Showing papers in "Canadian Mineralogist in 1998"

Recommended nomenclature for zeolite minerals: report of the subcommittee on zeolites of the International Mineralogical Association, Commission on New Minerals and Mineral Names

Abstract: This report embodies recommendations on zeolite nomenclature approved by the International Mineralogical Association Commission on New Minerals and Mineral Names. In a working definition of a zeolite mineral used for this review, interrupted tetrahedral framework structures are accepted where other zeolitic properties prevail, and complete substitution by elements other than Si and Al is allowed. Separate species are recognized in topologically distinctive compositional series in which different extra-framework cations are the most abundant in atomic proportions. To name these, the appropriate chemical symbol is attached by a hyphen to the series name as a suffix except for the names harmotome. pollucite and wairakite in the phillipsite and analcime series. Differences in space-group symmetry and in order-disorder relationships in zeolites having the same topologically distinctive framework do not in general provide adequate grounds for recognition of separate species. Zeolite species are not to be distinguished solely on Si:Al ratio except for heulandite (Si:Al or =4.0). Dehydration, partial hydration, and over-hydration are not sufficient grounds for the recognition of separate species of zeolites. Use of the term "ideal formula" should be avoided in referring to a simplified or averaged formula of a zeolite. Newly recognized species in compositional series are as follows: brewsterite-Sr, -Ba: chabazite-Ca, -Na, -K; clinoptilolite-K, -Na, -Ca; dachiardite-Ca, -Na; erionite-Na, -K, -Ca: faujasite-Na, -Ca, -Mg; ferrierite-Mg, -K, -Na; gmelinite-Na, -Ca, -K; heulandite-Ca, -Na, -K, -Sr; levyne-Ca, -Na; paulingite-K, -Ca; phillipsite-Na, -Ca, -K; stilbite-Ca, -Na. Key references, type locality, origin of name, chemical data, IZA structure-type symbols, space-group symmetry, unit-cell dimensions, and comments on structure are listed for 13 compositional series. 82 accepted zeolite mineral species, and three of doubtful status. Herschelite, leonhardite, svetlozarite, and wellsite are discredited as mineral species names. Obsolete and discredited names are listed.

Oxide minerals of the separation rapids rare-element granitic pegmatite group, northwestern Ontario

Abstract: The newly discovered Separation Rapids pegmatite group, situated in mafic metavolcanic host-rocks that represent the eastern extremity of the Bird River metavolcanic-metasedimentary belt, contains Ontario's first occurrences of wodginite-group minerals (mainly wodginite and ferrowodginite), the pyrochlore-group minerals stibiomicrolite, stibiobetafite and yttropyrochlore, ferrotapiolite, and probably the first occurrence in North America of nigerite from a granitic pegmatite. This example of the rare-element class of granitic pegmatites hosts both beryl- and petalite-subtype pegmatites. Columbite-tantalite and cassiterite are the predominant oxide species. On the basis of columbite-tantalite compositions, the pegmatites have been divided into an Fe-suite and a Mn-suite. Both beryl and petalite pegmatites occur in each suite. On the basis of ferrocolumbite compositions, the associated Separation Rapids pluton is considered to be the parent of at least the Fe-suite of pegmatites. The Fe suite includes beryl pegmatites within and adjacent to the pluton, in which ferrocolumbite coexists with ferrowodginite, and, with increasing evolution, petalite-bearing pegmatites that contain ferrotantalite and wodginite. In individual pegmatites, columbite-tantalite variation is mainly in Ta/(Ta+Nb). Minor microlite, antimonian microlite and stibiomicrolite are found replacing earlier phases. Cassiterite is the final Nb-Ta-bearing oxide to crystallize. Pegmatites belonging to the Mn-suite follow a similar pattern of crystallization, with early manganocolumbite followed by manganotantalite, the latter coexisting with wodginite. Manganocolumbite within individual samples varies appreciably in Mn/(Mn+Fe), whereas the variation in manganotantalite is mainly in Ta/(Ta+Nb). In pods rich in "cleavelandite" and Li-mica within one of the beryl pegmatites, extreme Mn-enrichment has produced near-end-member manganotantalite and W-bearing wodginite. Microlite is an important late phase, which is either primary or forms as a replacement, mainly of wodginite. The presence of microlite, lithian mica and topaz in Mn-suite pegmatites (and aplites) indicates that they were derived from a more F-rich melt than that which produced the Fe-suite of pegmatites. Albitization also is more apparent in the Mn-suite of pegmatites. The wall zone of Marko's pegmatite, the largest body in the eastern subgroup and part of the Mn-suite, is unique in hosting titanowodginite, "ferrotitanowodginite", stibiobetafite and struverite. These Fe-, Ti- and Sb-phases are considered to have developed as a result of interaction of the pegmatite-forming melt with banded ironstones and Fe-Ti-rich metavolcanic host-rocks.

