Petr Černý

Fertile granites of Precambrian rare-element pegmatite fields : is geochemistry controlled by tectonic setting or source lithologies ?

Abstract A total of twenty fertile granites and their rare-element pegmatite aureoles was examined to correlate their tectonic position relative to the preceding orogenic event with their geochemical signature, namely Li, Rb, Cs, Be, Ga, Sn, Nb Ta, Y, REE, Sc, Ti, Zr, Be, Th, U, F (NYF). The range of ΔT (time difference) between metamorphism and/or deformation and granite intrusion is −20 to 465 Ma for the examined cases. There is some tendency toward syn- to late-orogenic origin of LCTs as opposed to post- to anorogenic timing of NYFs, but the correlation is poor compared to the previously accepted stereotype. Similarly, both metaluminous and peraluminous fertile granites are about equally spread among syn-, late- and anorogenic environments. LCT systems are largely associated with peraluminous granites, whereas all metaluminous to peralkaline plutons bear the NYF signature. Tectonic control on geochemistry of granite-pegmatite suites is poor and secondary to the fundamental influence of the source lithologies yielding granitic melts. The LCT-type suites are derived from undepleted upper-crustal lithologies suffering their first anatectic event, which mobilizes the most volatile components into low-temperature, low-percentage melts. Supracrustal sequences as well as ortho- and para-lithologies of their basement serve as protoliths, with variable degrees of involvement. The NYF-type suites whose plutons have the geochemical signature of A-granites, may be derived by melting of depleted lower-crustal sources, residual after preceding anatectic events, or possibly by melting of juvenile lithologies with short crustal residence. Differentiation from mantle-derived magmas is highly improbable. Volatile extraction of NYF elements from the mantle and their pre-enrichment in protoliths or melts seems to be required. The few mixed systems with variable expressions of a NYF⪢LCT signature are derived basically as NYF suites but contaminated by the LCT elements from undepleted upper crust via several possible mechanisms.

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.

Mineralogical and geochemical evolution of micas from miarolitic pegmatites of the anorogenic Pikes Peak batholith, Colorado

