ALBERT

Description: Laurentianite, [NbO(H 2 O)] 3 (Si 2 O 7 ) 2 [Na(H 2 O) 2 ] 3 , is a new mineral discovered in siderite-dominant pods in an altered syenite at the Poudrette quarry, Mont Saint-Hilaire, Quebec. Crystals are colorless, acicular, euhedral, and elongate along [001] with average dimensions of 0.012 x 0.012 x 0.25 mm. The mineral generally occurs in loose, randomly oriented groupings (‘nests’) of crystals. Associated minerals include quartz, pyrite, franconite, rutile, lepidochrocite, and an unidentified Fe-bearing mineral. Laurentianite is transparent to translucent with a vitreous luster and is non-fluorescent under long-, medium-, and short-wave radiation. The Mohs hardness could not be measured owing to the small size of the crystals. No partings or cleavages were observed, although crystals do exhibit a splintery fracture. The calculated density is 2.464 g/cm 3 . Laurentianite is nonpleochroic and uniaxial negative, with 1.612(2) and 1.604(2). The average of 12 analyses from several crystals is: Na 2 O 8.88 (4.54–12.80), K 2 O 0.26 (0.14–0.44), CaO 0.22 (0.10–0.43), TiO 2 0.58 (0.31–0.83), Nb 2 O 5 43.64 (36.43–49.90), SiO 2 26.87 (22.81–29.07), and H 2 O (calc.) 17.93, total 98.38 wt.% on the basis of 26 anions, corresponding to [(Nb 0.99 Ti 0.01 ) 1.00 O(H 2 O)] 3 (Si 2.00 O 7 ) 2 [(Na 0.86 0.10 K 0.02 Ca 0.01 ) 0.99 (H 2 O) 2 ] 3 or, ideally, [NbO(H 2 O)] 3 (Si 2 O 7 ) 2 [Na(H 2 O) 2 ] 3 . The presence of H 2 O in laurentianite is inferred from Raman spectroscopy and results from refinement of the crystal structure. The mineral crystallizes in space group P 3 (#143) with a 9.937(1), c 7.004(1) Å, V 599.0(1) Å 3 , and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [ d in Å (I) ( hkl )] are: 8.608 (100) (010), 7.005 (19) (001), 4.312 (25) (020), 3.675 (25) (201, 021), 3.260 (31) (120, 210), and 2.870 (20) (030). The crystal structure of laurentianite, refined to R = 2.78% for 2347 reflections ( F o 〉 4 F o ) contains one Na , two Nb , and four Si sites. The two Nb sites are coordinated in distorted Nb O 5 (H 2 O) octahedra, with four equatorial bonds of typical Nb–O bond distances (~2 Å) and two highly asymmetric ones (one long, ~2.5 Å and one short, ~1.8 Å). Each site is each only partially occupied (~50%) and because of the short distance between them (~0.7 Å), they are not simultaneously occupied. A novel cation-anion coordination scheme involving the apical oxygens, Nb, and disordered H 2 O groups is developed: when one of the Nb sites is occupied, the other is vacant, resulting in one of the apical O sites being occupied by O 2– and the other by H 2 O. The opposite situation occurs when the occupancy and vacancy of the Nb sites are reversed, leading to both apical O sites having an equal, mixed (O 2– /H 2 O) composition. A minor charge understaturation at both apical O sites is remedied by each of these O sites receiving a single H-bond from one of the H 2 O groups associated with the Na cation. The crystal structure of laurentianite is based on five-membered pinwheels of composition [Nb 3 Si 2 O 17 (H 2 O) 3 ] –11 , consisting of three Nb O 5 (H 2 O) octahedra linked to two SiO 4 tetrahedra. Individual Nb–Si pinwheels are attached to form a layer composed of 18-membered rings of composition [Nb 6 Si 12 O 54 (H 2 O) 6 ] 30– perpendicular to [001]. The crystal structure is also layered along [001], with a silicate layer composed of (Si 2 O 7 ) dimers and a layer of isolated Nb O 5 (H 2 O) octahedra. Sodium atoms are positioned within the silicate layer, occupying sites that almost directly overly the Nb sites but are displaced ~ z + 1/2. Laurentianite is a late-stage mineral intergrown with lepidocrocite, both of which overgrow franconite and quartz. The mineral is believed to have precipitated from a late-stage aqueous fluid enriched in Na, Si, and Nb, possibly arising through the breakdown of franconite, sodalite, and quartz.

