Arsenoflorencite-(La), a new mineral from the Komi Republic, Russian Federation: description and crystal structure

Abstract

Arsenoflorencite-(La) [Арсенфлоренсит-(La)], ideally LaAl 3 (AsO 4 ) 2 (OH) 6 , is a new mineral (IMA2009–078), from the Grubependity Lake cirque, Maldynyrd range, upper Kozhim River basin, Prepolar Ural, Komi Republic, Russia. It occurs in direct association with zircon, quartz, hematite, ardennite-(As), andalusite, anorthite, sericite, clinochlore, chernovite-(Y), manganiandrosite-(La), spessartine and monazite–gasparite-group minerals in Mn-rich nodules in metasediments. Arsenoflorencite-(La) forms orange–red to pink rhombohedral, pseudocubic or tabular crystals up to about 0.2 mm across. The dominant forms observed are {001} and {102}. The crystals have a vitreous lustre, are transparent to translucent, have a very light pink streak and are non-fluorescent. Mohs hardness is about 3.5 (estimated). The fracture is uneven and the tenacity is brittle. Arsenoflorencite-(La) has fair cleavage on {001}. Crystals are uniaxial (+), with the indices of refraction ω = 1.740(5) and e = 1.750(5), measured in white light, and are nonpleochroic. The empirical formula (based on 14 O atoms) is: (La 0.56 Ce 0.18 Nd 0.12 Pr 0.04 Sr 0.09 Ca 0.03 ) ∑1.02 (Al 2.94 Fe 0.06 ) ∑3.00 (As 1.80 P 0.21 ) ∑2.01 H 5.95 O 14 . The simplified formula is LaAl 3 (AsO 4 ) 2 (OH) 6 . Arsenoflorencite-(La) is trigonal, space group R 3 m , a = 7.0316(3), c = 16.5151(8) A, V = 707.16(5) A 3 and Z = 3. The crystal structure was solved from single-crystal X-ray diffraction data and refined to R 1 = 0.0112 on the basis of 287 unique reflections with F o > 4 σF and R 1 = 0.0116 for all 293 reflections. The five strongest lines in the powder X-ray diffraction pattern are [ d obs in A, ( I ), ( hkl )]: 2.982, (100), (113); 3.538, (55), (110); 1.914, (38), (303, 033); 2.211, (28), (122); 5.755, (27), (101). The name is for its relationship to florencite [and arsenoflorencite-(Ce)] with La as the dominant REE.