Argandite, Mn7(VO4)2(OH)8, the V analogue of allactite from the metamorphosed Mn ores at Pipji, Turtmann Valley, Switzerland

Abstract

Argandite, Mn 7 (VO 4 ) 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 Émile Argand . 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 Si 2 O 27 H 38 ; these minerals result from the remobilization of ore components during the Tertiary Alpine metamorphism under upper greenschist facies conditions (∼450 °C, 4-6 kbar). Argandite forms isolated anhedral grains up to 60 µm in diameter. The mineral is transparent, orange in color with a pale orange streak and vitreous luster, Mohs hardness ∼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 α ∼1.74, β = 1.762(4), γ ∼1.77 (white light), 2V (calc) = ∼62°. It shows a distinct pleochroism under polarized light, orange-yellow to orange. The empirical chemical formula is (Mn 6.54 Mg 0.38 Ni 0.04 Ca 0.02 Zn 0.01 Sr 0.01 ) ∑=7.00 (V 1.46 ,As 0.54 ) ∑=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) Å, β = 95.559(4)°; V = 679.04(5) Å 3 ; Z = 2. The six strongest lines measured in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 3.202). The crystal structure was solved with direct methods on the basis of 1661 unique reflections with I > 4σ F and refined to R 1 = 3.40%. Argandite is isostructural with allactite, Mn 7 (AsO 4 ) 2 (OH) 8 , and raadeite, Mn 7 (PO 4 ) 2 (OH) 8 .

H-bonding scheme in allactite: a combined single-crystal X-ray and neutron diffraction, optical absorption spectroscopy, FTIR and EPMA-WDS study

“…O1-H1, O3-H3, O5-H5, O6-H6; Table 4). This is in line with previous studies on raadeite (Chopin et al, 2001) and argandite (Brugger et al, 2011). The principal root-meansquare components of the H-site displacement parameters obtained in this study are fairly anisotropic (as deducible from the data reported in Tables 3b,c).…”

“…MO 4 (OH) 2 and M2,M3,M4 MO 2 (OH) 4 ; no MO 6 unit occurs, consistent with the previous findings for raadeite (Chopin et al, 2001) and argandite (Brugger et al, 2011). The optical absorption experiment of this study confirms that the Mn valence state is 2+, and no evidence of partial substitution, with e.g.…”

“…The reduced cell for allactite can be obtained from Moore's cell choice by the transformation [001/010/ 10 1]. Consistently, the structure of argandite, ideally Mn 7 (VO 4 ) 2 (OH) 8 (the V analogue of allactite) was later reported by Brugger et al (2011) in the space group P2 1 /n with a = 5.5038(2), b = 12.2665(5), c = 10.1055(5) Å, β = 95.559(4)°(Z = 2), and the isostructural relation with allactite and raadeite was revealed. In the structure refinements of raadeite and argandite, four independent H sites were located by inspection of a difference-Fourier map.…”

“…Our spectrum demonstrates that a number of OH environments, with different hydrogen-bond strengths, occur in the allactite structure. Based on observed relationships between O•••H distances as determined by diffraction studies and OH-stretching band energies (Libowitzky, 1999) (Chopin et al 2001;Brugger et al, 2011).…”

The crystal structure of gatehouseite

“…Strips of edge-sharing octahedra, topologically identical to those in the gatehouseite and arsenoclasite structures, are found in the structure of waterhouseite (Pring et al, 2005). Strips of edge-sharing octahedra, two octahedra wide, occur in the structures of the isotypic minerals allactite, Mn 7 (AsO 4 ) 2 (OH) 8 , (Moore, 1968), raadeite, Mg 7 (PO 4 ) 2 (OH) 8 , (Chopin et al, 2001) and argandite, Mg 7 (VO 4 ) 2 (OH) 8 , (Brugger et al, 2011).…”