Description: A bstract The crystal structure of faheyite, ideally Mn 2+ Fe 3+ 2 [Be 2 (PO 4 ) 4 ](H 2 O) 6 , trigonal, a 9.404(7), c 15.920(11) Å, V 1219(2) Å 3 , Z = 3, space group P 3 1 21, has been solved and refined to an R 1 index of 4.4% with single-crystal X-ray diffraction data collected from a twinned fiber. There are two P sites that are tetrahedrally coordinated by O atoms with 〈 P –O〉 distances of 1.52 and 1.54 Å, respectively, one Be site tetrahedrally coordinated by O atoms with a 〈 Be –O〉 distance of 1.63 Å, one Mn site occupied by Mn 2+ coordinated by four O atoms and two (H 2 O) groups with a 〈 Mn –O〉 distance of 2.22 Å, and one Fe site occupied by Fe 3+ coordinated by four O atoms and two (H 2 O) groups with a 〈 Fe –O〉 distance of 2.01 Å. Each vertex of the Be tetrahedron is shared with a vertex of a neighboring P tetrahedron, and two vertices of each P tetrahedron are shared with neighboring Be tetrahedra to form a corner-sharing [Be(PO 4 ) 2 ] chain, with P tetrahedra flanking the Be tetrahedra of the central spine in the sequence - P (1)/ P (1)- Be - P (2)/ P (2)- Be -. Faheyite has a chiral structure, with the [Be(PO 4 ) 2 ] chain twisting about the c -axis in a clockwise direction for the refined P 3 1 21 enantiomer. The Mn octahedron lies along the 3 1 screw axis within the core region of the [Be(PO 4 ) 2 ] chain, forming [MnBe 2 (PO 4 ) 4 ] spires that are wrapped by Fe octahedra that share vertices with P tetrahedra. The crystal structures of fransoletite and parafransoletite also contain beryllophosphate chains topologically identical to that found in faheyite, although the [Be(PO 4 )(PO 3 OH)] chain in fransoletite and parafransoletite is straight, whereas the [Be(PO 4 ) 2 ] chain in faheyite forms a helix about the central c -axis.

Description: Fontarnauite was discovered in cores recovered from the Kütahya-Emet 2 and 188 (named here as Doğanlar) boreholes drilled in the Emet borate basin near the village of Doğanlar, Kütahya Province, Western Anatolia, Turkey. The Emet (or Emet-Hisarcık) basin is one of the Neogene basins in western Turkey bearing a borate-rich unit intercalated with Miocene sediments. Fontarnauite is most commonly associated with probertite, glauberite, and celestine and occurs as isolated colorless to light-brown prismatic crystals or as clusters of crystals less than 5 mm long. Fontarnauite is brittle, with a Mohs hardness of 21/2–3, and perfect {010} cleavage. D calc = 2.533 g/cm 3 . The new mineral is optically biaxial (–), α 1.517(2), β 1.539(2), 1.543(2) (590 nm); 2 V meas = 46(1)°; 2 V calc = 46°; X ^ a 95.0° (β obtuse); Y // b , Z ^ c 81.9° (β acute). Dispersion is r 〉 v , medium to weak. The chemical composition (electron microprobe; B and H from the crystal-structure refinement) is as follows: SO 3 17.75, B 2 O 3 38.66, CaO 2.26, SrO 18.98, Na 2 O 12.65, K 2 O 1.70, H 2 O 10.01, total 102.01 wt.%. The empirical formula (based on 15 O atoms per formula unit) is (Na 1.84 K 0.16 ) 2.00 (Sr 0.82 Ca 0.18 ) 1.00 S 1.00 B 5 H 5 O 15 ; the endmember formula is Na 2 Sr(SO 4 ) [B 5 O 8 (OH)](H 2 O) 2 based on the crystal-structure refinement. Single-crystal X-ray studies gave the space group P 2 1 /c, a 6.458(2), b 22.299(7), c 8.571(2) Å, β 103.047(13)°, V 1202.5(1.0) Å 3 , Z = 4. Structure refinement ( R 1 = 2.9%) revealed that two BO 4 tetrahedra and three BO 3 triangles share vertices to form B 5 O 10 (OH) units that link to other B 5 O 10 (OH) units along [100] and [001] to give a [B 5 O 8 (OH)] sheet parallel to (010). Within the central cavities of opposing sheets are the H 2 O groups, SO 4 tetrahedra, and Na (1) sites; the Sr and Na (2) sites occupy the interstices of a given sheet. The region of the structure where opposing cusps of neighboring sheets approach each other is dominated by weaker H-bonding associated with the OH and H 2 O groups, in accord with the observed perfect {010} cleavage. The strongest lines in the powder X-ray diffraction pattern, obtained after profile fitting using the Le Bail method, are as follows [ d in Å ( I ) ( hkl )]: 11.1498 (100)(020), 3.3948 (8)(061), 3.3389 (20)(042), 3.1993, 3.1990 (10)(160, 1 42), 3.0458(10)(052), 3.0250(7)(220), 2.7500 (10)( 2 22,142), 2.3999 (8)(260), 2.2300, 2.2284(7)(0 10 0,222), 1.9241, 1.9237(7)(311, 2 24). The holotype is deposited in the mineralogy collection of the Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada, accession number M56745.

