Stanley H. Evans

Optic properties of centimeter-sized crystals determined in air with the spindle stage using EXCALIBRW

Abstract Extinction data sets for four centimeter-sized anisotropic crystals were collected in air with standard spindle-stage methods and submitted to a new Windows-based version of EXCALIBR, termed EXCALIBRW. EXCALIBRW solved these data sets with varying degrees of accuracy related to the external shape of the crystal: the more rounded the crystal, the more precise the results. For an olivine crystal ground into a sphere, the results were similar to those obtained for a crystal immersed in an index-matching fluid. However, even for samples bounded with growth or cleavage faces, the program determined the orientation of the optical indicatrix and 2V with an error of only 1−2°. Thus, this logical extension of spindle-stage methods is helpful: (1) to orient centimeter-size single crystals for various types of mineralogical measurements (e.g., spectroscopy or diffusion studies in which it might be undesirable to place the sample in a liquid); (2) as a non-destructive means of identifying gemstones based upon a determination of their optical class (i.e., isotropic vs. uniaxial vs. biaxial); and (3) for optical characterization by determination of 2V. In addition, the newest version of EXCALIBR is easier to use, mathematically more robust in its solution algorithms, and provides solutions for crystals in less favorable orientations than the earlier versions of EXCALIBR.

Crystal structure of uchucchacuaite, AgMnPb3Sb5S12, and its relationship with ramdohrite and fizélyite

Abstract Uchucchacuaite, ideally AgMnPb3Sb5S12, was originally reported as orthorhombic, with possible space group Pmmm, P222, or Pmm2, and unit-cell parameters a = 12.67, b = 19.32, and c = 4.38 Å obtained from powder X‑ray diffraction data (Moëlo et al. 1984a). Using single-crystal X‑ray diffraction, we examined two uchucchacuaite samples, one from the type locality, Uchucchacua, Peru, and the other from Hokkaido, Japan (designated as R100213 and R070760, respectively). Our results show that uchucchacuaite is isostructural with ramdohrite and fizélyite, with monoclinic symmetry (P21/n) and the unit-cell parameters a = 19.3645(11), b = 12.7287(8), c = 8.7571(6) Å, β = 90.059(3)° for R100213 and a = 19.3462(7), b = 12.7251(5), c = 8.7472(3) Å, β = 90.017(2)° for R070760. Both samples are pervasively twinned and the twin refinements yielded the final R1 factors of 0.037 and 0.031 for R100213 and R070760, respectively. The chemical compositions determined from electron microprobe analysis are Ag0.99(Mn0.92Pb0.03Sb0.02Bi0.01)Σ=0.98Pb3.00Sb5.00S12.00 for R100213 and Ag1.00(Mn0.82Sb0.11Ag0.04Bi0.02)Σ=0.99Pb2.98Sb5.00S12.00 for R070760. The key structural difference among uchucchacuaite, ramdohrite, and fizélyite lies in the cations occupying the M2 site, which can be expressed with a general structural formula as Ag(M2+2yAg½-ySb½-y)Pb3Sb5S12, where M2+ represents divalent cations with 0 ≤ y ≤ ½. From the current list of IMA-defined minerals, we consider M = Cd with y = 0.125 for ramdohrite, M = Pb with y = 0.25 for fizélyite, and M = Mn with y = 0.5 for uchucchacuaite. Associated with the variation in the average M2 cation size from fizélyite (1.078 Å) to ramdohrite (0.955 Å) and uchucchacuaite (0.83 Å) is the significant decrease in the average M2-S bond distance from 2.917 to 2.834, and 2.654 Å, respectively, as well as corresponding variations in the unit-cell b dimension from ~13.23 to 13.06 and 12.73 Å.

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.