The IMA Commission on New Minerals and Mineral Names; procedures and guidelines on mineral nomenclature, 1998

Abstract: An author wishing to introduce a new mineral name into the literature, or to redefine, discredit or rename an existing mineral, must obtain prior approval of the IMA Commission on New Minerals and Mineral Names. This paper outlines the procedure to be followed in the preparation and submission of a proposal for approval, and describes how such proposals are handled by the Commission. The paper also gives general guidelines on mineral nomenclature and provides a list of nomenclature changes approved since 1987.

Distinction of jarosite-group compounds by Raman spectroscopy

Abstract: Raman spectra (200-1300 cm-I) were measured for synthesized jarosite-group compounds [MFe,(S04h(OH)r" M+ = K+, NH4+,Na+, Ag+, and Y2Pb2+).The Raman spectra of jarosite-group compounds are characterized by a tendency for the wavenumbersassigned to two vibrational modes of S042-, VI(SOi-) and vj(SOi-), and three vibrational modes of Fe-O bonds, to decrease with increase in the c unit-cell parameter. The wavenumbers assigned to the v2(SOi-) and v4(SOi-) vibrational modes'are independent of the.value.of c. For plumbojarosite, thc peaks corresponding to the VI(S042-) and V,(S042-) vibrational modesare broad owing to two overlapping peaks assigned to two types of sulfate groups, S042- ions adjacent and not adjacent to Pb2+ ions. Raman spectra can serve to identify the specific type of jarosite-group compound in poorly crystalline or low-concentration geochemical samples.

Structural relations among schoepite, metaschoepite and dehydrated schoepite

Abstract: Schoepite, [(UOr)8O2(OH)rz](HzO),2, transforms slowly in air at ambient temperarure to metaschoepite, UOr,ngrg 1o - r't, and crystals commonly contain an intergrowth of both minerals. The transformation may be due to the loss of one-sixth of the interlayer HrO groups in schoepite, and a possible smrctural formula for metaschoepite is [(UOr)EOr(OfDrz](HzO)ro: The transformation of schoepite (a 14.337,, 16.813, c 14.731 A, P2rca) ro meraschoepite (a 13.99, b 16;72, c 14.73 A, Pbna) is characterized by a 2Vo decrease in the a cell dimension, a slight decrease in the 6 dimension, and little or no change in the c dimension. Unit-cell changes probably reflect the reorganization of H-bonds. Differences in unit-cell volumes induce strain in crystals in which the transformadon to metaschoepite is incomplete, and stored strain energy may be-sufficient to rapidly drive the transformation of the remaining schoepite to "dehydrated schoepite" la 6.86, b 4.26, c 10.20 A, Abcm (?)l when partly altered crystals are exposed to an external stress (e.g., heat, sunlight or mechanical pressure). Metaschoepite is apparently stable in air; canary yellow altered crystals commonly consist of a polycrystalline mixture of "dehydrated schoepite" and metaschoepite. The alteration of schoepite to "dehydrated schoepite" occurs in three steps: (l) loss of all interlayer H2O from schoepite, causing collapse of the layers, (2) abmic rezrrangement wirhin the struchral sheets to a configuration that may be similar !o that of metaschoepite, and (3) funher re-ilrangement to a defect o-UOr(OH)r-type sheet. The complete reaction is [(UOJ8Or(OFt)rzf(lfzO)n '+ 8 [(UOr)O65(OH),.] + l2HrO. We propose that "dehydrated schoepite" forms an omission solidsolution over the compositional range UO3.0.75H2O to UOr.11r9, represented by the general formula (UO2)O0.25 -"(OI{)r.5 .2 (0 <;r < 0.25).

New data on metamorphic chlorite as a petrogenetic indicator mineral, with special regard to greenschist-facies rocks

Abstract: For rock-forming minerals, the extent and the behavior of solid solution are understandable only if considered in ttre context of a rigorous petrological framework, as has been shown repeatedly in the met?morphic perology literature. Herein, we present a study of the composition of trioctahedral chlorites, with special focus on nen-limigillg minsl4l 455e6!lages in rocks of low metamorphic grade. The goal has been to discern and understand any systematic changes of chlorite composition due to changing T' P' and bulk-rock composition. The three main compositional variations shown by the metamorphic chlorite in this study include the ratio Fe/Mg, the extent ofTschermak substitution, and deviation from trioctahedral toward dioctahedral chlorite. Our database includes 2619 chlorite compositions, of which 450 are selected from literature covering the temperature range subgreenschist - amphibolite facies, and,2169 are newly determined compositions from greenschist-facies rocks. All samples used are classified according to metemorphic grade, pressure, and brtk-rock composition, thereby establishing groups and subgroups of analytical data. These data are plotted on diagrams aimed at enabling one to discern more specifically how bulk-rock composition, temperatue and pressure affect the three main compositional variations of chlorite from typical meramorphic rocks. Each of ttrese parameters controls chlorite composition to some extent, but the control by the bulk-rock chemistry is clearly dominant. Comm661y it largely obscures the systematic compositional changes caused by temperatue and pressure. Thus, despite numerous attempts, it is evident that chlorite composition by itself is non-viable for geothermobarometric purposes in the case of the non-lirniting assemblages tlpical of the greenschist facies. Attempts to use chlorite from such assemblages for geothermobarometry should be restricted solely to approaches involving cation exchange with some coexisting phase(s).