Description: A bstract The crystal structure of mammothite, Pb 6 Cu 4 AlSbO 2 (SO 4 ) 2 Cl 4 (OH) 16 , is monoclinic in acentric space group C 2, with a 18.959(4), b 7.3398(19), c 11.363(3) Å, β 112.428(9){ring}, V 1461.6(1.0) Å 3 , and Z = 2. It has been refined to an R index of 0.019 on the basis of 3878 observed reflections. There are three crystallographically distinct Pb sites with two different co-ordinations: [Pb1O 8 Cl 1 ] is a mono-capped square antiprism polyhedron, while [Pb21O 7 Cl 2 ] and [Pb22O 7 Cl 2 ] are tri-capped trigonal prisms. Both Cu 2+ sites have distorted [4 + 2] octahedral coordination due to the Jahn-Teller effect. The Al and Sb sites are regular-octahedral co-ordination with oxygen atoms. The [SO 4 ] tetrahedron is quite distorted, with S–O bond lengths varying from 1.45 to 1.52 Å and subtended O–S–O angles varying from 106 to 113{ring}. In the structure there are eight (OH) – anions. All eight H atoms pfu were located, and it is these structure sites that reduce the symmetry from centric to acentric. Although mammothite is classified as a framework structure, it has a distinct layering. There are two layer types in the mammothite structure that parallel (001). There are three octahedrally coordinated sites; two are occupied by Cu atoms and one by an Al atom, in the octahedral layer. The tetragonal dipyramids [CuØ 6 ] are linked forming ‘olivine-like’ chains parallel to the b -axis. The second layer, termed the cross-linked layer, has three [PbØ 9 ] polyhedra with shared edges forming chains parallel to the b -axis, like the [CuØ 6 ] tetragonal dipyramids. These chains are cross-linked by edge-sharing [SbO 6 ] octahedra and decorated with [SO 4 ] groups. The H atoms are in ‘holes’ within both layers.

Description: Billwiseite, ideally Sb 3+ 5 (Nb,Ta) 3 WO 18 , is an oxide mineral from a granitic pegmatite on the eastern margin of the Nanga Parbat – Haramosh massif at Stak Nala, 70 km east of Gilgit, Pakistan. It is transparent, pale yellow (with a tinge of green), has a colorless to very pale-yellow streak, a vitreous luster, and is inert to ultraviolet radiation. Crystals are euhedral with a maximum size of 0.5 x 0.25 x 0.15 mm and show the following forms: {100} pinacoid {011} pinacoid {410} prism; contact twins on (100) are common. Cleavage is {100} indistinct, Mohs hardness is 5, and billwiseite is brittle with a hackly fracture. The calculated density is 6.330 g/cm 3 . The indices of refraction were not measured; the calculated index of refraction is 2.3, 2 V ( obs ) = 76(2)°. Billwiseite is colorless in transmitted light, non-pleochroic, and the optic orientation is as follows: X || b , Y ^ c = 72.8° (in β acute). It occurs scattered across the surface of a large (5 x 2.5 x 1.3 cm) crystal of lepidolite from a miarolitic cavity. The most abundant minerals in the cavities at Stak Nala are albite, quartz, K-feldspar, tourmaline, muscovite or lepidolite, topaz and fluorite, and billwiseite can be partly mantled by B-rich muscovite. Billwiseite is monoclinic, space group C 2/ c , a 54.116(5), b 49143(5), c 5.5482(5) Å, β 90.425(2)°, V 1475.5(2) Å 3 , Z = 4, a:b:c = 11.012 : 1 : 1.131. The strongest seven lines in the X-ray powder-diffraction pattern [ d in Å( I ) hkl ] are as follows: 3.147(100)(91, 911), 3.500(55) (51, 511), 1.662(53)( 14 2), 3.017(48)( 18 00), 1.906(47)( 18 20), 1.735(30)(11, 113), 1.762(25)( 27 1, 27 11). Chemical analysis by electron microprobe gave Nb 2 O 5 12.03, Ta 2 O 5 19.31, Sb 2 O 3 48.34, TiO 2 0.99, WO 3 19.96, sum 100.63 wt.% where the valence state of Sb was determined by crystal-structure analysis. The resulting empirical formula on the basis of 18 O anions is Sb 3+ 4.87 (Nb 1.33 Ta 1.28 Ti 0.18 W 1.26 ) 4.05 O 18 . The crystal structure of billwiseite was solved by direct methods and refined to an R 1 index of 4.71% based on 2122 observed reflections collected on a three-circle diffractometer with Mo K α X-radiation. The structure consists of two distinct sheets of M (= Ta,Nb,W) octahedra and three distinct sheets of Sb 3+ polyhedra parallel to (100). These sheets alternate in the a direction to form a continuous structure.

