Chevkinite Group Research Articles

Refinement of the crystal structure of christofschäferite-(Ce) and the modular aspect of the chevkinite polysomatic series with the general formula of {A<sub>4</sub>B(T<sub>2</sub>O<sub>7</sub>)<sub>2</sub>}{C<sub>2</sub>D<sub>2</sub>O<sub>8</sub>}<sub>m</sub> (m = 1, 2)

Research subject. The crystal structure of christofschäferite-(Ce) was previously refined in terms of the P21/m low-symmetrical space group, which allowed the local features of cationic arrangements to be determined. In this work, we set out to refine the crystal structure of christofschäferite-(Ce) in terms of the P21/a high-symmetrical space group based on the previously collected diffraction data. A topology-symmetrical analysis of the members of the chevkinite group with the general formula of A4BC2D2(Si2O7)2O8 was conducted. Materials and methods. A magmatic rock sample with christofschäferite-(Ce) inclusions was found in the vicinity of the Laacher See volcano, near Mendig, Eifel Mountains, Rhineland-Palatinate (Rheinland-Pfalz), Germany. The crystal structure was studied using single-crystal X-ray analysis. Results. Despite an increase in the symmetry to the P21/a space group (in comparison with the previous data with the P21/m space group), the main patterns of cation distribution between the octahedral and tetrahedral sites are preserved. However, due to the lover number of cationic sites, this distribution becomes more disordered. Based on a crystal chemical analysis of the crystal structures of natural and synthetic members of the chevkinite group in the framework of the OD theory, it is possible to combine them into a united OD family with the same OD groupoid. Conclusions. According to the OD theory, there are two structural OD-subgroups of the chevkinite group (chevkinite and perrierite). The crystal structure and symmetry of possible MDO-polytypes are predicted.

Perrierite-(Ce) from the Laacher See tephra, Eifel, Germany, and the modular character of the chevkinite group of minerals

Perrierite-(Ce) crystals from ejecta of the Laacher See volcano, Eifel, in Germany were studied by X-ray single-crystal diffraction and electron microprobe analysis. The composition and crystal structure of this sample is discussed in relation to the known properties of the chevkinite group minerals and related synthetic compounds. Taking into account the modular character of the chevkinite minerals, based on a rutile and a silicate module, the preferred formation of either the perrierite or the chevkinite structure type is correlated with the formal charge of the rutile and silicate modules. The rutile module is expected to carry a negative charge, compensated by a positive charge of the silicate module. On average, the charge modulus is observed to be larger for the chevkinite-type module stacking. It can drop to zero in perrierite-type structures rich in Sr or Ca. In such cases, it is generally not expected to rise above two. The perrierite-(Ce) described in this study crystallizes in space group C2/m. The anisotropic oxygen and rare-earth element displacements observed in this crystal indicate a local domain structure of P2_1/a symmetry, when compared to the P2_1/a symmetry of the synthetic perrierite hbox {La}_4hbox {Mg}_2hbox {Ti}_3hbox {O}_8(hbox {Si}_2hbox {O}_7)_2.

Volcanic-Derived Placers as a Potential Resource of Rare Earth Elements: The Aksu Diamas Case Study, Turkey

Rare earth elements (REE) are essential raw materials used in modern technology. Current production of REE is dominated by hard-rock mining, particularly in China, which typically requires high energy input. In order to expand the resource base of the REE, it is important to determine what alternative sources exist. REE placers have been known for many years, and require less energy than mining of hard rock, but the REE ore minerals are typically derived from eroded granitic rocks and are commonly radioactive. Other types of REE placers, such as those derived from volcanic activity, are rare. The Aksu Diamas heavy mineral placer in Turkey has been assessed for potential REE extraction as a by-product of magnetite production, but its genesis was not previously well understood. REE at Aksu Diamas are hosted in an array of mineral phases, including apatite, chevkinite group minerals (CGM), monazite, allanite and britholite, which are concentrated in lenses and channels in unconsolidated Quaternary sands. Fingerprinting of pyroxene, CGM, magnetite and zircon have identified the source of the placer as the nearby Gölcük alkaline volcanic complex, which has a history of eruption throughout the Plio-Quaternary. Heavy minerals were eroded from tephra and reworked into basinal sediments. This type of deposit may represent a potential resource of REE in other areas of alkaline volcanism.

