Description and Unique Crystal-Structure of Waterhouseite, a New Hydroxy Manganese Phosphate Species from the Iron Monarch Deposit, Middleback Ranges, South Australia

Home , Allactite, Arsenoclasite, Collinsite, Gatehouseite, Nissonite, Seamanite

1401

The Canadian Mineralogist Vol. 43, pp. 1401-1410 (2005)

DESCRIPTION AND UNIQUE CRYSTAL-STRUCTURE OF WATERHOUSEITE, A NEW HYDROXY MANGANESE PHOSPHATE SPECIES FROM THE IRON MONARCH DEPOSIT, MIDDLEBACK RANGES, SOUTH AUSTRALIA

ALLAN PRING§ Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, and School of Earth & Environmental Science, University of Adelaide, Adelaide, S.A. 5005, Australia

UWE KOLITSCH Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstr. 14, A–1090 Wien, Austria

WILLIAM D. BIRCH Department of Mineralogy, Museum of Victoria, GPO Box 666E, Melbourne, Victoria, 3000, Australia

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 m 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 1.730(3), ~1.738 and 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 Mn7.29[(P1.81As0.07V0.04)1.92O7.68](OH,O)8.32, calculated on the basis of 16 O atoms. The simplifi ed formula is Mn7(PO4)2(OH)8, in agreement with the crystal-structure determination. The strongest fi ve 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 refi ned from powder-diffraction data, a 11.364(6), b 5.570(2), c 10.455(3) Å, 96.61(3)°, V 657.4(2) Å3 (Z = 2), agree very well with those refi ned from the single-crystal data. 2 The crystal structure was solved by direct methods and refi ned in space group P21/c to R1(F) = 5.15% and wR2all(F ) = 16.28% using data from a twinned crystal (by non-merohedry) with 1400 “observed” refl ections with Fo > 4(Fo). 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 corners. 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 fi nite chains of edge-sharing octahedra (fragments of infi nite rutile-type chains). The PO4 tetrahedra provide a connection between the strips and the chains. Single-crystal Raman spectra confi rm 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, Mg7(PO4)2(OH)8.

Keywords: new mineral species, waterhouseite, manganese phosphate, crystal structure, Iron Monarch mine, Australia.

SOMMAIRE

La waterhouseïte, nouvelle espèce de phosphate de manganèse hydroxylé découverte à la mine Iron Monarch, à Iron Knob, en Australie du Sud, possède une structure cristalline très inhabituelle. Elle a été découverte dans une zone carbonatée associée à gatehouseïte, seamanite, rhodochrosite, shigaïte, barite, hausmannite et hématite. Elle se présente en amas divergents de cristaux en lames orange-brun à brun foncé, atteignant 1 mm en longueur mais seulement 20 m en épaisseur. Les cristaux sont transpar-

