ALBERT

Description: Zavalíaite, (Mn 2+ ,Fe 2+ ,Mg) 3 (PO 4 ) 2 , belongs to the sarcopside group of minerals and corresponds to the Mn 2+ equivalent of sarcopside and chopinite. It occurs at the La Empleada pegmatite, San Luis Province, Argentina. Zavalíaite forms colorless and transparent exsolution-lamellae up to 70 μm thick and 1.5 mm long in lithiophilite. Owing to hydration, the mineral partially hydrates to reddingite, (Mn 2+ ,Fe 2+ ) 3 (PO 4 ) 2 ·3H 2 O. Lamellae of zavalíaite display a perfect cleavage along the {100} or {001} direction. The luster is vitreous to resinous, and the streak is white. Optically, the mineral is biaxial (–), with an average index of refraction of 1.66(1) (for = 589 nm), 2 V meas = 15(5)°. It is non-pleochroic, without dispersion; the cleavage plane shows an angle of ca . 45° with the elongation of the lamellae; X is perpendicular to this cleavage plane. Zavalíaite is non-fluorescent under short-wave and long-wave ultraviolet light, and its calculated density is 3.68 g/cm 3 . The mineral is brittle, with a Mohs hardness of 4. The mean chemical composition, determined with an electron microprobe from 27 point analyses (wt.%), is: P 2 O 5 41.38, FeO 24.94, MgO 6.09, MnO 27.08, total 99.49. The empirical formula, calculated on the basis of eight atoms of oxygen per formula unit, is (Mn 2+ 1.31 Fe 2+ 1.19 Mg 0.52 ) 3.02 (P 1.00 O 4 ) 2 . Zavalíaite is monoclinic, space group P 2 1 / c , a 6.088(1), b 4.814(1), c 10.484(2)Å, β 89.42(3)°, V 307.2(1) Å 3 , and Z = 2. The crystal structure is identical to that of sarcopside and chopinite, with the M 1 site occupied by 0.5 Fe 2+ + 0.35Mn 2+ + 0.15 Mg, and the M 2 site occupied by 0.475 Mn 2+ + 0.35 Fe 2+ + 0.175 Mg. The strongest eight lines in the X-ray powder pattern [ d in Å (intensities estimated visually) ( hkl )] are: 6.75 (very strong) (100), 3.54 (weak) (012)(11), 2.964 (strong) (200), 2.816 (weak) (013), 2.537 (strong) (11), 1.894 (very strong) (220), 1.848 (very strong) (311), and 1.652 (very strong) (313)(31). The mineral has been approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association [IMA 2011–012], and is named in honor of María Florencia de Fátima Márquez Zavalía [or Márquez-Zavalía], investigator of Argentinian minerals and ore assemblages and Head of the Department of Mineralogy, Petrography and Geochemistry, IANIGLA, CCT Mendoza, CONICET (National Research Council of Argentina).

Description: A sample from the Koktokay No. 3 granitic pegmatite, Altai Mountains, northwestern China, which shows a transition from lithiophilite [LiMn 2+ PO 4 ] to sicklerite [Li 1– x Mn 2+ PO 4 ], was investigated by single-crystal X-ray diffraction, electron-microprobe analysis, and secondary-ion mass spectrometry (SIMS) techniques. Under the polarizing microscope, the sample shows colorless lithiophilite and deep-orange sicklerite, as well as several intermediate phases. The chemical compositions change from Li 0.96 (Mn 2+ 0.81 Fe 2+ 0.09 Fe 3+ 0.08 )PO 4 to Li 0.69 (Mn 2+ 0.62 Mn 3+ 0.19 Fe 3+ 0.16 )PO 4 , and show a progressive decrease of the lithium content from lithiophilite to sicklerite. Five crystals were extracted from the thin section, and their crystal structures were refined in space group Pbnm , with unit-cell parameters from a 4.736(1), b 10.432(2), c 6.088(1) Å (lithiophilite), to a 4.765(1), b 10.338(1), c 6.060(1) Å (sicklerite). The olivine-type structures of these phosphates are identical to that of triphylite, and are characterized by two chains of edge-sharing octahedra parallel to the c axis. The first chain consists of M 1 octahedra containing Li atoms and vacancies, and the second chain consists of M 2 octahedra occupied by Fe and Mn. The significant Mn 3+ content in sicklerite necessitates a more careful interpretation of the electron-microprobe data, and demonstrates the different mechanisms of oxidation affecting the lithiophilite–sicklerite and triphylite–ferrisicklerite series, respectively.