Rigid-body character of the SO4 groups in celestine, anglesite and barite

Abstract: The crystal srucnres of natural celestine (Srt.m)S04, anglesite @be.eSre.er)SO+, and barite (Bao.geSro.or)SOa have been refined in space group Pbnm lu{lizing rotating anode, Iv[o X-ray diffraction data from single crystals. Unit-cell parameten for celestine are a 6.8671(7), b 8.3545(8), and c 5.3458(6) A, for anglesite, a 6.9549(9), b 8.472(l), and c 5.3973(8) A, and for baite, a 7.154(l), b 8.879Q), and c 5.454(1) A. Strucnral data are presented for these sulfates with geatly improved precision over previous studies owing to high peak+o-background intensity ratios and precise analytical absorption corrections. The final model R(F) values are 0.025, 0.041, and 0.019, for celestine, anglesite, and barite, respectively. The average bond-distance from divalent cation to the nearest twelve oxygen atoms is 2.827(l) A in celestine, 2.864(5) A in anglesite, and 2.951(2) A in barite. Theavera9e sulfur-to-oxygen bond distance is 1.475(2) Ancelestine, 1.476(6) A in anglesite, arndl.476(2) Anbarite. The sulfate tetrahe&a in each structwe show very similar distortions that are attributed to the bonding of the various oxygen atoms to the divalent cations, which is similal in each structure. Thus, the different metal cations do not seem to a.ffect the size or shape of the sulfate tetrahe&a. An analysis of the displacement parameters suggests that the SOa groups behave as rigid molecular units, with an apparent shortening of the S-O bonds of 0.008-O.010 A.

The Richemont rhyolite dyke, Massif Central, France; a subvolcanic equivalent of rare-metal granites

Abstract: A rhyolitic dyke at Richemont, Haute-Vienne, France, is shown to be the subvolcanic counterpart of rare-metal-bearing granites and pegmatites. Textural relationships provide evidence for a quenched silicate melt with less than 5% phenocrysts, consisting of albite (50%), quartz (20%), K-feldspar (20%), and muscovite (10%). The affinity with pegmatites arises from the ore mineralogy, with "uran-euxenite" and "wolframo-ixiolite" as the main rare-metal carriers. An affinity with rare-metal granites arises from the geochemistry. The composition of the rhyolite is quite similar to that of the Beauvoir Ta-Li-bearing granite, corresponding to the high-phosphorus, high-fluorine class of strongly peraluminous leucogranites, enriched in Ta, Nb, Sn, Li, and Be. The melt belongs to the family that typically crystallizes as LCT granitic pegmatites. Analyses of muscovite phenocrysts provide estimates of muscovite-melt partition coefficients, allowing an explanation of some geochemical characteristics of this type of magmatism, e.g., Li, Cs, Ta and Sn increase during differentiation, and patterns of fractionation involve the ore elements W, Nb and Ta. Isolated aggregates of phosphate with inclusions of sulfosalts and of Nb, W, and Sn oxides are interpreted as signaling the onset of silicate-phosphate melt unmixing processes. Microcrystalline facies of the rhyolite are tentatively identified as the result of complete fluid unmixing, allowing a qualitative assessment of element extraction by fluids escaping from high-F melts. Although Sn, Ti, Th, Nb, and Ta are not depleted relative to the melt, other elements are removed, mildly in the case of Rb, Mn, S, Be, and Zn, strongly in the case of U, Li, B, As, F, and Ba, and very strongly in the case of W, Sb, Sr, P, and Ca.

Niobium-tantalum oxide minerals from complex granitic pegmatites in the moldanubicum, czech republic : primary versus secondary compositional trends