§ E-mail address: [email protected] 1402 THE CANADIAN MINERALOGIST ents, avec un éclat nacré sur les clivages, mais plutôt vitreux à nacré sur les faces des cristaux. Le minéral est cassant, à fracture conchoïdale, et à rayure brun jaunâtre. Il y a un clivage parfait sur (100) et un clivage probable sur (001). Les cristaux montrent les formes principales {100} (dominante), {010}, {011} et {001}. Tous les cristaux sont maclés sur (100) par non-méroédrie. La dureté de Mohs serait environ 4, et la densité mesurée est 3.55(5) g/cm3 (la densité calculée est 3.591 g/cm3). Les cristaux sont biaxes négatifs et à allongement négatif, avec 1.730(3), ~1.738 et 1.738(4), mais nous nʼavons pas pu mesurer lʼangle 2V. Les couleurs dʼinterférence sont normales, ce qui implique lʼabsence de dispersion optique. Lʼorientation optique est XYZ = bac (pseudo-orthorhombique), et le pléochroïsme est: X brun pâle, Y brun-jaune, Z brun pâle, avec absorption Z = X > Y. Les analyses effectuées avec une microsonde électronique ont mené à la formule empirique Mn7.29[(P1.81As0.07V0.04)1.92O7.68](OH,O)8.32, calculée sur une base de seize atomes dʼoxygène. La formule simplifi ée serait Mn7(PO4)2(OH)8, en accord avec les résultats de lʼébauche de la détermination de la structure. Les cinq raies les plus intenses du spectre de diffraction X (méthode des poudres) [d en Å(I)(hkl)] sont: 4.436(70)(111), 3.621(100)(202), 3.069(50)(311),¯ 2.941(40)(013),¯ et 2.780(35)(020). Les paramètres réticulaires affi nés à partir des données acquises sur poudre, a 11.364(6), b 5.570(2), c 10.455(3) Å, 96.61(3)°, V 657.4(2) Å3 (Z = 2), concordent très bien avec celles qui résultent de lʼaffi nement sur monocristal. Nous avons résolu la structure par 2 méthodes directes, et nous lʼavons affi né dans le groupe spatial P21/c jusquʼà un résidu R1(F) de 5.15% [wR2toutes(F ) = 16.28%] en utilisant les données obtenues dʼun cristal maclé (par non méroédrie), et les 1400 réfl exions “observées” ayant Fo > 4(Fo). La structure possède une trame dense dʼoctaèdres Mn(O,OH)6 et de tétraèdres PO4, qui sont liés à la fois par partage dʼarêtes et de coins. La structure contient deux sous-unités: il y a des rubans dʼoctaèdres à arêtes partagées, comme dans lʼarsénoclasite (des fragments de feuillets dʼoctaèdres comme dans la brucite ou la pyrochroïte) et des chaînes fi nies dʼoctaèdres à arêtes parta- gées (des fragments de chaînes infi nies comme dans le rutile). Les tétraèdres PO4 assurent une connexion entre les rubans et les chaînes. Les spectres de Raman obtenus sur monocristaux confi rment la présence de faibles liaisons hydrogène. Un aspect unique de la structure concerne le tétraèdre PO4, qui partage deux de ses arêtes avec des octaèdres Mn(O,OH)6. Ce partage inhabituel nʼest connu que dans deux arsenates anhydres synthétiques. La structure de la waterhouseïte nʼa aucun équivalent, quoique les paramètres réticulaires révèlent certains liens avec les analogues du point de vue chimique, allactite, Mn7(AsO4)2(OH)8, et raadeïte, Mg7(PO4)2(OH)8.

Mots-clés: nouvelle espèce minérale, waterhouseïte, phosphate de manganèse, structure cristalline, mine Iron Monarch, Australie.