Reavaliação do Complexo Jequié: Um Olhar Geocronológico Sobre a Geologia da Área Jequié-Itagi (Bahia, Brasil)

New geological and litogeochemical data plus La-ICP-MS U-Pb isotopic study on the Jequie Complex, or Jequie “Block”, a granulite facies terrain of the Northeastern Sao Francisco Craton, allowed to propose that a new lithodemic unit, the Volta do Rio Plutonic Suite, should be created as a lower rank unit. The Jequie Complex is defined here as an intrusive complex metamorphosed in the granulite facies, containing an assemblage of plutonic calc-alkalic mafic to intermediate rocks, fractionated trondhjemites, tonalites and granodiorites, besides normal calc-alkaline tonalites/ granodiorites and rare metasedimentary remnants plus two different sets of leucogranites. The Volta do Rio Plutonic Suite is proposed as a lower range lithodemic unit belonging to the Jequie Complex. It is composed of granodiorites and monzogranites with porphyroclastic texture, even- -grained granitoids and fine-grained granitoids, besides an association of amphibole-bearing leucogranites and mafic to intermediate rocks. The metamorphosed mafic, intermediate and felsic rocks of the Jequie Complex compose a Cordilleran-type magnesian calc-alkalic association, which age is 2.7 Ga. In contrast, all the metagranitoids of the Volta do Rio Plutonic Suite show a distinctive ferroan (“A-type”) geochemical signature and the mafic and intermediate rocks associated to metaleucogranites show alkaline characteristics and host locally high-grade REE mineralizations contained in chevkinite group minerals. In the Volta do Rio Plutonic Suite, the porphyroclastic granites were dated at 2.6 Ga.The provisional age of alkaline mafic rocks with magmatic-hydrothermal REE mineralizations and of possibly coeval leucogranites is 2.5 Ga. These mineralizations are inedit in the world and the obtained time frame indicates the need to re-evaluate the geology and metallogenic potential of the Jequie Complex by considering its primordial igneous nature and by screening out its “granulitic” or “charnockitic” nature.

Composition, paragenesis, and alteration of the chevkinite group of minerals

The chevkinite-group minerals (CGM) are dominantly monoclinic REE-Ti-Fe sorosilicates [(REE, Ca)(4)Fe2+(Fe2+, Fe3+, Ti)(2)Ti-2(Si2O7)(2)O-8)], with REE2O3 contents up to similar to 50 wt%, but members with predominant Mg, Al, Mn, Cr, Sr, or Zr in one of the cation sites are also known. Twelve members have been approved by the Commission of New Minerals, Nomenclature and Classification International Mineralogical Association (CNMNC-IMA) but more will undoubtedly be identified. Minerals of the group are known from hundreds of terrestrial localities and have also been recorded in lunar and martian rocks. The main occurrences are in igneous rocks ranging from diamond in kimberlites through mafic and intermediate lithologies to metaluminous and peralkaline felsic rocks. They also occur in metamorphic rocks, including granulites, metacarbonates, and jadeitites, and in metasomatic rocks, such as skarns and fenites, and in rare-metal deposits. Chevkinite-group minerals may form over the pressure range 50 to The stability of CGM vis-a-vis other REE-Ti-bearing accessories is poorly understood. They are often the major carriers of REE and actinides, and they have a high potential for fractionating the light lanthanides and Th from U. Very little systematic work has been done in determining CGM-melt partition coefficients, yet such data are critical in, inter alia, geochemical modeling. Similarly, CGM are amenable to geochronology due to their high Th abundances, commonly at the several percent level. In common with other REE-bearing accessories, CGM are prone to alteration by hydrothermal fluids. The nature and extent of the alteration are primarily determined by the composition of the fluids. Fluids poor in ligands tend to generate a Ti-enriched phase whose nature is unknown but is probably amorphous. With increasing F + CO2 levels, complex replacement assemblages are formed, usually in more than one step. Although observational evidence of the effects of alteration and element mobility is accumulating and chemical equations can be constructed to approximate the reactions, there is still no firm geochemical basis for understanding element redistribution during these processes.