Abstract: Compositional trends of Nb, Ta-oxide minerals in four complex granitic pegmatites (lepidolite and petalite subtypes) in the Moldanubicum, Czech Republic, covering primary (magmatic) and secondary (hydrothermal replacement) stages of crystallization, were investigated. The primary stage is characterized by ferrocolumbite to manganocolumbite, followed after Fe- and Mn-depletion by Ca- or Sb-rich minerals: microlite (Nova Ves), rynersonite (Chvalovice) and stibiotantalite (Dobra Voda, Lastovicky). Compositional paths in columbite are similar to those of lepidolite pegmatites; the other primary Nb, Ta-oxide minerals exhibit Ta/(Ta+Nb) values significantly higher relative to columbite. Decreasing Nb/Ta and the cation sequence Fe-Mn-(Sb,Ca) are typical of the primary stage. The secondary stage displays a broad spectrum of mainly fracture-filling secondary phases, such as diverse microlite-group minerals, cesstibtantite, manganotantalite, ferrotantalite and ferrotapiolite, which replace stibiotantalite, microlite and rynersonite; columbite in outer pegmatite units remains unaffected. The Ta/(Ta+Nb) values of the secondary phases are comparable to those of their precursors. The general sequence of major A-site cations in hydrothermal Nb, Ta-oxide minerals, (Sb)-Ca-Mn-Fe, is reversed relative to that of the primary stage, and it may represent a universal pattern of hydrothermal replacement of primary Nb, Ta-oxide minerals in comparable granitic pegmatites. The composition of microlite-type minerals and textural relations indicate that the secondary hydrothermal stage includes a broad range of P-T-X conditions from early subsolidus replacement at approximately 500-350 degrees C, approximately 2.5-2.0 kbar (Nova Ves, Dobra Voda) to near-surface weathering at

Compositional variations in Cu-Ni-PGE sulfides of the Dunka Road Deposit, Duluth Complex, Minnesota; the importance of combined assimilation and magmatic processes

Abstract: The Dunka Road deposit is one of ten occurences of Cu-Ni sulfides bearing platinum-group elements (PGE) on the northwestern margin of the Duluth Complex, in Minnesota. Mineralization has been linked to contamination of the host troctolitic magma through assimilation of argillaceous rocks from the Virginia Formation. On the basis of texture and composition, the sulfide mineralization is divided into five types: 1) norite-hosted disseminated sulfides, 2) troctolite-hosted disseminated sulfides, 3) PGE-rich disseminated sulfide horizons, 4) pyrrhotite-rich massive sulfides, and 5) chalcopyrite-rich disseminated sulfides. The norite-hosted sulfides exhibit featues suggestive of the magma's substantial contamination, such as high proportions of pyrrhotite and arsenide minerals, and high mean values of S/Se (9,700) and δ34S (ll.2%₀). They are also generally metal-poor, implying that the sulfides interacted with a relatively low volume of silicate melt (i.e., low R factor). The troctolite-hosted sulfides formed at moderate degrees of contamination, as indicated by their intermediate mean values of S/Se (4,600) and δ34S (7.8%₀). The PGE-rich sulfide horizons show little sign of contamination, and have mantle-like mean values of S/Se (2,600) and δ34S (2.1%o). Their very high PGE contents suggest that they formed at elevated R factors. The pyrrhotite-rich massive sulfides and associated chalcopyrite-rich disseminated sulfides have relatively high mean values of S/Se (8,000) and δ34S (10.2%o), indicative of significant contamination. The former are interpreted to represent a cumulate of monosulfide solid-solution (mss), whereas the chalcopyrite-rich sulfides represent the fractionated sulfide liquid. A general increase in the degree of contamination is observed toward the base of the intrusion, associated with a decrease in R factor and metal concentration of the sulfides. This likely results from the introduction of partial melt from the metasedimentary country-rocks, which was cooler than the mafic magma and led to the early crystallization of the sulfide liquid.

Composition of complex lepidolite-type granitic pegmatites and of constituent columbite-tantalite, chedeville, massif central, france

Abstract: The Chedeville field of granitic pegmatites extends over ca. 1200 m along a SW-NE trend in the southeastern part of the Saint-Sylvestre granite, northern Massif Central, France. The occurrence of lepidolite as the main lithium mineral, and the geochemical characteristics of the pegmatites, enriched in Li, Rb, Cs, Ta, and Sn, point to an affiliation with the rare-element class, complex type, lepidolite subtype. Vein-like pegmatite bodies have a well-developed internal structure, with a coarse upper zone, a fine-grained banded zone, an aplitic zone, and a lower layered zone with vertical wedge-shaped crystals of feldspar. Flow banding is common, and a repetition of the zonation provides evidence of multiple batches of magma. Metasomatism at the margins between units or of enclaves leads to the development of lepidolite-rich rocks. The geochemical characteristics of associated aplites match those of other occurrences of rare-metal granite at several localities in the northern Massif Central. However, Nb-Ta mineral characteristics are different from those of the granitic rocks. Uranmicrolite is restricted to the purple lepidolite zones, and columbite-tantalite is more Mn-rich than in granites. The Mn/(Fe+Mn) value actually varies over a narrow range (+ or -0.01) within each sample, but the variation between different bodies of pegmatite (from 0.90 to 0.96 and 0.99) again suggests multiple intrusions. Metasomatic units (accounting for less than 5% of the bodies) are characterized by a lower Nb/Ta in the whole rock and a higher Mn/Fe in the columbite-tantalite than in corresponding aplitic parts. Such rocks may be strongly enriched in Zr, Hf, and Th, suggesting a significant mobility of these elements at the metasomatic stage.

