Robert A. Jenkins

Phosphohedyphane, Ca2Pb3(PO4)3Cl, the phosphate analog of hedyphane: Description and crystal structure

Abstract Phosphohedyphane, Ca2Pb3(PO4)3Cl, space group P63/m, a = 9.857(1), c = 7.130(2) Å, V = 599.9(2) Å3, Z = 2, is a new mineral from the Capitana mine, Copiapó, Atacama Province, Chile and has been identified from numerous other deposits world-wide. At the Capitana mine, it occurs as transparent, colorless, tapering hexagonal prisms, as individuals up to about 0.5 mm in length and 0.1 mm in diameter, which are commonly doubly terminated. Crystals often occur in subparallel intergrowths and irregular clusters. Phosphohedyphane forms as a secondary mineral in the oxidized zone of the Capitana mine, a Cu-Pb-Ag deposit, where it closely associated with quartz, duftite, and bayldonite. Crystals exhibit core-to-rim chemical zonation and electron analyses of cores/rims yielded CaO 9.24/7.76, PbO 67.60/69.35, P2O5 18.40/17.00, As2O5 2.73/3.68, Cl 3.32/3.22, .O = Cl .0.75/.0.73, total 100.54/100.28 wt%. The name phosphohedyphane is for the relationship of the mineral to hedyphane. The mineral has an apatite structure with ordering of Ca and Pb in the two non-equivalent large cation sites, as in hedyphane. The structure refinement indicates that the Ca2(6h) site is completely occupied by Pb and the Ca1(4f) site is occupied by 92% Ca and 8% Pb. The tetrahedral site refines to 91% P and 9% As. The refinement indicates the 0,0,0 position to be fully occupied by Cl. The ordering of Ca and Pb in phosphohedyphane has important implications with respect to the chlorapatite-pyromorphite solid solution series. An analysis of compositions of natural members of the pyromorphite-mimetite-turneaureite-chlorapatite system suggests the existence of complete solid solution among pyromorphite, mimetite, hedyphane, and phosphohedyphane. No stable solid solutions appear to exist between the joins phosphohedyphane-hedyphane and chlorapatite-turneaureite in natural systems.

Markascherite, Cu3(MoO4)(OH)4, a new mineral species polymorphic with szenicsite, from Copper Creek, Pinal County, Arizona, U.S.A.

Abstract A new mineral species, markascherite (IMA2010-051), ideally Cu3(MoO4)(OH)4, has been found at Copper Creek, Pinal County, Arizona, U.S.A. The mineral is of secondary origin and is associated with brochantite, antlerite, lindgrenite, wulfenite, natrojarosite, and chalcanthite. Markascherite crystals are bladed (elongated along the b axis), up to 0.50 × 0.10 × 0.05 mm. The dominant forms are {001}, {100}, and {010}. Twinning is found with the twofold twin axis along [101̄]. The mineral is green, transparent with green streak and vitreous luster. It is brittle and has a Mohs hardness of 3.5~4; cleavage is perfect on {100} and no parting was observed. The calculated density is 4.216 g/cm3. Optically, markascherite is biaxial (-), with nα >1.8, nβ > 1.8, and nγ >1.8. The dispersion is strong (r > v). It is insoluble in water, acetone, or hydrochloric acid. An electron microprobe analysis yielded an empirical formula Cu2.89(Mo1.04O4)(OH)4. Markascherite, polymorphic with szenicsite, is monoclinic, with space group P21/m and unit-cell parameters a = 9.9904(6), b = 5.9934(4), c = 5.5255(4) Å, β = 97.428(4)°, and V = 328.04(4) Å3. Its structure is composed of three nonequivalent, markedly distorted Cu2+(O,OH)6 octahedra and one MoO4 tetrahedron. The Cu1 and Cu2 octahedra share edges to form brucite-type layers parallel to (100), whereas the Cu3 octahedra share edges with one another to form rutile-type chains parallel to the b axis. These layers and chains alternate along [100] and are interlinked together by both MoO4 tetrahedra and hydrogen bonds. Topologically, the structure of markascherite exhibits a remarkable resemblance to that of deloryite, Cu4(UO2)(MoO4)2(OH)6, given the coupled substitution of [2Cu2+ + 2(OH−)]2+ for [(U6+ + □) + 2O2-]2+. The Raman spectra of markascherite are compared with those of two other copper molybdate minerals szenicsite and lindgrenite.