Temperature and pressure induced phase transitions in chevkinite group. A joint XRD, XPS and EPMA structural studies

Temperature and pressure induced phase transitions in chevkinite group. A joint XRD, XPS and EPMA structural studies

Structure of Sr-Zr-bearing perrierite-(Ce) from the Burpala Massif, Russia

AbstractSr- and Zr-bearing perrierite-(Ce) occurring in aegirinized syenite pegmatites of the Burpala massif, Russia, is compositionally intermediate between perrierite-(Ce) and hezuolinite and occupies a compositional gap in minerals of the chevkinite group. Its crystal structure has been determined using a single-crystal diffractometer fitted with a CCD detector and MoKα X-ray radiation. The mineral is monoclinic; a = 13.815(1), b = 5.668(1), c = 11.842(1) Å , β = 113.843(3)º, V = 848.18(4) Å3, space group C2/m, Z = 2. The crystal structure was refined with the occupancies [(Ce1.2La1.0Nd0.15) (Sr1.0Ca0.5Na0.15)]4(Zr0.5Fe0.3Mn0.2)(Ti1.3Fe0.7)2Ti2(Si2O7)2O8 on the basis of chemical composition although the allocation of cations to particular sites was performed on the basis of the number of refined electrons in each unique site. The dominance of Zr in the B site links the Burpala perrierite-(Ce) to more Sr-Zr-rich members of the chevkinite group, such as hezuolinite and rengeite. As in all of the perrierite members, there is a distortion of the D site octahedra, which is interpreted as due to the packing of the REE ions.

The occurrence and composition of chevkinite-(Ce) and perrierite-(Ce) in tholeiitic intrusive rocks and lunar mare basalt

Chevkinite-(Ce) and perrierite-(Ce) are the most common members of the chevkinite group of minerals. They are dimorphs, and both have the general formula A4BC2D2Si4O22, where A = REE, Y, Ca, Sr, Th; B = Fe2+, (Mn, Mg); C = Ti, Al, Fe3+, Fe2+, Cr, Mn, Mg, Zr, Hf, Nb; and D = Ti. Both have been reported from a wide range of igneous, metamorphic, and hydrothermal rocks types, but occurrences in mafic rocks are rare, with minimal chemical and crystallographic documentation. Chevkinite-(Ce) and/or perrierite-(Ce) occur with other Ti-, Zr-, and REE-bearing accessory phases in eight suites of tholeiitic dolerite from Western Australia, and in lunar mare basalt 10047. They are more abundant than has been recognized previously in mafic igneous rocks, and they are significant hosts of incompatible elements. Chevkinite-(Ce) and perrierite-(Ce) from mafic rocks have distinctive chemical compositions with higher Zr than recorded in examples from most other common rock types. Among mafic rocks, two groups are recognized based on total Fe contents in electron microprobe analyses: crystal structural analysis by electron diffraction indicates that the high-Fe group (>8 wt% FeO) is chevkinite-(Ce), while the low-Fe group (<8 wt% FeO) is consistent with perrierite-(Ce), and both minerals can occur within a single hand specimen. A previously proposed chemical discriminant is not applicable to chevkinite-group minerals from typical mafic igneous rocks and crystal structural information is required to unequivocally distinguish between the two dimorphs.

The chevkinite group: underestimated accessory phases from a wide range of parageneses

Abstract Chevkinite-group minerals are widespread in a very wide range of igneous and metamorphic parageneses, forming important components of accessory mineral assemblages. Their presence in a rock may be difficult to establish by standard optical techniques, which has contributed to their importance being underestimated; a combination of SEM and EMPA is recommended here. Currently, there are eleven IMAapproved members of the group but undoubtedly several more will be described in the near future. There is considerable compositional variation in the group, which can be expressed as: REE + M2+ C + M3+ C = Ca2+ A + Sr + Ti4+ C + Zr4+ C where A and C are structural sites. Chevkinite-group minerals strongly fractionate geochemically coherent pairs, such as LREE-HREE, Nb-Ta, Zr-Hf and Th-U, and thus play a critical role in geochemical modelling.