The composition of chrysotile and its relationship with lizardite

Abstract: Mdssbauer data were obtained from 14 specimens of chrysotile taken from three geologically well-characterized serpentinites. These data were used in conjunction with results of electron-microprobe (major elements) and uranium-extraction analyses (HrO) to generate a comprehensive set of compositions for chrysotile. Chrysotile contains both Al and Feh as secondary tetrahedrally coordinated cations, c/ith Al dominating over Feh. The proportion of t41Fe3+ and t6lFelr shows an inverse correlation that preserves a relatively constant total Fe3+ content. Most specimens have low FeryFe2*. The incorporation of trivalent cations is greater in the sheet of octatredra than in the sheet of terahedr4 suggesting the presence of H* vacancies; this result is consistent with measured HrO contents. The M6ssbauer parameters for chrysotile are similal but more scattered than those for lizardite, suggesting minimal differences in coordination polyhedra between the two minerals. However, chrysotile and lizardite are not polymorphs in nanrral systems. Compared to the associated lizardite, chrysotile conlains more Fe2* and I4]AI and fewerlalFeh ions arnd H* vacancies. These data support the hypothesis that high Fe2* content and H* vacancies contribute to the replacement of lizardite by chrysotile, and vice versa, duing serpentine replacement.

The role of Ti in hydrogen-deficient amphiboles; sodic-calcic and sodic amphiboles from Coyote Peak, California

Abstract: The crystal structues of eleven sodic--calcic and sodic amphiboles from lithic-wacke inclusions in the alkali ultramafic diaEeme at Coyote Peak, Humboldt County, Californi4 have been refined to R valtes of l--2%o ttsing single-crystal MoKcr X-ray data. The crystals used in the collection of the intensity data were subsequently analyzed by electronand ion-microprobe techniques. They were analyzed for H and Li, and the unit formulae were calculated on the basis of 24(O,OH,F). Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. These amphiboles range in composition from fluororichterite and fluorian eckermannite through titanian fluorian potassic-richterite, titanian fluorian richterite and titanian fluororichterite to titanian oxygenian arfuedsonite; Ti contents range from 0.121o 0.75 apfu, and (OH + D contenrs range from 2.00 ta 0.84 apfu. Where Ti <0.13 apfu, (OH + D * Z.O apfu. Where Ti > 0.13 apfu, Ti varies linearly with rhe amount of 02at the O(3) sire [= 2 (OH + DJ, with a slope of 0.52. Thus Ti is incorporated into these amphiboles vra the substirution Ti4 + 2 02(Mg,F"2*) + 2 (OH)-. Variation in MQ)aQ), M(3)O(3) nd M(llM(l) distances as a function of Ti content of the amphibole indicates that all Ti in excess of O.l3 apfu ocatrs at theM(l)s i te,wherei t isassociatedwithashortM(l ) -O(3)distance;Tiupto0.13 apfuocrwsLttheM(z)s i re.Thepresenceof 02at the O(3) site in these amphiboles induces a range of values of both iie OtSXiiOFo(7) angle, and the difference between the M( )-O(S) md M(4)4(6) bond-lengths nor found in normal tO(3) = 611,ry amphiboles. Some fearures of the formula derived from electron-microprobe results can be used as diagnostic indicators of the presenc€ of extensive t6ll-i and 02substitutions in monoclinic amphiboles.

Apatite as a monitor of fractionation, degassing, and metamorphism in the Sudbury igneous complex, Ontario

Abstract: Apatite occurs as an acc€ssory phase throughout the Sudbury Igneous Complex (SIC), Ontario, a layered igneous complex consisting deminsntly of norite, quartz gabbro, and granophfe. Apatite also is present in the overlying tuffaceous rocks of the Onaping Formation. Representatiys samples from tbree traverses (NW, NE, SW) across the complex have been studied in detail; the NW suite is the most pristine. Apatite is found primarily as a post-cumulus (or intergranular) phase, but becomes a cumulus mineral in the quartz gabbro. Despite textural signs ofrapid growrtr, the apatite is homogeneous. It is F-rich; Cl and OH contents decrease from the base of the complex upward, as in several other layered intrusions. In the SIC, Cl and OH probably decreased relative to F upon vapor saturation of the evolving melt. The concentrations of LREE, up to 2 wt%o I-a2O3 + Ce2O3 + Nd2O3, are highest in apa-tite nerlr the base of the complex. The chondrite-normalizd kEE patterns and ETSrF6Sr values (0.707-0.708) are similar in apatite from each rock type of the SIC. The complex evolved normally by fractional crystallization of a single barch of highly contaminated basic magrna\" The high initial tTSrF6Sr values in apatite indicate that the relatively silica-rich basic magura had a substantial crustal component, or possibly consisted entirely ofremehed crust. The apatite ftom the norite and that from the granophyre had a common magmatic source. Apatite from metamorphosed pans of the complex has signi-ficantly lower levels of Cl and LREE, compared to primary apatite from the NW section; furthermore, its rsr/t6sr value has been reset (up to 0.739) by exchange of strontium with adjacent K-rich minerals during recrystallization.