INTRODUCTION Waterhouse (1810–1888), who was Keeper of Geology at the British Museum (London) from 1845 until 1880. The Iron Monarch mine, a Precambrian sedimentary The name also celebrates the continuing work of the iron ore deposit in the Middleback Ranges, at Iron Waterhouse Club in their support of the South Austra- Knob, South Australia, is one of the most productive lian Museum. The mineral and the name were unani- sources of rare and unusual mineral species in Australia. mously approved by the IMA Commission on New Broken Hill Proprietary Limited worked the mine as a Minerals and Minerals Names (no. 2004–035). The source of high-grade iron ore for many years. In May type specimen and additional material are held in the 1987, Mr Glyn Francis, then a Quality Control Offi cer collections of the South Australian Museum, Adelaide at the mine, submitted a group of specimens to one of (SAM 28408 and 28409). the authors (AP) for identifi cation. One of the specimens contained some small clusters of very small and thin OCCURRENCE brownish, blade-like crystals which energy-dispersion X-ray analysis showed to contain manganese, phos- The Iron Monarch deposit (32°45ʼS, 137°08ʼE) is the phorus and a minor amount of arsenic. The powder largest of a number of Precambrian sedimentary iron ore X-ray-diffraction pattern could not be matched to any deposits in the Middleback Ranges of South Australia, known mineral, although similarities were noted to some 500 km northwest of Adelaide. The Middleback a number of manganese arsenate minerals, including Ranges are composed primarily of a sequence of Early allactite (Mn7(AsO4)2(OH)8). Although Mr. Francis was Proterozoic chemical metasediments (the Middleback able to locate several more small specimens containing Group), which are interpreted to have been deposited the mineral, there was still insuffi cient material for a full about 1950 to 1850 million years ago (Yeates 1990). characterization. Only the later crystal-structure deter- The orebody lies within a carbonate-facies iron forma- mination from data collected on a modern single-crystal tion, composed primarily of iron carbonate, silica and diffractometer equipped with a CCD area detector led iron oxide, with primary iron silicates such as iron-rich to a complete characterization of this mineral. The talc and cummingtonite–grunerite. Hematite and quartz new species has been named waterhouseite in honor of become predominant at the top of the sequence and Frederick George Waterhouse (1815–1898), naturalist constitute the orebody. There appear to have been at and first Director of the South Australian Museum least two periods of mobilization of iron-rich fl uids, (Adelaide), in recognition of his contribution to the the fi rst being in the Middle Proterozoic and the second preservation of the natural history of South Australia. during a period of intense weathering in the Tertiary Waterhouse was the youngest brother of George Robert (Miles 1955, Yeates 1990). The extensive suite of THE CRYSTAL STRUCTURE OF WATERHOUSEITE 1403 secondary phosphate and other oxysalt minerals appears outline of the blades results, and the angle enclosed to have been formed as a result of deep Tertiary and by the left and right boundary faces of the fl at “spear post-Tertiary weathering of the deposit. tip” is 18 ± 1°. Single-crystal studies provide evidence The new mineral occurs in cavities in a matrix that the mineral invariably shows contact twinning (by consisting of hematite, hausmannite, barite, manganoan non-merohedry) on the platy face (100). If a bladed calcite and rhodochrosite, from the 130-meter level on twin crystal is seen edge-on under the polarizing the eastern side of the Iron Monarch open cut. Shigaite, microscope, the twinning may be recognizable from a gatehouseite, seamanite, rhodochrosite, barite, haus- strongly re-entrant angle between two twin indiduals. mannite and hematite are found in close association The crystals have splintery to conchoidal fracture, with with waterhouseite. Metaswitzerite, sussexite, arseno- a perfect cleavage on (100) and a probable cleavage clasite, collinsite, pyrobelonite, triploidite and a new, on (001). Waterhouseite is transparent, and the color unnamed hexagonal calcium–manganese carbonate– ranges from resinous orange-brown to dark clove-brown phosphate have also been noted on specimens from the with a yellowish brown streak. The luster is pearly on same immediate area of the open cut, but not in direct cleavages and vitreous to pearly on crystal faces, and association with waterhouseite (Pring et al. 1989, 1990, the Mohs hardness is estimated to be 4. The density 1992a, b, 2000, Pring & Birch 1993). Mining operations was measured to be 3.55(5) g/cm3 by suspension in ceased at Iron Monarch in April 1998, and the mine a mixture of Clerici solution and water, whereas the site has been partially rehabilitated. Unfortunately, it is calculated density (with Z = 2) is 3.591 g/cm3, based on no longer accessible for collecting. To date, over 140 the chemical formula derived from the structure refi ne- mineral species have been identifi ed from this locality. ment, and 3.49 g/cm3 based on the simplifi ed formula, Pring et al. (2000) provided a full list. Waterhouseite Mn7(PO4)2(OH)8. was noted in Pring et al. (2000) as an unidentifi ed phase Optically, waterhouseite is biaxial negative and designated UK02. length-slow, with 1.730(3), ~1.738 and 1.738(4) (measured in white light). The optic angle could not be PHYSICAL AND OPTICAL PROPERTIES measured, but it must be small because is very close to . Interference colors are normal, implying absence Waterhouseite occurs as divergent sprays of bladed of optical dispersion. The optical orientation is XYZ = crystals up to 1.0 0.2 0.02 mm with an elongation bac (pseudo-orthorhombic), and the pleochroism is X parallel to [001] (Fig. 1). The principal forms observed pale brownish, Y brown-yellow, Z pale brownish, with are {100} (dominant), {010}, {011} and {001} (not absorption Z = X > Y. on all crystals); if {001} is missing, a spear-shaped

