalit beziehungsweise 0,01–0,04 und 0,84–0,89 für den Ferrotapiolit. Dies gilt insbesondere für den Vergleich mit Zusammensetzungen solcher Mineralphasen mit jenen von Fundarten, die primäre Verwachsungstrukturen aufweisen. Bis zu einem gewissen Ausmass ist diese homogene Zusammensetzung möglicherweise auf die Rekristallisation zurück zuführen. Diese Rekristallisation könnte auch den hohen strukturellen Ordnungsgrad der beiden Mineralphasen erklären. An anderen Fundorten zeigen diese Minerale strukturell merklich geringeren Ordnungsgrad. Intensive metamorphe überprägung, wie sie für die Pegmatite in den südlichen Ostalpen und insbesondere für jenen von Spittal typisch sind, kann wahrscheinlich als Ursache der Rekristallisationsphänomene der Ta-Nb-Sn Mineralparagenese angenommen werden.