Compositional variation in the chevkinite group: new data from igneous and metamorphic rocks

AbstractElectron microprobe analyses are presented of chevkinite-group minerals from Canada, USA, Guatemala, Norway, Scotland, Italy and India. The host rocks are metacarbonates, alkaline and subalkaline granitoids, quartz-bearing pegmatites, carbonatite and an inferred K-rich tuff. The analyses extend slightly the range of compositions in the chevkinite group, e.g. the most MgO-rich phases yet recorded, and we report two further examples where La is the dominant cation in theAsite. Patchily- zoned crystals from Virginia and Guatemala contain both perrierite and chevkinite compositions. The new and published analyses are used to review compositional variation in minerals of the perrierite subgroup, which can form in a wide range of host rock compositions and over a substantial pressure- temperature range. The dominant substitutions in the various cation sites and a generalized substitution scheme are described.

Chevkinite-group minerals from granulite-facies metamorphic rocks and associated pegmatites of East Antarctica and South India

Electron microprobe data are presented for chevkinite-group minerals from granulite-facies rocks and associated pegmatites of the Napier Complex and Mawson Station charnockite in East Antarctica and from the Eastern Ghats, South India. Their compositions conform to the general formula for this group, viz. A4BC2D2Si4O22 where, in the analysed specimens A = (rare-earth elements (REE), Ca, Y, Th), B = Fe2+, Mg, C = (Al, Mg, Ti, Fe2+, Fe3+, Zr) and D = Ti and plot within the perrierite field of the total Fe (as FeO) (wt.%) vs. CaO (wt.%) discriminator diagram of Macdonald and Belkin (2002). In contrast to most chevkinite-group minerals, the A site shows unusual enrichment in the MREE and HREE relative to the LREE and Ca. In one sample from the Napier Complex, Y is the dominant cation among the total REE + Y in the A site, the first reported case of Y-dominance in the chevkinite group. The minerals include the most Al-rich yet reported in the chevkinite group (≤9.15 wt.% Al2O3), sufficient to fill the C site in two samples. Conversely, the amount of Ti in these samples does not fill the D site, and, thus, some of the Al could be making up the deficiency at D, a situation not previously reported in the chevkinite group. Fe abundances are low, requiring Mg to occupy up to 45% of the B site. The chevkinite-group minerals analysed originated from three distinct parageneses: (1) pegmatites containing hornblende and orthopyroxene or garnet; (2) orthopyroxene-bearing gneiss and granulite; (3) highly aluminous paragneisses in which the associated minerals are relatively magnesian or aluminous. Chevkinite-group minerals from the first two parageneses have relatively high FeO content and low MgO and Al2O3 contents; their compositions plot in the field for mafic and intermediate igneous rocks. In contrast, chevkinite-group minerals from the third paragenesis are notably more aluminous and have greater Mg/Fe ratios.

Hydroxyl groups in nonmetamict chevkinite-(Ce): a crystal chemical discussion

The minerals of chevkinite group were commonly considered to be anhydrous minerals. The infrared absorption spectrum of natural nonmetamict chevkinite-(Ce) from the aegirine-alkali granite, Mianning, Sichuan Province, China, exhibited two broad peaks in the 3600–2800 cm −1 region owing to the OH stretching. The corresponding H 2O content required for the charge balance in formula was 1.27%. The O-H ··· O bond lengths maight cover from 0.2658 to 0.2794 nm by the correlated OH stretching energies. An electrostatic charge balance for chevkinite-(Ce) based on the assigned site-population from chemical data was calculated without the hydrogen contribution. The resulting empirical bond-valence sum on O6, O8, O2, O3, O5, and O4 ranged from 1.73 to 1.95 vu. The partial substitution of O by OH may occur in four atom sites: O6, O2, O4, and O5. The small differences in the bond-valence sums between the supposed donors and acceptors may mean a mixed donor/acceptor role of the involved oxygen atoms. The IR spectral features between 3394 and 3035 cm −1 consisted of various hydrous species at different structural sites and orientations. The OH groups in the chevkinite-(Ce) appeared to be involved in local charge imbalance in the structure and to be present when the mineral crystallized hydrothermally.