FIG. 1. SEM micrograph of waterhouseite showing the blade-like habit of the crystals. The crystals show the principal forms {100} (dominant), {010}, {011}, and {001}. 1404 THE CANADIAN MINERALOGIST

CHEMICAL AND POWDER-DIFFRACTION DATA Powder X-ray-diffraction data for waterhouseite (Table 2) were obtained using a Guinier–Hägg camera The chemical composition of waterhouseite was 100 mm in diameter, CrK radiation ( 2.28970 Å) and established using a Cameca SX50 electron micro- silicon (NBS SRM 640a) as an internal standard. The probe, at 15 kV and a specimen current of about 20 Guinier–Hägg fi lms were scanned in TPU/Pos mode nA. We used pure metals (Mn, V and Cu), wollastonite using an Epson fi lm scanner, and the powder-diffrac- (Ca), hematite (Fe), corundum (Al), fl uorapatite (P), tion profi le over the 2 range 10 to 90° was extracted sphalerite (Zn) and arsenopyrite (As) as standards. using the program SCION IMAGE and the Universal-Si- Electron-microprobe analysis showed that the mineral Calibration, a macro function based on IGOR PRO 4.0 is essentially a pure manganese phosphate with minor (WaveMetrics Inc., 2000). The unit-cell parameters substitution of arsenic for phosphorus, traces of Al and were refi ned by treating the whole powder pattern with Zn, and variable, very minor amounts of Si, but gave the Le Bail profi le-fi tting method (Le Bail et al. 1988), consistently low and somewhat variable totals (80 to starting from the unit-cell parameters measured with 84 wt.% without H2O). No other elements with Z > single-crystal techniques. The fi nal unit-cell parameters, 9 were detected during a wavelength-dispersion scan; a 11.364(6), b 5.570(2), c 10.455(3) Å, 96.61(3)°, Mg was sought specifi cally but not found. Raman spec- V 657.4(2) Å3, are very similar to those obtained from troscopy (see below) showed no carbonate or borate the single-crystal refinement of the structure (see groups. There was insuffi cient material to determine below). The axial ratios calculated from these cell H2O directly, so the amount was calculated from the parameters are 2.0402:1:1.8770. crystal-structure determination, which gave the ideal formula Mn7(PO4)2(OH)8 (see below). Further probe RAMAN SPECTROSCOPY mounts were made (fi ve grains in all), but the analytical total remained low. It was noted, however, that the Single-crystal laser-Raman spectra of waterhouseite crystal fragments were very thin. We conclude that (Fig. 2) were recorded in the range from 3800 to 100 the low totals were mostly due to part of the electron cm–1 with a Renishaw Ramascope 1000 using a laser beam passing though the crystals into the resin matrix. wavelength of 518 nm (green laser) and excitation The analytical results were thus normalized to the through a Leica DMLM optical microscope (Si stan- –1 ideal Mn content, and the H2O calculated to give eight dard, spectral resolution ±4 cm , minimum lateral OH groups. Seven analyses were made from multiple resolution ~2 mm, unpolarized laser light, random crystals, with the calculated oxide averages and ranges sample orientation). Very similar spectra were recorded shown in Table 1. These results demonstrate that with a blue laser (488 nm) on a Renishaw M1000 there is only very limited substitution of As and V MicroRaman Imaging System in Vienna, but they are for P. The empirical formula, calculated on the basis of poorer quality (strong background that increases of 16 O atoms, as shown by the crystal structure, is toward higher wavenumbers, and relatively low inten- Mn7.29[(P1.81As0.07V0.04)1.92 O7.68](OH,O)8.32, and the sity of all bands), and therefore are not shown here. simplifi ed formula is Mn7(PO4)2(OH)8. The empirical The spectrum obtained with the green laser (Fig. 2) formula shows an excess of Mn and a defi cit in the P shows two medium strong bands due to O–H stretching site; we believe that this may be partly due to small vibrations at 3439 and 3510 cm–1, and two shoulders at but variable amounts of Si-for-P substitution (rarely up to 2.5 wt% SiO2) that could not be reproducibly established. The Gladstone–Dale compatibility index is 0.010, indicating “superior” agreement between the chemical and physical data (Mandarino 1981). THE CRYSTAL STRUCTURE OF WATERHOUSEITE 1405

FIG. 2. Single-crystal laser-Raman spectra of waterhouseite. See text for discussion.