Anorpiment, As2S3, the triclinic dimorph of orpiment

Abstract The new mineral anorpiment, As2S3, the triclinic dimorph of orpiment, has space group P1 and cell parameters a = 5.7577(2), b = 8.7169(3), c = 10.2682(7) Å, α = 78.152(7), β = 75.817(7), γ = 89.861(6)°, V = 488.38(4) Å3 and Z = 4. It occurs at the Palomo mine, Castrovirreyna Province, Huancavelica Department, Peru. It is a low-temperature hydrothermal mineral associated with dufrénoysite, muscovite, orpiment, pyrite and realgar. It occurs in drusy crusts of wedge-shaped, transparent, greenish yellow crystals. The streak is yellow. The lustre is resinous on crystal faces, but pearly on cleavage surfaces. The Mohs hardness is about 1½. The mineral is sectile with an irregular fracture and one perfect and easy cleavage on {001}. The measured and calculated densities are 3.33 and 3.321 g cm-3, respectively. All indices of refraction are greater than 2. The mineral is optically biaxial (-) with 2V = 35-40° and no observed dispersion. The acute bisectrix (X) is approximately perpendicular to the {001} cleavage. Electron microprobe analyses yielded the averages and ranges in wt.%: As 58.21 (57.74-59.03), S 38.72 (38.33-39.00), total 96.94 (96.07-97.75), providing the empirical formula (based on 5 atoms) As1.96S3.04. The strongest powder X-ray diffraction lines are [d (hkl) I] 4.867(002) 97, 4.519 (110,11̅1) 77, 3.702 (1̅1̅1) 46, 3.609 (022,11̅2) 82, 2.880 (201,02̅2,1̅2̅1,023) 75, 2.552 (1̅13,1̅31,132) 100, 2.469 (114,130,13̅1) 96. The structure of anorpiment [R 1 = 0.021 for1484 reflections with F o > 4σ(F)] consists of layers of covalently bonded As and S atoms. Each S atom bonds to two As atoms at As-S -As angles between 100.45 and 104.15°. Each As atom is strongly bonded to three S atoms at S-As -S angles between 91.28 and 103.59°, forming an AsS3 pyramid with As at its apex. The As - S linkages within the layers form rings of six AsS3 pyramids. Interlayer bonding forces are interpreted as van der Waals. The structure of anorpiment is similar to that of orpiment in that it is composed of layers of As2S3 macromolecules, but arranged in a different stacking sequence.

Redetermination of conichalcite, CaCu(AsO4)(OH)

The crystal structure of conichalcite [calcium copper(II) arsenate(V) hydroxide], with ideal formula CaCu(AsO4)(OH), was redetermined from a natural twinned specimen found in the Maria Catalina mine (Chile). In contrast to the previous refinement from photographic data [Qurashi & Barnes (1963 ▶). Can. Mineral. 7, 561–577], all atoms were refined with anisotropic displacement parameters and with the H atom located. Conichalcite belongs to the adelite mineral group. The Jahn–Teller-distorted [CuO6] octahedra share edges, forming chains running parallel to [010]. These chains are cross-linked by eight-coordinate Ca atoms and by sharing vertices with isolated AsO4 tetrahedra. Of five calcium arsenate minerals in the adelite group, the [MO6] (M = Cu, Zn, Co, Ni and Mg) octahedron in conichalcite is the most distorted, and the donor–acceptor O—H⋯O distance is the shortest.

