ALLANITE-(La) FROM BUCA DELLA VENA MINE, APUAN ALPS, ITALY, AN EPIDOTE-GROUP MINERAL

Abstract

Allanite-(La) is a member of the epidote group. It was sampled at Buca delia Vena mine, near Stazzema, Apuan Alps, Tuscany, Italy. It occurs as prismatic crystals up to 2-3 mm in length along [0101 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, nonfluorescent, brittle with a conchoidal fracture. The observed forms are {001}, {IOO}, {IOI}, {IOI}, {21O}, {Oil}, and the cleavage is imperfect along ZOO I}. The hardness (Mohs) is about 6, D(obs.) 3.93( I), D(calc.) 3.94 g/cm3. Optically, allanite-(La) is biaxial, with n, = 1.755(5), ny = 1.760(5), n, = 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: Ca 1.000[(L~J.288CeO.206Ndo.116PrO.106 Y0.00IhO.7I7Cao.279 Tho.006h: 1.002 (Alo.899Fe3+ 0.10I)l1.000Al 1.000 (Fe2+0669Fe3+o 279Mgo.044Tio 007ho 999 [(Sio.978Aloo22)nooo041 (Si207) 0 (OH). The simplified formula is Ca(REE,Ca)AI2(Fe2+, Fe3+)(Si04)(Si207)O(OH) (2 = 2). The six strongest lines in the X-ray powder-diffraction pattern [d in A(l)(hkl)] are: 3.506(20)(211), 2.90 I (100)(113), 2.860(40)(020), 2.692(60)(013), 2.611 (50)(311), and 2.174(25)(40 I). Allanite-(La) is monoclinic, P211m, with a 8.914(4), h 5.726(1), c 10.132(6) A, 13114.87(5)°, V 469.1(3) A3. 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 delia Vena mine, which now has.to be considered the type locality of the mineral, fill a gap in the systematics of the epidote group.