~3411 and ~3555 cm–1. The existence of these bands of waterhouseite and the low site-symmetries of nearly agrees with the presence of four different OH groups all its atom positions. in the structure. From the measured O–H band positions, OH–H...O distances between about 2.8 and 3.0 CRYSTAL-STRUCTURE DETERMINATION Å can be calculated using the correlation established by Libowitzky (1999); these distances would correspond Single-crystal studies were done with a Nonius to the weak hydrogen bonds provided by the four OH Kappa CCD four-circle diffractometer (MoK radia- groups (OH…O distances between 2.89 and 3.12 Å, tion, CCD area detector) at 293 K. All waterhouseite vide infra). A very weak, broad peak at ~1612 cm–1 crystals studied were found to be twinned by non-mero- could indicate the presence of impurities of molecular hedry, with the twin plane parallel to the dominant face water (H–O–H bending mode), although it must be of the platy crystal (100). The crystal structure has been pointed out that Raman spectroscopy is not very sensi- determined using a nearly full sphere of single-crystal tive for the detection of H2O-bending modes; this peak intensity data collected from a twinned crystal; see could also represent an overtone of the band at 809 Table 3 for experimental details. A large detector–crystal cm–1. Below 1100 cm–1, there is a group of very strong distance (40 mm) was chosen in order to avoid refl ection to very weak bands (the strong ones are italicized; sh: overlap as much as possible. The structure was solved shoulder) 1076, ~1050, ~1018, 984, 929, 809, 667, by direct methods (SHELXS97; Sheldrick 1997a) and ~595 (sh), 571, ~540 (sh), ~513 and 433 cm–1, charac- refi ned, using SHELXL97 (Sheldrick 1997b), in space teristic of vibrational modes of a PO4 tetrahedron in a group P21/c (no. 14) to a preliminary R1(F) of about low-symmetry environment (all four fundamental PO4 12%. The structure model obtained corresponds to the vibrations are Raman active). Lattice modes, including chemical formula Mn7(PO4)2(OH)8, in agreement with vibrations of Mn–O bonds, cause most of the strongly the idealized formula derived from the chemical data. overlapping, weak bands below 400 cm–1 (~342, ~321, Hydrogen atoms could not be located owing to the ~267, 174 and ~137 cm–1). A strong overlap could occur effects of the twinning by non-merohedry. A total of in the 500–400 cm–1 region between the bands attributed 153 refl ections most strongly affected by this twinning to vibrations of the PO4 tetrahedron and of the Mn–O (refl ections with Fo >> Fc) were omitted during the bonds (see Hatert et al. 2003, 2005). Accurate band- fi nal steps of the refi nement, which caused R1(F) to assignments are diffi cult owing to the low symmetry drop from about 12% to about 5.5%. Final residuals 1406 THE CANADIAN MINERALOGIST

