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New data on pyrochlore and perovskite-group minerals from the Lovozero alkaline complex, Russia

Anton R. Chakhmouradian and Roger H. Mitchell
New data on pyrochlore and perovskite-group minerals from the Lovozero alkaline complex, Russia
European Journal of Mineralogy (August 2002) 14 (4): 821-836

Abstract

Pyrochlore-and perovskite-group minerals are relatively common accessory constituents of agpaitic murmanite lujavrites at the Lovozero alkaline complex (Russia). These rocks contain euhedral crystal of niobian calcian loparite-(Ce) and, more commonly, ceroan lueshite that occurs as discrete oikocrysts and rims on the loparite-(Ce). The overall compositional range exhibited by these phases is (Na (sub 0.44-0.79) REE (sub 0.14-0.37) Ca (sub 0.02-0.12) Sr (sub 0.04-0.09) Th (sub 0-0.01) ) (Nb (sub 0.12-0.66) Ti (sub 0.33-0.85) Fe (sub 0-0.01) Ta (sub 0-0.01) ) O (sub 3) ; it agrees well with the evolutionary trend established previously for perovskite-group minerals from Lovozero. The murmanite lujavrites also contain early-crystallizing uranoan pyrochlore that subsequently underwent alteration to uranpyrochlore (5.0-26.4 wt.% UO (sub 2) for both) through interaction with a deuteric fluid. The alteration pattern involves a decrease in Na, Ca and Sr contents from the core outward, increasing ionic deficiency in the A and Y sites, and progressive hydration. The proportion of relatively higher-charged cations does not change or slightly decreases toward the rim. The occurrence of lueshite and U-bearing pyrochlore in the murmanite lujavrites indicates that these rocks crystallized from the most evolved portion of parental phonolitic magma. Pyrochlore-group minerals also occur in albite-rich, magnesio-arfvedsonite- and aegirine-bearing metasomatic rocks. These parageneses typically contain "silicified" varieties of pyrochlore exhibiting an oscillatory zoning pattern and, in some case, superimposed secondary zoning. The metasomatic pyrochlore ranges from nearly stoichiometric Na-Ca-rich compositions to cation-deficient strontiopyrochlore (up to 10.3 wt.% SrO) and plumbopyrochlore (up to 37.9 wt.% PbO). The oscillatory zoning involves variations in cation occupancy of the A site (primarily Na, Ca and Sr), and Si content. Elevated levels of Si (up to 16.8 wt.% SiO (sub 2) ) are invariably associated with the zones having the highest cation deficiency and H (sub 2) O contents. A negative correlation observed between the Si and (Nb+Ti) contents is interpreted to result form changes in pH, a(SiO (sub 2) ), a(Na (super 1+) ), a(Ca (super 2+) ), and activities of minor components during the crystal growth. Primary pyrochlore-group minerals from both lujavrites and albitites are characteristically poor in Ta (<2.6 wt.% Ta (sub 2) O (sub 5) ), and enriched in Sr and light rare-earth elements. Comparative data on pyrochlore-group minerals from other alkaline-rock occurrences are presented.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 14
Serial Issue: 4
Title: New data on pyrochlore and perovskite-group minerals from the Lovozero alkaline complex, Russia
Affiliation: University of Manitoba, Department of Geological Sciences, Winnipeg, MB, Canada
Pages: 821-836
Published: 200208
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 49
Accession Number: 2002-064893
Categories: Igneous and metamorphic petrologyMineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
N67°46'60" - N67°46'60", E34°45'00" - E34°45'00"
Secondary Affiliation: Lakehead University, CAN, Canada
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200220
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