Description: Aspedamite, ideally 12 (Fe 3+ ,Fe 2+ ) 3 Nb 4 [Th(Nb,Fe 3+ ) 12 O 42 ]{(H 2 O),(OH)} 12 , is a new heteropolyniobate mineral species from the Herrebøkasa quarry, Aspedammen, Østfold, southern Norway. It occurs as small euhedral crystals of dodecahedra and cubes to a maximum of 50 μm across, perched on a white mat of an Al–Nb–Fe–Ti–Ca–K-bearing silicate on a partly altered 12 x 12 x 6 mm crystal of monazite penetrated by plates of columbite-(Fe) and muscovite. Aspedamite is brownish orange with a very pale orange streak and an adamantine luster; it does not fluoresce under ultraviolet light. The Mohs hardness is 3–4, and it is brittle with a hackly fracture. The calculated density is 4.070 g/cm 3 , and the calculated index of refraction is 2.084. Aspedamite is cubic, space group Im , a 12.9078(6) Å, V 2150.6(3) Å 3 , Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [ d in Å( I ) hkl ] are: 9.107(100)011, 2.635(36)224, 2.889(33)024, 1.726(29)246, 3.233(28)004, 3.454(18)123, 4.567(15)022. A chemical analysis with an electron microprobe gave Nb 2 O 5 65.64, Ta 2 O 5 1.78, SiO 2 0.78, ThO 2 5.64, TiO 2 2.15, Fe 2 O 3 10.56, FeO 2.73, MnO 0.82, CaO 0.28, K 2 O 0.16, La 2 O 3 0.52, Ce 2 O 3 1.62, Nd 2 O 3 0.44, H 2 O (calc) 7.20, sum 100.32 wt.%; the H 2 O content was determined by crystal-structure analysis. The empirical formula of aspedamite on the basis of 54 anions with Fe 3+ /(Fe 3+ + Fe 2+ ) = 0.67 (estimated from crystal-chemical arguments) and O(4) = (H 2 O) 9 + (OH) 3 is K 0.09 Ca 0.13 Ce 0.26 La 0.08 Nd 0.07 Fe 2+ 1.00 Mn 0.30 Fe 3+ 3.48 Th 0.56 Ti 4+ 0.71 Si 0.34 Nb 12.98 Ta 0.21 O 42 (H 2 O) 9 (OH) 3 . The crystal structure of aspedamite was solved by direct methods and refined to an R 1 index of 1.6% based on 596 observed reflections collected on a three-circle rotating-anode (Mo K α X-radiation) diffractometer equipped with multilayer optics and an APEX-II detector. The structure is based on the heteropolyanion [ DA 12 O 42 ] ( D = Th, sum A = Nb 9 Fe 3+ 2 Ti), which consists of twelve face- and corner-sharing AO 6 octahedra that surround the [12]-coordinated D cation. There are eight heteropolyanions at the corners of the unit cell, with an additional heteropolyanion at the center, forming an I -centered arrangement. Each heteropolyhedral cluster is decorated by eight B octahedra, each of which bridges two adjacent clusters along the body diagonals of the cell. Further intercluster linkage is provided by the C octahedra, which link pairs of adjacent clusters in the a direction. Aspedamite is isostructural with menezesite.