Evolution of Nb,Ta-oxide minerals in the Prasiva granitic pegmatites, Slovakia; II, External hydrothermal Pb,Sb overprint

Abstract: Titanite, fersmite, pyrochlore-group minerals and romeite in two small dikes of relatively poorly fractionated pegmatite, located in the parent Hercynian Prasiva biotite granodiorite-granite, in central Slovakia, were generated by hydrothermal alteration of primary columbite, titanian ixiolite and niobian-tantalian rutile. The early titanite has elevated contents of Nb, Ta, Al and Fe, and the fersmite is very REE-poor. Pyrochlore, microlite, betafite, uranpyrochlore, uranmicrolite, stibiomicrolite, stibiobetafite, plumbomicrolite and several transitional compositions were identified in the next generation, most of them Si-bearing. The late romeite is considerably enriched in Nb, Ta, U and Si. The hydrothermal fluids mobilized Nb, Ta, Ti, U, Fe, Si, and probably also Ca and Na from the primary minerals of the pegmatite, but Sb and Pb must have been imported from an external source. The hydrothermal overprint is attributed to the same metamorphic-magmatic solutions that caused the nearby Sb+ or -Au and Pb-Zn-Sb sulfide deposits.

Compositional variation of tourmaline in the granitic pegmatite dykes of the Cruzeiro mine, Minas Gerais, Brazil

Abstract: Schorl-dravite and schorl-elbaite from different zones of the rare-element-enriched complex granitic pegmatite dykes of the Cruzeiro mine, in Minas Gerais, Brazil, were analyzed by electron and ion (H, Li, B) microprobes. General mechanisms of substitution (common to schorl and to elbaite independently of their position in the pegmatites) drive compositions to "proton-deficient schorl", "alkali-free schorl", and "alkali-free elbaite" end-members, each involving Al enrichment. Among the specific mechanisms (i.e., those characterizing tourmaline within each separate zone), "octahedral-defect" and "alkali-free proton-rich" types are particularly important in OH-rich, pocket elbaite. The substitution x []+OH (super -) x Na (super +) +O (super 2-) , driving compositions to an "alkali-free proton-rich elbaite", is proposed to explain both the lack of Na and the excess of (OH+F). Bond-valence calculations based on results of unpublished structure-refinement data allow the OH excess to be assigned to the O(2) site. The composition of the tourmaline in the Cruzeiro suite (high Al content; Mg, Fe, Zn, Mn, and Li covariation; antithetic behavior of OH and F) records the evolution of the pegmatite-forming melt.

Sm-nd isotope systematics and the derivation of granitic pegmatites in southwestern maine

Abstract: The Nd isotopic compositions of monazite and apatite are used to assess the initial isotope systematics of approximately 270 Ma granitic pegmatites in the Topsham area of southwestern Maine. The isotopic compositions are compared to values of spatially associated granites and country-rock migmatites to constrain potential sources of the pegmatites. The pegmatites form two groups: (1) the Northern series, which comprise the majority of the pegmatites exposed in the area, lack abundant rare-earth-element-enriched minerals, and have epsilon Nd (270 Ma) in the range -2.2 to -1.4; (2) the Standpipe Hill series, distinguished by an enrichment in rare-earth-element minerals, displays a epsilon Nd (270 Ma) in the range -3.4 and -3.3. Data for each group are internally consistent and suggest that the different pegmatite series were not derived from a single isotopically uniform source. The source of the Standpipe Hill series resembles adjacent biotite leucogranite [epsilon Nd (270 Ma) between -3.9 and -3.7]. The Northern series pegmatites have Nd isotopic characteristics similar to both migmatites that they intrude [epsilon Nd (270 Ma) between -2.9 and +0.8], and fine-grained biotite granites located ca. 15 km east of the pegmatites [epsilon Nd (270 Ma) between -2.5 and -1.7]. The isotopic data demonstrate that spatially and temporally related pegmatites need not be derived from identical sources.

Scandium substitution in columbite-group minerals and ixiolite

Abstract: Columbite-group minerals and ixiolite with extremely variable concentrations of scandium are widespread in moderately to highly fractionated rare-element granitic pegmatites. Columbite-group minerals with 1-3 wt.% Sc 2 O 3 are referred to as scandian columbite-tantalite and show degrees of structural order similar to columbite-tantalite lacking Sc. Disordered structures remain orthorhombic and become ordered upon heating. Pseudo-orthorhombic stannian (Sn-rich), titanian (Ti) and wolframian (W) variants of ixiolite may contain as much as 3.7 wt.% Sc 2 O 3 and revert to monoclinic phases upon heating. Similarly, scandian ixiolite that contains Sc in excess of 3.0 and up to 18.8 wt.% Sc 2 O 3 converts from orthorhombic to monoclinic symmetry upon heating and has a stoichiometry that approaches Sc(Nb, Ta)O 4 . Scandium-bearing columbite-tantalite and ixiolite show similar ranges in Mn/(Mn+Fe) and Ta/(Ta+Nb) values, but noticeably different Sn, Ti and Sc contents. Scandium is incorporated into the columbite and ixiolite structures via the coupled substitution Sc (super 3+) +(Ti,Sn) (super 4+) = (Fe,Mn) (super 2+) +(Nb,Ta) (super 5+) ; it is strongly partitioned into the (Fe,Mn) site, whereas the Ti and Sn prefer the (Nb,Ta) site. Within the (Fe,Mn) site, the substitution of Sc for Fe is more prevalent. In most pegmatites, Sc fractionation in columbite-tantalite, stannian ixiolite, titanian ixiolite and wolframian ixiolite is erratic, unlike scandian ixiolite, which shows strong enrichment in Sc with increasing Mn, Ta and Sn.

