Very few chemical and structural data are available for clinoholmquistite, A BLi2 C(Mg3 Al)2 Si8 O22 (OH)2, and related solid-solution terms. The holotype specimen is from the Tastyg spodumene deposit, Tuva, Siberia, Russia, and was reported to have the following crystal-chemical formula: A(Na0.45 Ca0.05 K0.04) B(Li1.79 Ca0.21) C(Mg1.93 Fe2+0.68 Mn 0.05 Al2.21 Fe3+0.05 Li0.08) Si8 O22 (OH1.25 F0.75) [1]. Later, a possible bias in the EMP analyses due to the presence of calcite has been suggested, and the oddity of some distances and the adp values has been noted [2]. A small piece of the holotype specimen was obtained (courtesy of M. Generalov, Moscow), and studied with X-ray single-crystal diffractometry and EMP analyses. Two different amphibole compositions were found in this sample. The first is a Li-bearing tremolite, A(Na0.23 K0.05) B(Ca1.83 Li0.17) C(Mg4.58 Fe2+0.06 Mn0.01 Fe3+0.08 Al0.07 Zn0.01 Li0.19) Si7.93 Al0.07 O22 (OH1.67 F0.33), and the second is a new composition, A(Na0.61 K0.01) B(Li1.93 Ca0.03 Na0.04) C(Al1.98 Mg1.83 Li0.61 Fe2+0.54 Mn2+0.02 Cr3+0.01 Zn0.01) T(Si7.97 Al0.03) O22 (F1.14 OH0.86), which should be referred as fluoro-sodic-pedrizite (approval by the IMA-CNMMN still pending). This is the first occurrence of amphiboles with dominant CLi in Fe-poor geologic environments; therefore, Li-rich amphiboles can be more abundant than previously thought. Discredit of former clinoholmquistite from Tuva confirms the failure of experimental synthesis in the LMASH system, which had been attempted in order to characterize the stability and crystal-chemistry of this end-member (Iezzi et al, this meeting). The structure refinements are of reasonably good quality (R3 = 2.3%), so that both geometry and cation order can be considered reliable. The above formulae were derived using all the available information. This work confirms that the structure refinement is an effective method to detect the presence of Li as well as its site preference. The exchange vectors observed in clinoamphiboles from Tastyg are: (1) Na1-1 at the A site; (2) Li1Ca-1 at the B sites; (3) Li1Mg-1 at the octahedral M3 site; (4) F1OH-1 at the O3 site. Similarly to what noted in other Fe-rich Al-poor localities [3], the CLi content is correlated with the A site occupancy and the O3F content. [1] Litvin et al. (1975) Konst. Svoistva Mineral. 7:18 [2] Hawthorne (1983) Can. Mineral. 21:173 [3] Oberti et al. (2003) Eur. J. Mineral. 15:309
Structure refinement and Li crystal-chemistry: new Li-bearing amphiboles from the holotype rock-specimen of clinoholmquistite
OBERTI ROBERTA
2004-01-01
Abstract
Very few chemical and structural data are available for clinoholmquistite, A BLi2 C(Mg3 Al)2 Si8 O22 (OH)2, and related solid-solution terms. The holotype specimen is from the Tastyg spodumene deposit, Tuva, Siberia, Russia, and was reported to have the following crystal-chemical formula: A(Na0.45 Ca0.05 K0.04) B(Li1.79 Ca0.21) C(Mg1.93 Fe2+0.68 Mn 0.05 Al2.21 Fe3+0.05 Li0.08) Si8 O22 (OH1.25 F0.75) [1]. Later, a possible bias in the EMP analyses due to the presence of calcite has been suggested, and the oddity of some distances and the adp values has been noted [2]. A small piece of the holotype specimen was obtained (courtesy of M. Generalov, Moscow), and studied with X-ray single-crystal diffractometry and EMP analyses. Two different amphibole compositions were found in this sample. The first is a Li-bearing tremolite, A(Na0.23 K0.05) B(Ca1.83 Li0.17) C(Mg4.58 Fe2+0.06 Mn0.01 Fe3+0.08 Al0.07 Zn0.01 Li0.19) Si7.93 Al0.07 O22 (OH1.67 F0.33), and the second is a new composition, A(Na0.61 K0.01) B(Li1.93 Ca0.03 Na0.04) C(Al1.98 Mg1.83 Li0.61 Fe2+0.54 Mn2+0.02 Cr3+0.01 Zn0.01) T(Si7.97 Al0.03) O22 (F1.14 OH0.86), which should be referred as fluoro-sodic-pedrizite (approval by the IMA-CNMMN still pending). This is the first occurrence of amphiboles with dominant CLi in Fe-poor geologic environments; therefore, Li-rich amphiboles can be more abundant than previously thought. Discredit of former clinoholmquistite from Tuva confirms the failure of experimental synthesis in the LMASH system, which had been attempted in order to characterize the stability and crystal-chemistry of this end-member (Iezzi et al, this meeting). The structure refinements are of reasonably good quality (R3 = 2.3%), so that both geometry and cation order can be considered reliable. The above formulae were derived using all the available information. This work confirms that the structure refinement is an effective method to detect the presence of Li as well as its site preference. The exchange vectors observed in clinoamphiboles from Tastyg are: (1) Na1-1 at the A site; (2) Li1Ca-1 at the B sites; (3) Li1Mg-1 at the octahedral M3 site; (4) F1OH-1 at the O3 site. Similarly to what noted in other Fe-rich Al-poor localities [3], the CLi content is correlated with the A site occupancy and the O3F content. [1] Litvin et al. (1975) Konst. Svoistva Mineral. 7:18 [2] Hawthorne (1983) Can. Mineral. 21:173 [3] Oberti et al. (2003) Eur. J. Mineral. 15:309I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