2 were R1(F) = 5.15% and wR2all(F ) = 16.28% [1400 analysis for waterhouseite is presented in Table 6. A “observed” refl ections with Fo > 4(Fo)]. Before the table of structure factors is available from the Deposi- fi nal cycles of refi nement, the atom coordinates were tory of Unpublished Data, CISTI, National Research standardized using the program STRUCTURE TIDY (Gelato Council, Ottawa, Ontario K1A 0S2, Canada. & Parthé 1987). Refi ned unit-cell parameters of the measured twin, a 11.364(2), b 5.569(1), c 10.455(2) Å, DESCRIPTION OF THE STRUCTURE 96.61(3)°, V 657.3(2) Å3, are basically identical with those calculated from the X-ray powder-diffraction data The asymmetric unit of waterhouseite contains four (Table 2). Atom coordinates and selected bond-lengths non-equivalent Mn atoms, one P atom, eight O atoms and angles (including probable hydrogen bonds) are (four of which represent OH groups) and four H atoms; given in Tables 4 and 5, respectively. A bond-valence the latter were not detectable owing to the effects of twinning. 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 corners (Figs. 3, 4). The framework is built of two types of subunits based on the Mn(O,OH)6 octahedra: fi rstly, there are strips of edge-sharing octahedra built from Mn1O2(OH)4 and Mn4O2(OH)4 octahedra. Topo- logically identical strips occur in arsenoclasite and can be visualized as fragments of brucite–pyrochroite-type sheets of octahedra. The plane of these strips of octahedra is parallel to (102), and they extend along [010] (Fig. 3). A single strip is connected to two adjacent THE CRYSTAL STRUCTURE OF WATERHOUSEITE 1407 and parallel, but offset strips via common OH6 atoms. RELATED COMPOUNDS AND CRYSTAL STRUCTURES Secondly, there are slightly twisted, fi nite chains of edge-sharing Mn2O3(OH)3 and Mn3O3(OH)3 octa- The fact that the PO4 tetrahedron in waterhouseite ¯ hedra, which run approximately parallel to [102] and shares two of its edges with Mn(O,OH)6 octahedra is have a length comprising four octahedra (sequence highly unusual. This feature of the crystal structure of Mn3–Mn2–Mn2–Mn3, Fig. 3). These chains of octa- waterhouseite is unique in the mineral kingdom. We hedra can be considered as fragments of infi nite rutile- are presently aware of only two (synthetic and anhy- type chains. A single chain is corner-linked to four drous) arsenates in which an equivalent edge-sharing adjacent and parallel, but offset chains via common is present: (K4Ni7(AsO4)6 (Ben Smail et al. 1999), and OH7 and OH8 atoms. KNi3(AsO4)(As2O7) (Ben Smail & Jouini 2000). Both The unique PO4 tetrahedron shares two of its of these arsenates contain fragments of brucite-type neighboring edges with the Mn2- and Mn3-centered sheets of octahedra, and thus show some relation to Mn(O,OH)6 octahedra of the chains of octahedra waterhouseite. mentioned (Figs. 3, 4). The fourth apex of the PO4 There are several, mostly anhydrous phosphates and tetrahedron is connected to the arsenoclasite-type strips arsenates in which the XO4 (X = P, As) tetrahedra share of octahedra. Thus, PO4 group provides a direct connec- one of their edges with MO6 (M = di- or trivalent metal) tion between the strips of octahedra and the chains of octahedra. The mineral representatives include gate- octahedra. The strongest bond-length distortions are houseite, Mn5(PO4)2(OH)4 (Pring & Birch 1993; see shown by the Mn2- and Mn4-centered Mn(O,OH)6 also Ruszala et al. 1977) and the isotypic arsenoclasite, octahedra (cf. Table 5). The Mn2 atom in fact has a [5 Mn5(AsO4)2(OH)4 (Moore & Molin-Case 1971), which + 1] coordination to oxygen, and the Mn4 atom, a [4 both also occur at the type locality of waterhouseite. + 2] coordination. Average Mn–O bond lengths show Among the synthetic compounds, there are, for instance, values within an unexpectedly close range: 2.190 (Mn1) K3Fe2(PO4)3 (Pintard-Scrépel et al. 1983), NaV3P3O12 Å, 2.228 Å (Mn2), 2.228 Å (Mn3) and 2.235 Å (Mn4). (Kinomura et al. 1989), K6V2P4O16 (Benhamada et al. The longer average Mn–O bond lengths for Mn(2), 1991), K3Fe3(PO4)4•H2O (Lii 1995) and SrFe3(PO4)3 Mn(3) and Mn(4) are also refl ected in the slightly low (Korzenski et al. 1999). bond-valence sums for these atoms (Table 6). Although the structure of waterhouseite is unique, In the PO4 tetrahedron, the mean P–O bond length Table 7 shows that the unit-cell parameters reveal is 1.559 Å [range: 1.543(5) to 1.571(5) Å, Table 5], a some metric relations with the two chemical analogues value that is considerably longer than the grand mean allactite, Mn7(AsO4)2(OH)8 (Moore 1968) and raadeite, P–O bond length observed in phosphates (1.537 Å; Baur Mg7(PO4)2(OH)8 (Chopin et al. 2001). The latter two 1981, Huminicki & Hawthorne 2002). This anomaly is are isotypic, with space group P21/n. Unlike water- explained, fi rstly, by the presence of small amounts of houseite, they contain face-sharing MO6 (M: Mn or Mg) impurities on the P site (As, V, possibly Si; cf. Table 1), octahedra, and the PO4 tetrahedra in their frameworks which all lead to an increased metal–oxygen distance are linked only via their corners, but not edges. It would and decrease the bond-valence sum (Table 6). A refi ne- seem interesting to synthesize further compounds with 2+ 5+ ment of the occupancy of the P site gave an occupancy the general formula M 7(X O4)2(OH)8 in order to factor of 1.01, which indicates that the number of study their topology. possible impurity elements at this site appears to be very small, however. A tentative P versus V refi nement ACKNOWLEDGEMENTS gave the occupancy ratio 0.984:0.016. Secondly, there is the already-mentioned fact that two of the four edges of Glyn Francis kindly provided the specimens studied. the PO4 tetrahedron are shared with the adjacent Mn2- David Beattie of the Ian Wark Research Institute, and Mn3-centered Mn(O,OH)6 octahedra (the edges University of South Australia, is thanked for assistance O2–O4 and O1–O4, respectively, Fig. 4). As expected with collecting the Raman spectra of waterhouseite. from the repulsion of the metal cations across the two We also thank Drs F. Hatert, R.S.W. Braithwaite and shared edges, the Mn–Oshared and P–Oshared bond lengths A.-M. Fransolet for their constructive reviews of the are distinctly enlarged, with the exception of the P–O1 manuscript. The Alexander-von-Humboldt Foundation and P–O2 bonds (cf. Table 5). The oxygen atoms with (Bonn, Germany) and the Australian Research Council the lowest bond-valence sums are O1, O2, and O4, are thanked for a Feodor–Lynen Fellowship to UK. A which again refl ects the fact that all three belong to the grant from the Mark Mitchell Foundation is also grate- two shared edges of the PO4 tetrahedron. In summary, fully acknowledged. geometric constraints lead to considerable distortion around the PO4 tetrahedron. 1408 THE CANADIAN MINERALOGIST

