Fe-dominated samarskite - a new member of the samarskite 'group'?
Abstract
Davis and Nicolescu (2011) alluded to the fact that Fe-dominated samarskite from the pegmatite at the Spinelli prospect in Glastonbury, CT is a potential new member of the samarskite mineral 'group'. Further work on untreated material from the same occurrence confirms Fe dominance at the A site, and based on 29 microprobe and four LA-ICP-MS analyses the following empirical formula is obtained: (Fe3+0.384[Y0.239Dy0.026Gd0.018Sm0.012Nd0.007Er0.006Tb0.005Yb0.005Ho0.003Ce0.003Pr0.001Tm0.001]Σ0.326U0.090Mn0.052Th0.047Ti0.033Ca0.009Pb0.006Mg0.003Al0.003Si0.001Sc0.001)0.956(Nb0.814Ta0.149Sn0.026W0.019)1.007O4. The corresponding simplified formula is (Fe3+,Y,U,REE)(Nb,Ta)O4. Due to its UO2 and ThO2 content (9.12 and 4.67 wt. %, respectively) and age of ~ 260 Ma, Spinelli prospect samarskite is completely metamict. Literature data (Simmons et al., 2006 and references therein) suggest that annealing metamict samarskite in a slightly reducing atmosphere of 95 % Ar - 5 % H2 for 12 hrs. at 1100° C will result in recrystallized samarskite. Repeated experiments conducted on powdered Spinelli prospect material under these conditions, at gas flow rates of 100 cm3/min. and 200 cm3/min., resulted in sintered material composed of an ill-defined mixture of fergusonite, columbite, possible samarskite and an unidentified phase. Moreover, in each of the 1100° C annealing experiments a black radioactive sublimate was deposited at the cold end of the annealing apparatus, suggesting significant changes in the chemistry of the annealed material. Annealing the powdered metamict phase for 21 hrs. at 650° C (Tomašić et al., 2004) in a 95 % Ar - 5 % H2 atmosphere at a flow rate of 100 cm3/min. resulted in a mineral phase yielding broad X-ray diffraction peaks, especially at elevated diffraction angles. Because of the broad diffraction peaks and one ambiguous peak impossible to index, attempts to calculate the unit cell parameters of the annealed phase were unsuccessful for either monoclinic or orthorhombic symmetry. Hence, the unambiguous structural characterization of the mineral phase failed. Fe-dominated samarskite does not seem to be restricted to the Spinelli prospect pegmatite. Of the 19 samarskite specimens discussed by Warner and Ewing (1993), 13 are Fe-dominated. Efforts to fully characterize the Spinelli prospect Fe-rich samarskite mineral phase are continuing. REFERENCES Davis & Nicolescu, 2011, Bull. Peabody Museum of Nat. History, 52(1), 135-152 Simmons et al., 2006, Can. Min., 44(5), 1119-1125 Tomašić et al., 2004, N. Jb. Miner. Mh., 2004(4), 163-175 Waren & Ewing, 1993, Am. Min., 78(3-4), 419-424
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V23A2767N
- Keywords:
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- 3620 MINERALOGY AND PETROLOGY Mineral and crystal chemistry