SummaryA suite of 29 micas from miarolitic pegmatites associated with granitic units of the anorogenic Pikes Peak batholith (1.08–1.02 Ga), Colorado range in composition, and follow in paragenetic sequence, from 1M siderophyllite (N = 1), and 3T or 2M1 lithian biotite (N = 5) to 1M zinnwaldite (N = 20) and 1M ferroan lepidolite (N = 1). Locally, 1M (?) phlogopite (N = 1) and ferroan 2M1 muscovite (N = 1) are also present. Pervasive, late-stage hydrothermal alteration along with possible supergene weathering of many of these micas produced vermiculite. Additionally, some vugs and cavities were filled with chlorite and/or smectite. Early crystallized micas form tapered columnar crystals in graphic pegmatite, growing toward, and adjacent to the miarolitic cavity zone which contains the later crystallized micas. Principal associated minerals are quartz, microcline perthite (mostly amazonite), and albite, with local topaz or fluorite, and rarely tourmaline (schorl-elbaite).Progressively younger micas of the main crystallization sequence display increasing Si, Li, F, and Al/Ga, and decreasing total Fe, Mg, and octahedral occupancy. The zinc content of all micas is considerably elevated, whereas Mn, Rb, Cs, and Sc are moderate and T1 is very low. Early siderophyllite and lithian biotite show a narrow range of FeO/Fe2O3 (5.6–8.0), whereas later zinnwaldite is much more variable (2.4–40.3). Annite of the host granite and early graphic pegmatite is compositionally homogeneous, but most mica crystals from cavities show remarkable compositional and abrupt, sharp and distinct color zoning. Most cavity-grown zinnwaldite crystals show a decrease, from core to rim, in total Fe and Mg, whereas Si, Li and F increase and Mn, Rb, Cs and Na are essentially constant. A few to more than 100 color zones have been identified in some mica crystals. The zones are well correlated with the Ti content (<0.2 wt. % TiO2 colorless, 0.4–0.6 wt.% TiO2 red-brown). The total Fe content may or may not correlate with color zoning, whereas Zn variations (up to 1.1 wt. %) are entirely independent. The dark color zones probably reflect Fe-Ti charge transfer.The mica composition sequence described here is typical of the extreme fractionation observed in pegmatites of the NYF family, associated with anorogenec granites. Elevated Fe, Zn, and enhanced Sc contents are characteristic of this family. Strong enrichment in Li, Rb, and F is present, particularly in the micas of the miarolitic cavities. Sharp color zonation and compositional variation in cavity-grown zinnwaldite and ferroan lepidolite crystals suggest rapid changes in the intensive parameters, particularly the f(O2), of the parent fluid during the final stages of pegmatite consolidationZusammenfassung29 Glimmer aus miarolithischen Pegmatiten, die mit den Graniten des anorogenen Pikes Peak Batholiten (1.08–1.02 Ga) in Colorado vorkommen, schwanken in ihrer Zusammensetzung, und folgen in paragenetischer Abfolge, von 1M Siderophylit (N = 1) und 3T oder 2M1 Lithium Biotit (N = 5) bis zu 1M Zinnwaldit (N = 20) und 1M Eisen-Lepidolit (N = 1). Lokal kommt auch 1M (?) Phlogopit (N = 1) und 2M1 Eisen-Muskovit (N = 1) vor. Anhaltende hydrothermale Umwandlung während später Stadien der granitischen Entwicklung und möglicherweise auch oberflächennahe Verwitterung mancher dieser Glimmer führte zur Entstehung von Vermiculit. Außerdem wurden einzelne Hohlräume mit Chlorit und/oder Smectit gefüllt. Die früh gebildeten Glimmer sind säulige Kristalle in graphischem Pegmatit, die in Richtung auf, und in Nähe der blockigen Zone wachsen; der Groβteil der später gebildeten Glimmer ist in der blockigen Zone und in an diese anschlieβenden miarolithischen Hohlräumen lokalisiert. Die wichtigsten assoziierten Minerale sind Quarz, Mikroklin-Perthit (hauptsächlich Amazonit), sowie Albit, mit lokal etwas Topas oder Fluorit, und selten Turmalin (Schörl-Elbait).Die Hauptabfolge der Glimmer-Kristallisation zeigt zunehmende Si, Li, F und Al/Ga, aber abnehmende Gesamtwerte für Fe, Mg, und oktaedrische Besetzung. Der Zinkgehalt ist beträchtlich erhöht, während Mn, Rb, Cs, und Sc mittlere und Tl sehr niedrige Werte zeigen. Frühgebildete Siderophylite und Lithium-Biotite zeigen beschränkt Variationen der FeO/Fe2O3 Verhältnisse (5.6–8.0), während späterer Zinnwaldit mehr variabel ist (2.4–40.3). Annit im Wirts-Granit und in frühem graphischen Pegmatit ist seiner Zusammensetzung nach homogen, aber die meisten Glimmerkristalle aus Hohlräumen zeigen bemerkenswerte Zonierung der Zusammensetzung und gut entwickelte Farb-Zonierung. Die meisten in Hohlräumen gewachsenen Zinnwaldit-Kristalle zeigen eine Abnahme an Fe und Mg von Kern zum Rand, während Si, Li, und F zunehmen, sowie Mn, Rb, Cs und Na konstant bleiben. Es können einige wenige, aber auch mehr als 100 Farbzonen entwickelt sein, und diese sind gut mit dem Ti-Gehalt (< 0.2 Gew. % TiO2 farblos, 0.4–0.6 Gew. % TiO2 rot-braun) zu korrelieren. Der Gesamteisengehalt kann, aber muβ nicht mit der Farb-Zonierung korrelierbar sein, während Variationen des Zinkgehaltes (bis zu 1.1 Gew %) vollkommen unabhängig von optischen Aspekten sind. Das Auftreten tieferer Farbtöne dürfte einem Fe-Ti Ladungstransfer zuzuschreiben sein.Die hier diskutierte Abfolge von Glimmern ist typisch für extreme Fraktionierung in Pegmatiten der NYF Familie, die mit anorogenen Graniten assoziiert sind. Hohe Gehalte von Fe, Zn und erhöhte Sc Gehalte sind charakteristisch. Deutliche Anreicherung an Li, Rb und F ist besonders in den Glimmern aus miarolithischen Hohlräumen festzustellen. Der gut entwickelte optische und chemische Zonenbau in Zinnwaldit und Eisen-Lepidolith Kristallen, die in Hohlräumen gewachsen sind, weist auf intensiven Wechsel der intensiven Parameter der Mineral-bildenden Fuide in den Endstadien der Pegmatit-Bildung hin; dies trifft besonders auf f(O2) zu.