FIG. 3a. Polyhedral representation of the crystal structure of waterhouseite, ideally Mn7(PO4)2(OH)8. The Mn(O,OH)6 octahedra and PO4 tetrahedra are linked by both edges and corners to form a dense framework. The Mn1- and Mn4-centered octahedra are unmarked, whereas the Mn2- and Mn3-centered octahedra are striped. The unit cell is outlined. View approximately along [201], approximately perpendicular to the arsenoclasite-type strips of octahedra (Mn1- and Mn4-centered octahedra) and perpendicular to the ﬁ nite chains of octahedra (Mn2- and Mn3-centered octahedra); note the single PO4 tetrahedron sharing two of its edges with Mn(O,OH)6 octahedra (see also Fig. 3b).

FIG. 3b. Polyhedral representation of the crystal structure of waterhouseite, ideally Mn7(PO4)2(OH)8. The Mn(O,OH)6 octahedra and PO4 tetrahedra are linked by both edges and corners to form a dense framework. The Mn1- and Mn4-centered octahedra are unmarked, whereas the Mn2- and Mn3-centered octahedra are striped. The unit cell is outlined. View along the b axis, parallel to the plane and elongation of the arsenoclasite-type strips of octahedra and perpendicular to the ﬁ nite chains of octahedra. All structure drawings were done with ATOMS (Shape Software, 1999). THE CRYSTAL STRUCTURE OF WATERHOUSEITE 1409

FIG. 4. Detailed view of bonding environment of the single PO4 tetrahedron, which shares two of its edges with Mn(O,OH)6 octahedra.

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