Te-rich raspite, Pb(W0.56Te0.44)O4, from Tombstone, Arizona, U.S.A.: The first natural example of Te6+ substitution for W6+

Abstract Te-rich raspite, Pb(W0.56Te0.44)O4, from the Grand Central mine, Tombstone, Arizona, U.S.A., was studied with single-crystal X-ray diffraction, Raman spectroscopy, and electron microprobe analysis. The mineral represents the first natural example of Te6+ substitution for W6+. It displays monoclinic symmetry with space group P21/a and unit-cell parameters a = 13.621(3), b = 5.019(1), c = 5.586(1) Å, b = 107.979(5)°, and V = 363.2(2) Å3. Its structure consists of distorted MO6 (M = W + Te) octahedra sharing edges to form zigzag chains running parallel to [010]. These octahedral chains are linked together by seven-coordinated Pb2+ cations. In addition, a refinement of the regular raspite structure with measured chemistry Pb1.00W1.00O4, P21/a symmetry, and unit-cell parameters a = 13.5773(8), b = 4.9806(3), c = 5.5670(3) Å, b = 107.658(3)°, and V = 358.72(4) Å3 is presented. Compared with regular raspite (PbWO4), the partial substitution of the small radius Te6+ for larger W6+ results in a decrease in the MO6 octahedral distortion, with a concomitant increase in the MO6 octahedral volume and the average Pb-O bond length. In addition, as should be expected for mixed occupancy compounds, most Raman bands for the mixed Te-rich raspite are broader than the corresponding ones for the end-member regular raspite. High-temperature annealing experiments reveal that Te-rich raspite transforms irreversibly to the stolzite structure at 590(10) °C, which is considerably higher than the reported transformation temperature of 395(5) °C for regular raspite.

Rongibbsite, Pb2(Si4Al)O11(OH), a new zeolitic aluminosilicate mineral with an interrupted framework from Maricopa County, Arizona, U.S.A.

Abstract A new zeolitic aluminosilicate mineral species, rongibbsite, ideally Pb2(Si4Al)O11(OH), has been found in a quartz vein in the Proterozoic gneiss of the Big Horn Mountains, Maricopa County, Arizona, U.S.A. The mineral is of secondary origin and is associated with wickenburgite, fornacite, mimetite, murdochite, and creaseyite. Rongibbsite crystals are bladed (elongated along the c axis, up to 0.70 × 0.20 × 0.05 mm), often in tufts. Dominant forms are {100}, {010}, {001}, and {101̄}. Twinning is common across (100). The mineral is colorless, transparent with white streak and vitreous luster. It is brittle and has a Mohs hardness of ~5; cleavage is perfect on {100} and no parting was observed. The calculated density is 4.43 g/cm3. Optically, rongibbsite is biaxial (+), with nα = 1.690, nβ = 1.694, nγ = 1.700, cZ = 26°, 2Vmeas = 65(2)°. It is insoluble in water, acetone, or hydrochloric acid. Electron microprobe analysis yielded an empirical formula Pb2.05(Si3.89Al1.11)O11(OH). Rongibbsite is monoclinic, with space group I2/m and unit-cell parameters a = 7.8356(6), b = 13.913(1), c = 10.278(1) Å, β = 92.925(4)°, and V = 1119.0(2) Å3. Its structure features an interrupted framework made of three symmetrically distinct TO4 tetrahedra (T = Si + Al). The framework density is 17.9 T per 1000 Å3. Unlike many known interrupted frameworks in zeolite-type materials, which are usually broken up by OH or F, the framework in rongibbsite is interrupted by O atoms. There are various corner-shared tetrahedral rings in the framework of rongibbsite, including two types of 4-membered, three 6-membered, and one 8-membered rings. The extraframework Pb and OH reside alternately in the channels formed by the 8-membered rings. The Pb cations are disordered over two split sites, Pb and Pb′, with site occupancies of 0.8 and 0.2, respectively, and a Pb-Pb′ distance of 0.229 Å, providing a structural explanation for the two strong Raman bands (at 3527 and 3444 cm-1) attributable to the O-H stretching vibrations. The average bond lengths for the T1, T2, and T3 tetrahedra are 1.620, 1.648, and 1.681 Å, respectively, indicating that the preference of Al for the three tetrahedral sites is T3 >> T2 > T1. Rongibbsite represents the first natural aluminosilicate with Pb as the only extraframework cation.