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.

Cesstibtantite—a geologic introduction to the inverse pyrochlores

SummaryThe crystal structure of cesstibtantite has been solved from diffractometer data collected on samples from Leshaia, Russia and the Tanco pegmatite, Manitoba. Cesstibtantite from the Leshaia pegmatite (type locality) hasa 10.515(2) Å, space groupFd3m, composition Cs0.31(Sb0.57Na0.31Pb0.02Bi0.01)ΣO.91(Ta1.88Nb0.12)Σ2(O5.69[OH, F]0.31)Σ6(OH, F)0.69, Z 8; its structure was refined toR 3.8,wR 4.3% using 96 observed (F > 3σ[F]) reflections (MoKα). Cesstibtantite from the Tanco pegmatite hasa 10.496(1) Å, space groupFd3m, composition (Cs0.22K0.01)Σ0.23(Na0.45Sb0.39Pb0.14· Ca0.06Bi0.02)Σ1.06(Ta1.95Nb0.05)Σ2(O5.78[OH,F]0.22)Σ6(OH,F)0.55,Z 8; its structure was refined toR 3.9w R 3.7% using 104 observed reflections. Cesstibtantite differs from the normal pyrochlores in that it contains significant amounts of very large cations such as Cs. As these cations are too large (VIII[r] > 1.60 Å) for the conventional [8]-coordinated A site, they occupy the [18]-coordinatedϕ site, which normally contains monovalent anions. Natural cesstibtantite samples are non-ideal in that both Cs and monovalent anions occur at theϕ site; thus cesstibtantite is intermediate to thenormal pyrochlores (with only monovalent anions at theϕ site) and theinverse pyrochlores (with only large cations at theϕ site).ZusammenfassungDie Kristallstruktur von Cesstibtantit wurde auf der Basis von Diffraktometerdaten von Proben von Leshaia, Russland and dem Tanco Pegmatit, Manitoba, gelöst. Cesstibtantit aus dem Leshaia Pegmatit (Typlokalität) hat a 10.515(2) Å, RaumgruppeFd3m, die Zusammensetzung CS0.31(Sb0.57Na0.31Pb0.02Bi0.01)Σ0.91(Ta1.88Nb0.12)Σ2· (O5.69OH, F0.31)Σ6(OH, F)0.69Z 8; die Struktur wurde aufR 3.8,wR 4.3% verfeinert unter Benützung von 96 beobachteten Reflexen. Cesstibtantit vom Tanco Pegmatit hat a 10.496(1) Å, RaumgruppeFd3m, die Zusammensetzung (Cs0.22K0.01)Σ0.23(Na0.45· Sb0.39Pb0.14Ca0.06Bi0.02)Σ1.06(Ta1.95Nb0.05)Σ2(O5.78OH,F0.22)Σ6(OH,F)0.55,Z 8; seine Struktur wurde aufR 3.9wR 3.7% auf der Basis von 104 beobachteten Rettexen verfeinert. Cesstibtantit unterscheidet sich von normalen Pyrochloren insofern, als er signifikante Mengen von sehr großen Kationen, wie z.B. Cs enthält. Da these Kationen zu groß sind (VIIIr 1.60 Å) für eine konventionelle [8]-koordinierteA Stelle, nehmen she die [18]-koordiniertenϕ Positionen ein, welche normalerweise monovalente Anionen enthalten. Natürliche Cesstibtantitproben sind nicht ideal insofern als sowohl Cs als auch monovalente Anionen in derϕ Position vorkommen. Somit ist Cesstibtantit intermediär zu den normalen Pyrochloren (mit nur monovalenten Anionen auf derϕ Position) and den inversen Pyrochloren (mit ausschließlichen großen Kationen an derϕ Position).

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).