Instability of perovskite in a CO 2 -rich environment; examples from carbonatite and kimberlite

Abstract: Intricate multiphase pseudomorphs after perovskite (Nb-, LREE-poor) from calcite carbonatite (Sebljaw complex, Kola Peninsula, Russia) and serpentine catcite kimberlite (kon Mountain, Wyoming) are described. In the kimberlite, the major products of perovskite replacement are (in order of crystallization): kassite, anatase and 6itanils plu5 calcite, ilmenite, LREE-Ti oxide [? lucasite-(Ce)]. In the carbonatite, perovskite is initially replaced by anatase plus calcite and, subsequently, ilmenite and ancylite(Ce). In both instonces, the development of calcite and Ti-bearing phases after perovskite involved initial progessive leaching of Ca2* from the structure followed by crystallization of ilmenite and IREE ninetals in the final stages, after the precipitation of groundmass calcile. The formation of kassite and titanite in tle pseudomorphs in kimberlite was controlled by a lower Ca leachiate and higher activity of SiOz in this system, compared with the carbonatite. J[s similarity between the fwo types of pseudomorphs results from the instability of Nb--LREE-poor perovskite in a CO2-rich fluid at low temperatures. Perovskite is ionsidered an unsuitable host for radioactive sSr and REE isotopes owing to the low resistance of CaTiO: to leaching and incompatibility of Sr and the rare earths with the products of perovskite replacement.

Wodginite-group minerals from the Separation Rapids rare-element granitic pegmatite group, northwestern Ontario

Abstract: Wodginite (ideally MnSnTa 2 O 8 ), and the rarer species, ferrowodginite (ideally Fe (super 2+) SnTa 2 O 8 ) and titanowodginite (ideally MnTiTa 2 O 8 ), have been discovered in rare-element granitic pegmatites of the complex type, petalite subtype, which occur in the Separation Rapids pegmatite field, northwestern Ontario. Tungsteniferous varieties of wodginite and an unnamed wodginite ("ferrotitanowodginite") are also described from this locality. The pegmatites intrude a metavolcanic (greenstone) belt between the English River and Winnipeg River subprovinces of the Canadian Shield, where they are associated with a 2643+ or -2 Ma rare-element-enriched granitic intrusion, the Separation Rapids pluton. The tungsteniferous wodginite has an excess positive charge at the C site due to substitution of Ta by W, which is balanced by the presence of Fe (super 3+) and Mn (super 2+) at the B site via the substitutions: B [Sn (super 4+) ] + C [Ta (super 5+) ] B [Fe (super 3+) ] + C [W (super 6+) ], B [Sn (super 4+) ] + 2 C [Ta (super 5+) ] B [Mn (super 2+) ] + 2 C [W (super 6+) ], and 2 B [Sn (super 4+) ] + C [Ta (super 5+) ] B [Mn (super 2+) ] + B [Ta (super 5+) ] + C [W (super 6+) ]. These new schemes of substitution result in hypothetical tungsteniferous end-members Mn (super 2+) 4 (Fe (super 3+) 2 Sn 2 ) (W 2 Ta 6 )O 32 , Mn (super 2+) 4 (Mn (super 2+) 2 Sn 2 ) (W 4 Ta 4 )O 32 , and Mn (super 2+) 4 (Mn (super 2+) Sn 2 Ta) (WTa 7 )O 32 . At Separation Rapids, the rare-metal oxides follow two distinct evolutionary paths: (i) ferrocolumbite --> ferrocolumbite + ferrowodginite --> ferrotantalite + ferrowodginite --> microlite-group minerals, and (ii) manganocolumbite --> manganocolumbite + wodginite --> manganotantalite + wodginite --> microlite-group minerals. Sequence (i) is considered to have arisen from a relatively F-poor magma, and sequence (ii), from a magma richer in F, where extreme enrichment in Mn was perhaps achieved through F-complexing. Wodginite-group minerals are most often found in the albite-enriched regions of the pegmatites. Magmatic fractionation is considered to be the major process controlling concentration of the rare elements. Titaniferous wodginite compositions cannot be explained by simple magmatic fractionation (from a Fe- or Mn-rich wodginite starting composition); localized interaction of the pegmatite-forming magma with mafic metavolcanic rocks is proposed for their origin. Of wodginite-group minerals worldwide, only the "giant" Tanco pegmatite at Bernic Lake in Manitoba hosts wodginite with a spread of compositions comparable to that at Separation Rapids. On the basis of a striking similarity in geological setting, mineralogy and age of emplacement with the rare-element pegmatite groups commencing 40 km west in Manitoba (including Tanco), we contend that the Separation Rapids Pegmatite Group constitutes the eastern limit of the Cat Lake-Winnipeg River Pegmatite Field.