Description: Ferrochiavennite is a new beryllium silicate zeolite with chemical composition close to Ca 1–2 FeSi 5 Be 2 O 13 (OH) 2 ·2H 2 O. It is described from two syenite pegmatite localities in Norway: Blåfjell, Langangen, Telemark, and the AS Granit larvikite quarry, Tvedalen, Vestfold. The mineral is monoclinic, P 2 1 / c , with a 8.759(5), b 4.864(2), c 31.258(6) Å, β 90.31(6)°, V 1331.7(6) Å 3 , and Z = 4. The crystal structure was refined to R 1 = 0.048 for 3651 observed reflections. The zeolite structure is isostructural with chiavennite, consisting of intersecting channels of nine-, six-, five-, and four-fold rings. The strongest eight reflections of the X-ray powder-diffraction pattern [ d (obs.) in Å ( I ) ( hkl )] are: 15.555 (100) (002), 4.104 (29) (12, 112), 3.938 (36) (13, 113), 3.909 (60) (008), 3.820 (30) (04, 204), 3.251 (66) (017, 210, 11), 3.186 (27) (12, 212), 2.884 (64) (15, 215). The mineral is biaxial (+) with refractive indices α 1.583(1), β 1.589(1), 1.602(1), measured at 590 nm. 2 V (meas.) = 62(4)° from extinction curves 2 V = 76(5)°; 2 V (calc.) = 69°. The optical orientation is X ~ a , Y ~ c , and Z ~ b . The Mohs hardness is ~ 3; D (meas.) = 2.67(2) and D (calc.) = 2.709 g/cm 3 .

Description: Hydroxylgugiaite, ideally (Ca 3 1 ) 4 (Si 3.5 Be 2.5 ) 6 O 11 (OH) 3 , is a new mineral species from two localities in the Larvik plutonic complex in Porsgrunn, Telemark, Norway, and one locality in Ilímaussaq, Greenland. Hydroxylgugiaite crystals occur as squat dipyramids {111} (30 x 50 μm) or as elongate tetragonal prisms. The crystals are translucent, white to pale grey in color, with a white streak and vitreous luster. It is brittle, with no apparent cleavage. Hydroxylgugiaite is uniaxial positive with = 1.622 ± 0.002 and = 1.632 ± 0.002. There is no pleochroism and birefringence is low. The average of eight analyses of a single grain of type material (oxide wt.%) gave Na 2 O 2.04, CaO 32.90, FeO 0.22, MnO 0.74, BeO 13.47 (LA-ICP-MS), Al 2 O 3 0.74, SiO 2 44.06, F 1.74, H 2 O (assuming 3 OH + F) 4.93, Total (–0.73 O = F) 100.10. Potassium, strontium, and magnesium were measured but not detected. The calculated density is 2.79 g cm –3 . The empirical formula on the basis of 14 anions including 3 OH – + F – is: (Ca 2.76 Na 0.31 Mn 0.05 Fe 0.01 ) 3.13 (Si 3.45 Be 2.53 Al 0.07 ) 6.05 O 11 [(OH) 2.57 F 0.43 ] 3 . The formula from crystal-structure analysis of the Saga specimen is: (Ca 3.02 0.98 ) 4 (Si 1.79 Be 0.21 ) 2 (Be 2.29 Si 1.71 ) 4 O 11 (OH) 3 . Combined structural and chemical data gives the following formula for the Nakkaalaaq specimen: (Ca 2.88 0.98 Na 0.12 Mn 0.02 ) 4 (Si 1.80 Be 0.17 Al 0.03 ) 2 (Be 2.32 Si 1.68 ) 4 O 11 [(OH) 2.70 F 0.30 ] 3 ; with simplified formula (Ca,) 4 (Si,Be) 2 (Be,Si) 4 O 11 (OH) 3 . The crystal structure of hydroxylgugiaite is tetragonal in acentric space group P 2 1 / m , with a 7.4151(2), b 7.4151, c 4.9652(1) Å, V 272.9(1) Å 3 , and Z = 1. It has been refined to an R index of 0.028 on the basis of 342 observed reflections and a correction for the {110} twin law. It is an H-bearing member of the melilite group. The structure has two distinct layers. The one crystallographically distinct Ca site with eight-fold coordination is a square antiprism polyhedron. The Ca polyhedra are in a layer with the H atoms. A second layer consists of corner-sharing Si/Be atoms in tetrahedral coordination with O. One H atom is bonded to an apical O atom that is not shared by two tetrahedra. This H atom is present only when there is a Ca -site vacancy. The other H atom is loosely bonded to the same O atom but at a different site. The IR spectrum supports this H-bonding scheme. Additional hydroxylgugiaite data is given for the other localities.

