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 refl ections, a brown streak and a vitreous luster, transparent to translucent, nonfluorescent, 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)0.717 Ca0.279Th0.006]1.002 (Al0.899Fe3+0.101)1.000 Al1.000(Fe2+0.669Fe3+0.279Mg0.044Ti0.007)0.999 [(Si0.978Al0.022)1.000O4] (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 [d in Å(I)(hkl)] 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, beta 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.

Allanite-(La) from Buca della Vena mine, Apuan Alps, Italy, an epidote group mineral

ORLANDI, PAOLO;PASERO, MARCO
2006

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 refl ections, a brown streak and a vitreous luster, transparent to translucent, nonfluorescent, 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)0.717 Ca0.279Th0.006]1.002 (Al0.899Fe3+0.101)1.000 Al1.000(Fe2+0.669Fe3+0.279Mg0.044Ti0.007)0.999 [(Si0.978Al0.022)1.000O4] (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 [d in Å(I)(hkl)] 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, beta 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.
Orlandi, Paolo; Pasero, Marco
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/202071
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