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In a series of timed experiments, monazite inclusions are induced to form in the Durango fluorapatite using 1 and 2 N HCl and H2SO4 solutions at temperatures of 300, 600, and 900 degrees C and pressures of 500 and 1,000 MPa. The monazite inclusions form only in reacted areas, i.e. depleted in (Y+REE)+Si+Na+S+Cl. In the HCl experiments, the reaction front between the reacted and unreacted regions is sharp, whereas in the H2SO4 experiments it ranges from sharp to diffuse. In the 1 N HCl experiments, Ostwald ripening of the monazite inclusions took place both as a function of increased reaction time as well as increased temperature and pressure. Monazite growth was more sluggish in the H2SO4 experiments. Transmission electron microscopic (TEM) investigation of foils cut across the reaction boundary in a fluorapatite from the 1 N HCl experiment (600 degrees C and 500 MPa) indicate that the reacted region along the reaction front is characterized by numerous, sub-parallel, 10-20 nm diameter nano-channels. TEM investigation of foils cut from a reacted region in a fluorapatite from the 1 N H2SO4 experiment at 900 degrees C and 1,000 MPa indicates a pervasive nano- porosity, with the monazite inclusions being in direct contact with the surrounding fluorapatite. For either set of experiments, reacted areas in the fluorapatite are interpreted as replacement reactions, which proceed via a moving interface or reaction front associated with what is essentially a simultaneous dissolution-reprecipitation process. The formation of a micro- and nano-porosity in the metasomatised regions of the fluorapatite allows fluids to permeate the reacted areas. This permits rapid mass transfer in the form of fluid-aided diffusion of cations to and from the growing monazite inclusions. Nano-channels and nano-pores also serve as sites for nucleation and the subsequent growth of the monazite inclusions
Quartz crystals from topaz-zinnwaldite-albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200 mum. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn-W mineralization. The combination of special experimental techniques with confocal laser Raman- microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H2O, F, and B in re-homogenized MIs. The contents of H2O and F were observed to increase from 3 to 30 and 1.9 to 6.4 wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H2O, with values up to 55 wt% H2O and an F concentration of approximately 3 wt%. Ongoing enrichment of volatiles H2O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H2O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid
Annite and Fe-rich siderophyllite constitute the rock-forming micas in the late-Variscan composite granite pluton of Konigshain, Lausitz, Germany. This multiphase pluton is composed of three fractionated, but not chemically specialized monzogranite types, which contain lithophile elements such as Li, Rb, Cs, Sn, and F in average quantities. Abundant miarolitic pegmatites of the NYF family with a broad diversity of rare minerals occur in the apical part of the pluton. These pegmatitic cavities locally contain di- and trioctabedral micas as well as cation-deficient micas. Trioctahedral micas comprise F-rich manganoan lithian siderophyllite to manganoan zinnwaldite, zinnwaldite, and minor lepidolite. The formula [calculated on the basis of 22 anion valencies and 2 (F + OH + Cl)] of the most Mn-rich siderophyllite is (K0.85Rb0.08Na0.04)(0.97)(Al0.99Li0.91Fe0.51Mn0.42Ti0.01Zn0.01)(2.85) (Si3.21Al0.79)(4)O- 10(F1.80OH0.19Cl0.01)(2). This mica constitutes one of the most Mn-rich siderophyllite compositions reported to date. The lithium micas poorer in Mn are distinguished by elevated concentrations of Rb (up to 2.5 wt % Rb2O), CS (UP to 1.2 wt % Cs2O), and F (up to 9.6 wt %). This fluorine content is probably consistent with the maximum possible F occupation of 2 of the (F,OH,Cl)-site. The structural formula of the most Li-rich lepidolite is (K0.83Rb0.07Cs0.03)(0.93) (Li1.62Al1.00Fe0.38)(3.00)(Si3.62Al0.38)(4) O-10(F1.91OH0.09)(2). During hydrothermal alteration, lepidolite and zinnwaldite became partially depleted in K, Li, Rb, Cs, and F and gradually transformed into cation-deficient micas (lithian phengite to illite of phengitic affinity)
The uranium deposit at Niederschlema-Alberoda, Germany, contains a rich variety of Bi minerals deposited between the Permian and the Cretaceous; these have been studied for paragenetic relations, composition, and conditions of formation. Particular attention is given to the rare Bi selenides watkinsonite, nevskite, and cuproan bohdanowiczite. Whereas watkinsonite and nevskite only occur intergrown with clausthalite, bohdanowiczite is more widespread and also is associated with Cu selenides. Watkinsonite from this second confirmed locality worldwide has an average composition (Cu1.47Ag0.49)(Sigma 1.96)(Pb1.01Hg0.01 Fe-0.01)(Sigma 1.03)Bi-3.98(Se7.98S0.05)(Sigma 8.03), ideally (Cu,Ag)(2)PbBi4Se8. These findings suggest that the empirical formula of watkinsonite originally proposed for the type specimen from the Otish Mountains uranium deposit in Quebec [CU2+xPb1+xBi4-xSe,S,Te)(8), x approximate to 0.3] requires revision. The composition of nevskite is (Pb0.06Bi0.95)(Sigma 1.01)Se-0.99, on average. Bohdanowiczite from the Cu- selenide assemblage shows extensive substitution of Cu+ for Ag+, expressed by the crystallochemical formula (Ag1.80- 0.94CU0.16-1.05Pb0.00-0.05)(Sigma 1.97-2.07)BiSigma 1.97-2.03SeSigma 3.96-4.04. This observation seems to argue for the natural existence of CU2Bi2Se4, the Se-dominant analogue of emplectite. The Bi selenides were deposited at temperatures of about 100 degrees C, in the Jurassic. The lack of thermodynamic data for all the Bi selenides limits reliable inferences on the fugacities of selenium and sulfur that prevailed during their formation. Other Bi minerals from this locality comprise members of the bismuthinite-aikinite solid-solution series of Permian age and, more importantly, native Bi and Bi sulfides (matildite, bismuthinite, wittichinite), deposited in the Cretaceous
In a sample from the Niederschlema-Alberoda U-Se-polymetallic deposit, western Erzgebirge, Germany, the entire PbSe-PbS solid-solution series was observed associated with uraninite, coffinite, hematite, acanthite, sphalerite, chalcopyrite, pyrite, and lollingite. Early deposited, Se-rich members of the Pb(Se, S) series occur as fracture fillings inside spherical uraninite or on its surface or form anhedral to subhedral grains precipitated in the immediate neighbourhood of the U minerals. Later crystallized, S-rich members of the series are affiliated with the sulfide minerals. The solid-solution series covers the range PbS1.00-Pb(S0.04Se0.96)(&USigma; 1.00) virtually free of gaps, consistent with a temperature of formation of &GE; 100° C. The PbSe-PbS solid solutions were likely deposited from hydrothermal fluids that became successively depleted in Se and enriched in S. The fugacities of selenium and sulfur covered the range -17 < logfSe(2) < -26 and -17 < logfS(2) < -22, respectively, implying fSe(2)/fS(2) &LE; 1. The spherical texture of the uraninite, as well as its U-Th-total Pb age (192 ± 21 Ma), imply deposition of the Pb(Se, S) series during the Jurassic, contemporaneous with the formation of the bulk of the other selenium minerals. The electron-microprobe data from this study confirm earlier inferences on complete miscibility between clausthalite and galena deduced from X-ray patterns of PbSe-PbS solid solutions from different uranium-vanadium deposits of the Colorado Plateau (COLEMAN 1959). In Niederschlema-Alberoda, the entire clausthalite-galena series occurs in a single section
Thermobarometrical and mineral-chemical investigations by electron microprobe and LA-ICP-MS on a sillimanite- bearing pegmatoid from the Reinbolt Hills provide important constraints on the P-T-X-age relations of part of East Antarctica during Pan-African tectonism. U-Th-total Pb ages of monazite imply that the pegmatoid of originally Grenvillan age (zircon U-Pb age of ca. 900 Ma) underwent a major, late Pan-African (Cambrian) regional, granulite-facies metamorphism between 500 and 550 Ma. Most of the monazite formed during this event, as result of apatite metasomatism owing to infiltration of high-grade metamorphic fluids. Apatite-biotite and other mineral thermobarometers define the peak metamorphic temperatures and pressures with 850-950 degrees C and 0.8-1.0 GPa. The F-Cl-OH relations in apatite, and biotite, the chemistry of fluid inclusions and the presence of K-feldspar microveins suggest that the metasomatising fluid was a CO2-bearing, diluted KCl brine. The pegmatoid is the first record of monazite-(Ce) formed from fluorapatite that is rich in U (up to 2.6 Wt% UO2) and possesses Th/U ratios <1 (0.09 on average). These chemical signatures are direct reflection of the U and Th concentration patterns in the parental fluorapatite
In the earliest emplaced granite subintrusion of the multiphase peraluminous Satzung pluton, Erzgebirge, Germany, a mineral aggregate was observed consisting of sekaninaite (X-Fe = 0.74-0.94), Zn-rich hercynite (X-Zn = 0.03- 0.11), tri- and dioctahedral layer silicates of different composition and color, and minor quartz. Geological, textural, and compositional criteria argue that the sekaninaite, hercynite, quartz, and the brown biotite are not primary or secondary granite minerals, but are of metamorphic origin representing a xenolith uptaken from the granite melt near its level of emplacement. The metamorphic origin is supported by the occurrence of this mineral assemblage in metamorphic rocks exposed locally in the Erzgebirge basement. Reaction of the polymineralic metamorphic aggregate with the surrounding melt and subsequent interaction with alkali-, F- and LILE-rich residual fluids account for the widespread decomposition of the sekaninaite and formation of several layer silicates including green biotite, muscovite, berthierine/Fe chlorite, and sericite. The observed enrichment of the relic sekaninaite and its replacement products in elements such as Na, Li, Be, Rb, Cs, and F is result of interaction of the metamorphic fragment with the surrounding melt/fluid, in accordance with the evolved nature of the Satzung magmatic-hydrothermal system