Chevkinite-group Minerals Research Articles

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.

Chevkinite-group minerals from salic volcanic rocks of the East African Rift

AbstractElectron microprobe analyses are presented of chevkinite-group minerals occurring as microphenocrysts in peralkaline rhyolites of the Greater Olkaria Volcanic Complex (Kenya) and as a groundmass phase in a peralkaline quartz trachyte lava from the Tarosero volcano (Tanzania), both in the East African Rift Valley. Their compositions conform closely to the formula: (REE, Ca, Th)4 Fe2+ (Fe2+, Al, Ti, Zr, Nb)2 Ti2 (Si4O22). Compared to published analyses of chevkinite-group minerals, the Olkaria phases are relatively enriched in Nb and the LREE; the Tarosero phase is more calcic and relatively Zr- and Nb-rich. The main substitution in the A site at Olkaria is Ca ⇌ Ce. The overall charge-balancing substitution seems to be (McDowell, 1979):Phenocryst/glass ratios are presented for Nb, REE, Sr, Th, U and Y in two, and Ba, Zr and Hf in one, Kenyan samples. Partition coefficients are lower in the more peralkaline rock, with the exception of Sr, which is higher. The lower values are consistent with a lower degree of polymerization of more peralkaline melts. The higher Sr value may be a function of Sr partitioning into phenocryst phases coexisting with chevkinite.

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.