Description and unique crystal-structure of waterhouseite, a new hydroxy manganese phosphate species from the Iron Monarch deposit, Middleback Ranges, South Australia

Author(s)

Allan Pring, Uwe Kolitsch, William D. Birch

Abstract

Waterhouseite from the Iron Monarch

mine, Iron Knob, South Australia, is a new hydroxy manganese phosphate

species that has a unique crystal-structure. The mineral was found in a

carbonate-rich zone with gatehouseite, seamanite, rhodochrosite,

shigaite, barite, hausmannite and hematite. It occurs as divergent

sprays of orange-brown to dark brown bladed crystals up to 1 mm in

length but only up to 20 μm in thickness. The crystals are transparent

with a pearly luster on cleavages, but it is vitreous to pearly on the

tabular faces. The mineral is brittle, with a conchoidal fracture and a

yellowish brown streak. There is a perfect cleavage on (100) and a

probable cleavage on (001). The crystals show the principal forms {100}

(dominant), {010}, {011} and {001}. All crystals are twinned on (100) by

non-merohedry. The Mohs hardness is estimated to be ∼4, and the

measured density is 3.55(5) g/cm³ (calculated density is 3.591 g/cm³).

Crystals are biaxial negative and length-slow, with α 1.730(3), β

∼1.738 and γ 1.738(4), but 2V could not be measured. Interference colors

are normal, implying the absence of optical dispersion. The optical

orientation is XYZ = bac (pseudo-orthorhombic), and the pleochroism is X

pale brownish, Y brown-yellow, Z pale brownish, with absorption Z = X

> Y. Electron-microprobe analyses yielded the empirical formula Mn_7.29[(P_1.81As_0.07 V_0.04)_∑1.92O_7.68](OH,O) _8.32, calculated on the basis of 16 O atoms. The simplified formula is Mn₇(PO₄)₂(OH)₈,

in agreement with the crystal-structure determination. The strongest

five lines in the powder X-ray-diffraction pattern [d in Å(I)(hkl)] are:

4.436(70)(111), 3.621(100)(202), 3.069 (50)(311), 2.941(40)(013), and

2.780(35)(020). Unit-cell parameters refined from powder-diffraction

data, a 11.364(6), b 5.570(2), c 10.455(3) Å, β 96.61(3)°, V 657.4(2) Å ³

(Z = 2), agree very well with those refined from the single-crystal

data. The crystal structure was solved by direct methods and refined in

space group P2₁/c to R1(F) = 5.15% and wR2_all(F²) = 16.28% using data from a twinned crystal (by non-merohedry) with 1400 "observed" reflections with F_o. > 4o-(F_o). The crystal structure is characterized by a dense, complex framework of Mn(O,OH)₆ octahedra and PO₄

tetrahedra, which are linked by both edges and comers. Two different

subunits can be recognized in the structure: arsenoclasite-type strips

of edge-sharing octahedra (fragments of brucite-pyrochroite-type sheets

of octahedra) and finite chains of edge-sharing octahedra (fragments of

infinite rutile-type chains). The PO₄ tetrahedra provide a

connection between the strips and the chains. Single-crystal Raman

spectra confirm weak hydrogen bonding. A unique feature of the structure

is that the single PO₄ tetrahedron shares two of its edges with Mn(O,OH)₆

octahedra. Only two synthetic anhydrous metal arsenates are known that

show a corresponding sharing of two edges. The structure of

waterhouseite has no equivalent, although the unit-cell parameters

reveal some relations with the two chemical analogues allactite, Mn₇(AsO₄)₂(OH)₈, and raadeite, M9₇(PO₄)₂ (OH)₈.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Flinders University, Museums Victoria

Journal

The Canadian Mineralogist

Volume

43

Pages

1401-1410

No. of pages

10

ISSN

0008-4476

Publication date

2005

Peer reviewed

Yes

Austrian Fields of Science 2012

1051 Geology, Mineralogy

Portal url

https://ucrisportal.univie.ac.at/en/publications/f2778b94-9703-467d-8215-aaea2042ba4f

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