Radiogenic 87Sr, its mobility, and the interpretation of RbSr fractionation trends in rare-element granitic pegmatites

Abstract The extent of fractionation of Rb and Sr is routinely used in petrogenetic modelling of igneous processes, including internal fractionation of individual pegmatites as well as large-scale evolution of pegmatite groups and fields. However, highly evolved granitic pegmatites may contain as much as 14000 ppm Rb and less than 150 ppm Sr. The total Sr in K-feldspar and micas from geologically old and Rb-rich pegmatites may consist predominantly of radiogenic 87 Sr, which obscures the original relationship of Rb to common Sr at the time of crystallization. A subtraction of radiogenic 87 Sr calculated from the Rb content and age of emplacement is possible, but it commonly results in negative concentrations of Sr. The relative immobility of Rb, analytically determined isotopic composition of Sr, apparent ages of the Rb, Sr-bearing minerals, high concentration of 87 Sr in coexisting Rb-poor phases, and experimental evidence indicate that post-crystallization migration of radiogenic 87 Sr is significant. Where isotopic data are not available, RbSr trends in geologically old and highly fractionated pegmatites are misleading and cannot be used for geochemical interpretation of pegmatite derivation or evolution.

The Archean Lac du Bonnet batholith, Manitoba: Igneous history, metamorphic effects, and fluid overprinting

The 2.6 Ga Lac du Bonnet batholith of ~ 1000 km2 surface exposure is emplaced along a longlived regional fault. The batholith was generated by successive intrusions that have undergone different degrees of modification by internal processes or by tectonic effects: (i) minor early porphyritic hornblendebiotite granodiorite; (ii) extensive silicic leucogranite (with interior barren and exterior Be, Nb > Ta, Y, REE, F, Zr, Th, U, Ti-bearing pegmatites); (iii) dominant biotite granite with xenoliths of (basement?) tonalite, containing extensive K-feldspar porphyroblastesis; (iv) minor undeformed but porphyroblastic late biotite granodiorite. The chemistry of the two granodiorites is closely related to that of the biotite granite but the geochemical features of the leucogranite are markedly different. Isotopic constraints (δ18Oof +8.1, RSriof 0.7003) and K2O content suggest LIL-depleted tonalite with <25% greenstone-belt metasediments as the probable source of biotite granite. Very modest negative Eu anomalies and low HREE of the granite are compatible with a tonalitic protolith. Melting of short-lived felsic volcanics would also satisfy all constraints but encounters a potential mass balance problem. Extensive K-feldspar porphyroblastesis had no significant effect on bulk compositions, alkali-feldspar-bound elements or Rb-Sr isotope systematics but it disturbed magmatic oxygen to +6.7–+11.0 and remarkably depleted P, Zr, Hf, Th and all REE except Eu. The highly evolved leucogranite could have been affected by liquid fractionation, was depleted in volatile components and rare lithophile elements extracted into its pegmatite aureole, and its disturbed Rb-Sr isotope systematics could have resulted from pervasive shearing and recrystallization. The overall geochemical features of the leucogranite (δ180 + 7.9, RSri ~ 0.7000, high FeMg, LREE > HREE with very prominent negative Eu anomaly, and Nb > Ta, Y, F signature of derived pegmatites) rank it with A-type granites.

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).

Pollucite from the Proterozoic petalite-bearing pegmatites of Utö, Stockholm archipelago, Sweden

Abstract Primary pollucite has been discovered in one of the two petalite-bearing pegmatites transecting the iron formation of Nykopingsgruvan, Uto, Stockholm archipelago, which have been the first lithium pegmatites ever discovered. The pollucite is associated with albite, K-feldspar, rubellite, indicolite, apatite, and quartz. It contains numerous microinclusions of acicular apatite arranged in a three-dimensional rectangular grid. Late veining of the pollucite carries sericite, albite, adularia and a calcium carbonate. The pollucite is isotropic with n=1.521(1), a=13.686(1) A, and D=2.92(3) g/cm3. Chemical analysis gives Poll76.7Anal23.3 to Poll78.5Anal21.5, and Si/Al from 2.43 to 2.49. The mineral assemblage of the parent pegmatite, examined for almost two centuries, is not particularly indicative of the presence of pollucite. However, the geochemical features of the dikes, virtually unexplored to date, indicate high fractionation levels that match those of other pollucite-bearing pegmatites: e.g. ave...