Changes in layer organization of Na- and Ca-exchanged smectite materials during solvent exchanges for embedment in resin

Abstract: The embedding process designed for impregnation i resin of hydrated clays for TEM observation comprises four steps of exchange by solvents and resin. Clay pastes of Naand Ca-exchanged Wyoming smectite were prepared at low suction pressures (3.2 and 100 kPa, respectively), and their layer organization was examined at different steps of the embedding p'rocess. X-ray diffraction was used in order to follow the evolution of layer distances and particle orientation during solvent exchanges. At 3.2 kPa as well as at 100 kPa the water-saturated clay exhibits interlayer distances of 1.9 nm. Alter methanol exchange, interlayer distrnces collapsed to 1.6-1.7 nm. With l,2-epoxypropane and resin saturation, clays behave in a similar way as with methanol. Examination of layer-stacking coherency by measurement of peak widths indicates that the fust exchange by methanol is the most critical step in the embedding process because it induces reduction of the layer distances and aggregation' especially in the case of dilute Na-exchanged clay. The final polymerization of resin inroduces further slight changes in organization of the clay.

Geological setting and petrogenesis of symmetrically zoned, miarolitic granitic pegmatites at Stak Nala, Nanga Parbat-Haramosh Massif, northern Pakistan

Abstract: Miarolitic granitic pegmatites in the Stak valley in the northeast part of the Nanga Parbat-Haramosh Massif, in northern Pakistan, locally contain economic quantities of bi- and tricolored tourmaline. The pegmatites form flat-lying sills that range from less than 1 m to more than 3 m thick and show symmetrical internal zonation. A narrow outer or border zone of medium-to coarse-grained oligoclase--K-feldspar-quartz grades inward to a very coarse-grained wall zone characterized by K-feldspar-oligoclase-quartz-schorl tourmaline. Radiating sprays of schorl and flaring megacrysts of K-feldspar (intermediate microcline) point inward, indicating progressive crystallization toward the core. The core zone consists of variable mixtures of blocky K-feldspar (intermediate microcline), oligoclase, quartz, and sparse schorl or elbaite, with local bodies of sodic aplite and miarolitic cavities or "pockets". Minor spessartine-almandine garnet and lollingite are disseminated throughout the pegmatite, but were not observed in the pockets. The pockets contain well-formed crystals of albite, quartz, K-feldspar (maximum microcline+ or -orthoclase overgrowths), schorl-elbaite tourmaline, muscovite or lepidolite, topaz, and small amounts of other minerals. Elbaite is color-zoned from core to rim: green (Fe (super 2+) - and Mn (super 2+) -bearing), colorless (Mn (super 2+) -bearing), and light pink (trace Mn (super 3+) ). Within approximately 10 cm of the pegmatites, the granitic gneiss wallrock is bleached owing to conversion of biotite to muscovite, with local quartz and albite added. Schorl is disseminated through the altered gneiss, and veins of schorl with bleached selvages locally traverse the wallrock up to 1 m from the pegmatite contact. The schorl veins can be traced into the outer part of the wall zone, which suggests that they formed from aqueous fluids derived during early saturation of the pegmatite-forming leucogranitic magma rich in H 2 O, F, B, and Li. Progressive crystallization resulted in a late-stage sodic magma and abundant aqueous fluids. Two late stages of volatile escape are recognized: the first stage caused pressure-quenching of the last magma, which produced aplite and caused albitization (An 3 to An 8 ) of earlier crystallized K-feldspar and oligoclase. The second stage, released during the rupture of miarolitic cavities, produced platy albite ("cleavelandite," An 1 ) locally associated with F-rich muscovite and elbaite. Albitization is likely due to cooling of alkali-fluoride-dominated fluids at less than 2 kbar pressure. The pegmatites are derived from Himalayan leucogranitic magma emplaced prior to 5 Ma into granulitic gneiss that was at 300 degrees to 550 degrees C and 1.5 to 2 kbar. The pegmatites were emplaced during uplift of the Haramosh Massif, since they cross-cut ductile normal faults but are cut by brittle normal faults. Economically important pink tourmaline mineralization formed in pockets concentrated near the crest of a broad antiform, as a result of trapping of late magmatic aqueous fluids that had become Fe-poor owing to the prior crystallization of schorl.