Dingdaohengite-(Ce) from the Bayan Obo REE-Nb-Fe Mine, China: Both a true polymorph of perrierite-(Ce) and a titanic analog at the C1 site of chevkinite subgroup

Dingdaohengite-(Ce), ideally Ce 4 Fe 2+ Ti 2 Ti 2 (Si 2 O 7 ) 2 O 8 , is a new member of the chevkinite group minerals from the world-famous Bayan Obo REE-Nb-Fe Mine near Baotou city, Inner Mongolian Autonomous Region, North China. It occurs in the magnesian skarn in the excontact of granite within dolomitic marble. Most individual crystals vary from 0.2 to 1.0 cm in length. Associated minerals are diopside, tremolite, richterite, allanite-(Ce), magnetite, ilmenite, spinel, titanite, pyrochlore, F-rich phlogopite, fluorapatite, quartz, and fluorite, etc. Dingdaohengite-(Ce) is probably of bimetasomatic origin among Ca-Mg-carbonate rock and/or carbonatite, and REE-, F-rich postmagmatic hydrothermal solutions. The mineral is black and becoming brown black in thin fragments. It is translucent to opaque with a submetallic-metallic luster, and a brown streak. It is brittle with conchoidal fracture. No cleavage or parting is observed. Its hardness is VHN 25g 606.0–717.4 kg/mm 2 (Mohs hardness near 5.9). The measured density is 4.83(7) g/cm 3 and the calculated density is 4.88(0) g/cm 3 . Its reflectance values (for λ = 589 nm) are 11.4–12.5%. It is biaxial negative. The strongest six X-ray diffraction lines in the powder pattern [ d in A ( I ) ( hkl )] are 2.7524(100)(121), 2.7263(98)(313), 3.1978(68)(212), 2.5460(54)(304), 2.8702(52)(020), and 3.1622(46)(312). An electron-microprobe analysis on the crystal used to collect X-ray intensity data for crystal-structure refinement gives SiO 2 19.29, TiO 2 18.26, Al 2 O 3 0.04, FeO 8.49, Fe 2 O 3 1.67, ThO 2 0.16, MgO 1.32, CaO 2.17, Nb 2 O 5 0.47, Ta 2 O 5 0.00, La 2 O 3 19.53, Ce 2 O 3 28.08, Nd 2 O 3 n.d., Y 2 O 3 0.00, Na 2 O 0.00, sum 99.48 wt%; the Fe 3+ /Fe 2+ ratio was converted by Mossbauer spectroscopy. The empirical formula is (Ce 2.13 La 1.49 Ca 0.48 Th 0.01 ) Σ4.11 Fe 2+ (Ti 0.88 Fe 2+ 0.47 Mg 0.41 Fe 3+ 0.26 Al 0.01 ) Σ2.03 (Ti 1.96 Nb 0.04 ) Σ2.00 (Si 2 O 7 ) 2 O 8 , based on 22 O atoms with prevalence of Ti in the C 1 site of the structure. Dingdaohengite-(Ce) is monoclinic, a = 13.4656(15) A, b = 5.7356(6) A, c = 11.0977(12) A, β = 100.636(2)°, V = 842.39(46) A 3 , and Z = 2. The crystal structure of dingdaohengite-(Ce) was refined with space groups P 2 1 / a and C 2/ m . Pseudo-extinction was found, i.e., reflections with h + k = 2n are systematically strong, while those with h + k = 2n + 1 are weak, which show that the true space group of dingdaohengite-(Ce) is P 2 1 / a (pseudo- C 2/ m) .

Crystal Chemistry of Iron in Non-Metamict Chevkinite-(Ce): Valence State and Site Occupation Proportions

The ideal formula for chevkinite can be expressed as A 4BC 2D 2Si 4O 22. It is important to determine the valence state and site occupation proportions for Fe among the B, C, and D octahedral sites as it may help to identify different species in the chevkinite group. Non-metamict chevkinite-(Ce) from Mianxi alkali feldspar-granite, Sichuan Province, China, was investigated using Mössbauer spectroscopy. The Fe 3+/ΣFe ratio was 39.2%. A significant increase of Fe 3+ occured during metamictization and annealing for chevkinite-group minerals. In metamict samples Fe tended to lower coordination. According to the correlation between bond length and isomer shift ( IS), the quadrupole doublets with IS = 1.10 and 0.94 mm·s −1 can be assigned to Fe 2+ in the B and C octahedral sites, respectively. Based on the correlation between octahedral distortion and quadrupole splitting ( QS), the quadrupole doublets with QS = 0.86 and 0.77 mm·s −1 can be assigned to Fe 3+ in the C and D sites, respectively. The simplified formula can be revised as: Ce 4Fe 2+ (Ti, Fe 2+, Fe 3+) 2(Ti, Fe 3+) 2Si 4(O, OH) 22. It indicated that the non-metamict chevkinite-(Ce) belonged to Fe 2+ end member of the chevkinite group because Fe 2+ was the predominant component in the B site.

