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Mineralogical ClassificationAllanite-(La): type-description abstract
1st May 2006 20:37 UTCMarco E. Ciriotti Manager
• Orlandi, P. & Pasero, M. (2006): Allanite-(La) from Buca della Vena mine, Apuan Alps, Italy, an epidote-group mineral. Canadian Mineralogist, 44, 63-68.
Abstract:
Allanite-(La) is a member of the epidote group. It was sampled at Buca della Vena mine, near Stazzema, Apuan Alps, Tuscany, Italy. It occurs as prismatic crystals up to 2–3 mm in length along <010> within barite veins cutting a dolomitic metamorphosed limestone. The mineral is black with brownish reflections, a brown streak and a vitreous luster, transparent to translucent, non-fluorescent, brittle with a conchoidal fracture. The observed forms are {001}, {100}, {101}, {10¯1}, {210}, {011}, and the cleavage is imperfect along {001}. The hardness (Mohs) is about 6, D(obs.) 3.93(1), D(calc.) 3.94 g/cm3. Optically, allanite-(La) is biaxial, with nx = 1.755(5), ny = 1.760(5), nz = 1.765(5), 2V = 90(2)°, very high dispersion and strong pleochroism from greenish to greenish brown. The chemical composition of allanite-(La) has been determined by electron-microprobe analysis and points to the following empirical formula: Ca1.000 <(La0.288Ce0.206Nd0.116Pr0.106Y0.001)S0.717Ca0.279Th0.006>S1.002 (Al0.899Fe3+0.101)S1.000 Al1.000(Fe2+0.669Fe3+0.279Mg0.044Ti0.007)S0.999 <(Si0.978Al0.022)S1.000O4> (Si2O7) O (OH). The simplified formula is Ca(REE,Ca)Al2(Fe2+,Fe3+)(SiO4)(Si2O7)O(OH) (Z = 2). The six strongest lines in the X-ray powder-diffraction pattern are: 3.506(20)(¯211), 2.901(100)(¯113), 2.860(40)(020), 2.692(60)(013), 2.611(50)(¯311), and 2.174(25)(¯401). Allanite-(La) is monoclinic, P21/m, with a 8.914(4), b 5.726(1), c 10.132(6) Å, b 114.87(5)°, V 469.1(3) Å3. The crystal structure of allanite-(La) has been refined to R = 0.0328 for 1985 independent reflections collected via single-crystal X-ray-diffraction measurements. The name allanite-(La) recalls the relationship with allanite-(Ce), the Levinson modifier indicating the dominant REE. The root name honors the Scottish mineralogist Thomas Allan (1777–1883). Although the name allanite-(La) has been already used in the literature, the mineral had not been officially approved. The recognition and complete characterization of allanite-(La) from Buca della Vena mine, which now has to be considered the type locality of the mineral, fill a gap in the systematics of the epidote group.
2nd May 2006 16:18 UTCPeter Haas
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