Segerstromite, Ca3(As5+O4)2[As3+(OH)3]2, the first mineral containing As3+(OH)3, the arsenite molecule, from the Cobriza mine in the Atacama Region, Chile

Abstract A new mineral species, segerstromite, ideally Ca3(As5+O4)2[As3+(OH)3]2, has been discovered at the Cobriza mine in the Sacramento district in the Copiapó Province, Chile. Crystals of segerstromite occur as tetrahedra, dodecahedra (up to 0.50 × 0.50 × 0.50 mm), or in blocky aggregates. Associated minerals include talmessite, vladimirite, and Sr-bearing hydroxylapatite. Similar to the associated minerals, segerstromite is a secondary mineral. The new mineral is colorless in transmitted light, transparent with a white streak and vitreous luster. It is brittle and has a Mohs hardness of ~4.5. No cleavage, parting, or twinning was observed. The measured and calculated densities are 3.44(3) and 3.46 g/cm3, respectively. Optically, segerstromite is isotropic, with n = 1.731(5). It is insoluble in water or hydrochloric acid. An electron microprobe analysis yielded an empirical formula (based on 14 O apfu) Ca2.98(AsO4)2.00[As(OH)3]2.00. Segerstromite is cubic, with space group I213 and unit-cell parameters a = 10.7627(2) Å, V = 1246.71(4) Å3, and Z = 4. Its crystal structure is constructed from three different polyhedral units: distorted CaO8 cubes, rigid As5+O4 arsenate tetrahedra, and neutral As3+(OH)3 arsenite triangular pyramids. The Ca-groups form layers of corrugated crankshaft chains that lie parallel to the cubic axes. These chains are linked by the isolated As5+O4 and As3+(OH)3 groups. Segerstromite is the first known crystalline compound that contains the As3+(OH)3 arsenite molecule, pointing to a new potential approach to remove highly toxic and mobile As3+(OH)3 from drinking water.

Calcioferrite with composition (Ca3.94Sr0.06)Mg1.01(Fe2.93Al1.07)(PO4)6(OH)4·12H2O.

Calcioferrite, ideally Ca4MgFe3+ 4(PO4)6(OH)4·12H2O (tetracalcium magnesium tetrairon(III) hexakis-phosphate tetrahydroxide dodecahydrate), is a member of the calcioferrite group of hydrated calcium phosphate minerals with the general formula Ca4 AB 4(PO4)6(OH)4·12H2O, where A = Mg, Fe2+, Mn2+ and B = Al, Fe3+. Calcioferrite and the other three known members of the group, montgomeryite (A = Mg, B = Al), kingsmountite (A = Fe2+, B = Al), and zodacite (A = Mn2+, B = Fe3+), usually occur as very small crystals, making their structure refinements by conventional single-crystal X-ray diffraction challenging. This study presents the first structure determination of calcioferrite with composition (Ca3.94Sr0.06)Mg1.01(Fe2.93Al1.07)(PO4)6(OH)4·12H2O based on single-crystal X-ray diffraction data collected from a natural sample from the Moculta quarry in Angaston, Australia. Calcioferrite is isostructural with montgomeryite, the only member of the group with a reported structure. The calcioferrite structure is characterized by (Fe/Al)O6 octahedra (site symmetries 2 and -1) sharing corners (OH) to form chains running parallel to [101]. These chains are linked together by PO4 tetrahedra (site symmetries 2 and 1), forming [(Fe/Al)3(PO4)3(OH)2] layers stacking along [010], which are connected by (Ca/Sr)2+ cations (site symmetry 2) and Mg2+ cations (site symmetry 2; half-occupation). Hydrogen-bonding interactions involving the water molecules (one of which is equally disordered over two positions) and OH function are also present between these layers. The relatively weaker bonds between the layers account for the cleavage of the mineral parallel to (010).