Crystal structure of non-metamict minerals of chevkinite group

Crystal structure of non-metamict minerals of chevkinite group

Compositional variation in minerals of the chevkinite group

AbstractThe composition of chevkinite and perrierite, the most common members of the chevkinite group, is closely expressed by the formulaA4BC2D2Si4O22, whereA= (La,Ce,Ca,Sr,Th),B= Fe2+,C= (Fe2+,Fe3+,Ti,Al,Zr,Nb) andD= Ti. TheAsite is dominated by a strong negative correlation between (Ca+Sr) and theREE. Chondrite-normalizedREEpatterns are very variable, e.g. inLREE/HREEand Eu/Eu*. TheCsite is dominated by Ti, Al and Fe2+, in very variable proportions. Most chevkinites and perrierites are close to stoichiometric, with cation sums between 12.9 and 13.5, compared to the theoretical 13. There is no single, generally applicable charge balancing substitution scheme in the group; however, the general relationshipdefines a linear array with r2= 0.91. Chevkinite and perrierite are shown to be compositionally distinct on the basis of CaO, FeO*, Al2O3and Ce2O3abundances. Chevkinite forms mainly in chemically evolved parageneses, such as syenites, rhyolites and fenites associated with carbonatite complexes. Perrierite is more commonly recorded from igneous rocks of mafic to intermediate composition. The compositional characteristics and possible structural formulae of other members of the chevkinite group are reviewed briefly.

Rengeite, Sr4ZrTi4Si4O22, a new mineral, the Sr-Zr analogue of perrierite from the Itoigawa-Ohmi district, Niigata Prefecture, central Japan

AbstractRengeite, Sr4ZrTi4Si4O22, is a new member of the perrierite–chevkinite group found in the jades from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P21/a, a = 13.97(1), b = 5.675(7), c = 11.98(1)Å, β = 114.26(8)°, V = 866 Å3 and Z = 2. The six strongest lines in the X-ray powder diffraction pattern are 3.12 (s) (40-3), 3.05 (vvs) (31-3), 2.99 (vs) (311), 2.84 (s) (020), 2.74 (s) (004), 2.20 (s) (31-5). Electron microprobe analysis gave SiO2 22.58, TiO2 29.88, ZrO2 9.49, Nb2O5 0.24, Ta2O5 0.07, Al2O3 0.20, FeO 0.10, CaO 0.43, SrO 34.32, BaO 0.13, La2O3 0.00, Ce2O3 0.38, Pr2O3 0.10, Nd2O3 0.29, Sm2O3 0.04, total 98.25 wt.%, corresponding to (Sr3.62Ca0.08Ce0.03Nd0.02Ba0.01Pr0.01)∑3.76 (Zr0.84Ti0.09Al0.04Fe0.02Nb0.02)∑1.01Ti4.00Si4.11O22 on the basis of O = 22. The unitcell parameters and chemical composition imply that rengeite is the Sr and Zr-analogue of perrierite or high-β analogue of strontiochevkinite. It is transparent, dark brown with adamantine lustre. Its streak is pale greenish brown, and no cleavage was observed. The hardness is VHN100 606–698 kg mm−2 (Mohs 5–5.5). The calculated density is 4.12 g cm−3. It is strongly pleochroic from pale green to pale greenish brown where the REE contents are &lt;1 wt.% and pale violet to greenish brown where the REE contents are between 3 and 10 wt.%. It occurs as anhedral grains in close association with titanite, zircon and tausonite in a pebble of blue titanian omphacite-jadeite rock from the seashore of Oyashirazu, Ohmi Town, in a boulder of lavender-coloured Ti-bearing jadeitite from the bed of the Kotaki-gawa river, Itoigawa City, and in a boulder of green jade from the bed of the Hime-kawa river, Itoigawa City, Niigata Prefecture, central Japan. Rengeite is considered to have crystallized by interaction between pre-existing minerals (rutile, anatase, titanite and zircon) and Sr-rich metamorphic fluid during later stage activity of high-P/T metamorphism. The name is for Mt. Renge near the locality and the Renge metamorphic belt where jadeitite deposits are found.

