Argandite, Mn-7(VO4)(2)(OH)(8), the V analogue of allactite from the metamorphosed Mn ores at Pipji, Turtmann Valley, Switzerland

作者：Brugger Joel^*; Elliott Peter; Meisser Nicolas; Ansermet Stefan

来源：American Mineralogist, 2011, 96(11-12): 1894-1900.

DOI：10.2138/am.2011.3841

• Argandite
• new mineral
• crystal structure
• metamorphosed exhalative Fe-Mn deposit
• Turtmann valley
• Central Alps
• Switzerland

Argandite, Mn-7(VO4)(2)(OH)(8), is a new mineral from the metamorphosed synsedimentary exhalative Mn deposit located underneath the Pipji glacier (Pipjigletscher) in the Turtmann valley, Central Alps, Switzerland. The mineral is dedicated to the Swiss geologist Emile Argand (1879-1940). Argandite occurs in manganosite-rich ores in association with the V-minerals pyrobelonite, reppiaite, and an unknown silico-vanadate with chemical formula (Mn,Mg)(24)(V,As,Si)(4)Si2O27H38; these minerals result from the remobilization of ore components during the Tertiary Alpine metamorphism under upper greenschist facies conditions (similar to 450 degrees C, 4-6 kbar). Argandite forms isolated anhedral grains up to 60 mu m in diameter. The mineral is transparent, orange in color with a pale orange streak and vitreous luster, Mohs hardness similar to 3.5-4, D-calc 3.67(1), and D-meas 3.71(5) g/cm(3). It is brittle with one distinct cleavage, probably parallel to {001}. Argandite is biaxial negative, with alpha similar to 1.74, beta = 1.762(4), gamma similar to 1.77 (white light), 2V(calc) = similar to 62 degrees. It shows a distinct pleochroism under polarized light, orange-yellow to orange. The empirical chemical formula is (Mn6.54Mg0.38Ni0.04Ca0.02Zn0.01Sr0.01)(Sigma=2.00)(V-1.46,As-0.54)(Sigma=2.00)O-8(OH)(8.00). Argandite is monoclinic, space group P2(1)/n, a = 5.5038(2), b = 12.2665(5), c = 10.1055(5) angstrom, beta = 95.559(4)degrees; V = 679.04(5) angstrom(3); Z = 2. The six strongest lines measured in the X-ray powder diffraction pattern are [d in angstrom (I)(hkl)]: 3.074 (100)(131), 2.687 (70)(140;113), 3.395 (60)(112), 3.708 (50) (11 (2) over bar ;121), 2.945 (50)(041;11 (3) over bar), and 2.522 (50)(004;20 (2) over bar). The crystal structure was solved with direct methods on the basis of 1661 unique reflections with I > 4 sigma(F) and refined to R-1 = 3.40%. Argandite is isostructural with allactite, Mn-7(AsO4)(2)(OH)(8), and raadeite, Mn-7(PO4)(2)(OH)(8).

• 出版日期2011-12