Description: Wopmayite, ideally Ca 6 Na 3 Mn(PO 4 ) 3 (PO 3 OH) 4 , is a new secondary mineral from the Tanco mine, Bernic Lake, Manitoba. It occurs in vugs in a single 5–10 cm mass of phosphate-carbonate mineralization in a spodumene-rich boulder found in the dumps of the Tanco Mine, Bernic Lake, Manitoba, Canada. It is a secondary mineral that crystallized together with rhodochrosite, quartz, whitlockite, apatite, and other phases after dissolution of primary lithiophosphate by hydrothermal solutions. The initial crystal of wopmayite was a corroded {101} rhomb ~150 microns across. Wopmayite is colorless to white to pale pink with a white streak and a vitreous luster, and does not fluoresce under ultraviolet light. It has a Mohs hardness of 5, is brittle, has an irregular to subconchoidal fracture, and shows no cleavage or parting. The calculated density is 3.027 g/cm 3 . It is uniaxial (-), = 1.617, = 1.613, both ± 0.002. Wopmayite is hexagonal-rhombohedral, space group R 3 c , a 10.3926(2), c 37.1694(9) Å, V 3476.7(2) Å 3 , Z = 6, a:c = 1:3.577. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (Å), I , ( h k l ): 2.858, 100, (02.10); 3.186, 88, (34); 2.589, 68, (40); 5.166, 33, (20); 6.421, 32, (14); 8.017, 31, (012); and 3.425, 29, (110). Chemical analysis by electron microprobe gave P 2 O 5 46.40, Al 2 O 3 0.38, Fe 2 O 3 0.80, FeO 0.96, MnO 3.74, MgO 0.41, CaO 37.65, SrO 0.91, Na 2 O 5.43, and H 2 O(calc) 2.00, sum 98.68 wt.%. The H 2 O content was determined by crystal-structure analysis. On the basis of 28 O apfu , the empirical formula is (Ca 7 .19 Na 1.88 Sr 0.09 ) 9.16 (Mn 0.56 Mg 0.11 Fe 2+ 0.14 Fe 3+ 0.11 Al 0.08 ) 1.00 (PO 4 ) 4.63 (PO 3 OH) 2.37 , and the endmember formula is Ca 6 Na 3 Mn(PO 4 ) 3 (PO 3 OH) 4 . The crystal structure of wopmayite was solved by direct methods and refined to an R 1 index of 2.21% based on 2288 unique observed reflections collected on a three-circle rotating-anode ( M o K α X-radiation) diffractometer equipped with multilayer optics and an APEX-II detector. Wopmayite has a structural unit consisting of an [M 2+ (PO 4 ) 6 ] arrangement that is topologically the same as the structural units in the whitlockite and merrillite structures. The [M 2+ (PO 4 ) 6 ] clusters are linked by Ca polyhedra and (PO 3 ) groups of the form {Ca 9 X (PO 3 )} where = O,OH and X = (, Na, Ca), depending on the mineral species. Wopmayite is related to whitlockite by the substitution Na + H -〉 Ca + , whereby Na is incorporated primarily at the Ca (3) site and H attaches to the P (1) tetrahedron to produce an acid-phosphate group. Thus merrillite contains no acid-phosphate group, whitlockite contains a single acid-phosphate group at P (3), and wopmayite contains acid-phosphate groups at both P (1) and P (3).

Description: The crystal structure of the mineral kurilite, a rare silver chalcogenide, was solved using intensity data collected from a twinned crystal of type material from the Prasolovskoe deposit, Kunashir Island, Kuril islands (Russia). The study revealed that the structure is trigonal, space group R 3{macron} , with cell parameters: a 15.8135(18), c 19.618(3) Å, and V 4248.6(9) Å 3 . The refinement had a final R index of 0.0218 for 2513 observed reflections [ I 〉 2( I )] and 0.0265 for all 2776 independent reflections and 193 parameters. Three Te sites, three Se sites, and ten Ag sites occur in the crystal structure of kurilite. The silver cations form various crystal-chemical environments, as is typical of many intermetallic compounds. The Ag positions correspond to the most pronounced probability density function ( pdf ) locations (modes) of diffusion-like paths. The d 10 silver ion distribution was determined by means of a Gram-Charlier development of the atomic displacement factors. Crystal-chemical features are discussed in relation to other copper and silver tellurides and pure metals. The disorder observed for the Ag atoms is compared to that of other natural fast ionic conductors, such as the pearceite-polybasite group minerals. On the basis of information gained from the structural characterization, the Z of the crystal-chemical formula was revised to 18 instead of 15, as previously reported.