Primary, agpaitic and deuteric stages in the evolution of accessory Sr, REE, Ba and Nb-mineralization in nepheline-syenite pegmatites at Pegmatite Peak, Bearpaw Mts, Montana

The pegmatites at Pegmatite Peak (Bearpaw Mts., Montana) crystallized from an evolved fraction of nepheline-syenitic melt enriched in Sr, Ba, light REE and Nb. These rocks are composed essentially of microcline (up to 1.1 wt.% Na2O and 1.0 wt.% BaO), altered nepheline (replaced by analcime, zeolites, muscovite and gibbsite), and prismatic aegirine set in an aggregate of fibrous and radial aegirine. The early accessory assemblage includes Mg-Fe mica, rutile, zircon, titaniferous magnetite and thorite. Precipitation of these phases was followed by crystallization of a plethora of rare minerals enriched in Sr, Ba, light REE and Nb. Three major stages are distinguished in the evolution of this mineralization: primary, agpaitic and deuteric. Primary repositories for Sr, REE and Nb included betafite, loparite-(Ce), crichtonite and ilmenite-group minerals. Betafite (Ta-poor, REE- and Th-rich) is present in very minor amounts and did not contribute significantly to the sequestration of incompatible elements from the nepheline-syenite melt. Loparite-(Ce) evolved predominantly by depletion in Sr and Ca and enrichment in Nb, Na and REE, i.e. from strontian niobian loparite (up to 22.0 wt.% SrO) to niobian loparite (up to 17.6 wt.% Nb2O5). Crichtonite contains minor Na, Ca and K, lacks detectable Ba and REE, and is unusually enriched in Mn (7.0–13.6 wt.% MnO). The ilmenite-group minerals evolved from manganoan ilmenite to ferroan pyrophanite, and have relatively low Nb contents (≤ 0.9 wt.% Nb2O5). During the agpaitic stage, the major repositories for incompatible elements were silicates, including lamprophyllite, titanite and chevkinite-group minerals. Lamprophyllite is generally poor in Ba, and contains relatively minor Ca and K; only few small crystals exhibit rims of barytolamprophyllite with up to 26.3 wt.% BaO. Titanite is devoid of Al and depleted in Fe, but significantly enriched in Nb, Sr, REE and Na: up to 6.4, 4.5, 4.4. and 2.9 wt.% oxides, respectively. The chemical complexity of titanite suggests involvement of several substitution mechanisms: Ca2++Ti4+⇐Na1++Nb5+, Ca2 ⇐Sr2+, 2Ca2+⇐Na1++REE3+, and Ca t++OZ-~--Nal+ + (OH)1−. Chevkinite group minerals evolved from Sr-rich (strontiochevkinite) to REE-rich compositions [chevkinite-(Ce)]. Strontiochevkinite from Pegmatite Peak is compositionally similar to the type material from Sarambi, and has high ZrO2 (up to 7.8 wt.%) and low FeOT (≤ 2.5 wt.%) contents. During the final stages of formation of the pegmatites, a deuteric F-bearing fluid enriched in Sr and REE precipitated carbonates and minor phosphates confined to fractures and cavities in the rock. In this youngest assemblage of minerals, ancylite-(Ce) is the most common Sr-REE host. Some discrete crystals of ancylite show significant enrichment in Th (up to 6.0 wt.% ThO2). Ancylite-(Ce) and bastnaesite associated with “metaloparite” and TiO2 (anatase?) comprise a replacement assemblage after primary loparite. The typical replacement pattern includes a loparite core with locally developed “metaloparite”, surrounded by a bastnaesite-anatase intermediate zone and an ancylite rim. Fluorapatite is rare, and has very high Sr, Na and REE contents, up to 21.4, 2.6 and 12.9 wt.% oxides, respectively. Compositionally, this mineral corresponds to the solid solution series between fluorapatite and belovite-(Ce). At this stage, hollandite-group minerals became a minor host for Ba; they demonstrate the evolutionary trend from priderite (5.2 wt. % K2O, 7.4 wt. % BaO) to Ba-Fe hollandite (19.2–21.4 wt. % BaO). Thus, the evolution of Sr, REE, Ba and Nb mineralization was a complex, multi-stage process, and involved primary crystallization, re-equilibration phenomena and late-stage deuteric alteration.