Scottyite, the natural analog of synthetic BaCu2Si2O7, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa

Abstract A new mineral species, scottyite, ideally BaCu2Si2O7, has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. The mineral appears to have formed as a result of a hydrothermal event and is associated with wesselsite, pectolite, richterite, sugilite, and lavinskyite. Scottyite forms blocky grains with striations parallel to the c axis. Crystals are found up to 0.4 × 0.3 × 0.3 mm. No twinning is observed. The mineral is dark-blue in transmitted and under incident lights, transparent with pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of 4-5; cleavage is perfect on {100} and {010} and no parting was observed. The calculated density is 4.654 g/cm3. Optically, scottyite is biaxial (-), with α = 1.750(1), β = 1.761(1), and γ = 1.765(1), 2Vmeas = 66(2)°. It is insoluble in water, acetone, or hydrochloric acid. An electron microprobe analysis produced an average composition (wt%) (8 points) of CuO 36.98(31), BaO 35.12(16), SiO2 27.01(61), SrO 0.28(5), and Na2O 0.06(2), and total = 99.45(65), yielding an empirical formula (based on 7 O apfu) Ba1.00Sr0.01Na0.01Cu2.04Si1.97O7. Scottyite is the natural analog of synthetic BaCu2(Si,Ge)2O7, which exhibits novel one-dimensional quantum spin-1/2 antiferromagnetic properties with tunable super-exchange interactions. It is orthorhombic, with space group Pnma and unit-cell parameters a = 6.8556(2), b = 13.1725(2), c = 6.8901(1) Å, and V = 622.21(6) Å3. The structure of scottyite is characterized by flattened CuO4 tetrahedra sharing corners with one another to form chains parallel to the c axis. These chains are interlinked by Si2O7 tetrahedral dimers and Ba2+. The Ba2+ cations are bonded to seven O atoms in an irregular coordination. The average Si-O, Cu-O, and Ba-O bond lengths are 1.630, 1.941, and 2.825 Å, respectively. Scottyite is topologically related to a group of compounds with the general formula BaM2+ 2 Si2O7, where M = Be (barylite and clinobarylite), Fe (andrémeyerite), Mg, Mn, Co, and Zn.

Lavinskyite, K(LiCu)Cu6(Si4O11)2(OH)4, isotypic with plancheite, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa

Abstract A new mineral species, lavinskyite, ideally K(LiCu2+)Cu62+(Si4O11)2(OH)4 (IMA 2012-028), has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. Associated minerals include wesselsite, pectolite, richterite, sugilite, and scottyite. Lavinskyite crystals are tabular [parallel to (010)]. The mineral is light blue, transparent with very pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of ~5; cleavage is perfect on {010} and no parting was observed. The measured and calculated densities are 3.61(3) and 3.62 g/cm3, respectively. Optically, lavinskyite is biaxial (+), with α = 1.675(1), β = 1.686(1), γ = 1.715(1), 2Vmeas = 64(2)°. An electron microprobe analysis produced an average composition (wt%) of SiO2 42.85(10), CuO 46.13(23), K2O 4.16(2), MgO 1.53(17), Na2O 0.27(4), BaO 0.18(6), and MnO 0.08(1), plus Li2O 1.38 from the LA-ICP-MS measurement and H2O 3.22 (added to bring the analytical total close to 100%), yielding a total of 99.79% and an empirical chemical formula (K0.99Ba0.01)Σ=1.00(Li1.04Cu0.93Na0.10)Σ=2.07 (Cu5.57Mg0.43Mn0.01)Σ=6.01(Si4.00O11)2(OH)4. Lavinskyite is isotypic with plancheite, Cu8(Si4O11)2(OH)4·H2O, an amphibole derivative. It is orthorhombic, with space group Pcnband unit-cell parameters α = 19.046(2), β = 20.377(2), γ = 5.2497(6) Å, and V = 2037.4(4) Å3. The key difference between lavinskyite and plancheite lies in the coupled substitution of K+ and Li+ in the former for H2O and Cu2+ in the latter, respectively. The structure of lavinskyite is characterized by the undulating, brucite-like layers consisting of three distinct octahedral sites occupied mainly by Cu. These layers are sandwiched by the amphibole-type double silicate chains extending along the c axis, forming a sheet structure of compact silicate-Cu-silicate triple layers. Adjacent sheets are linked together by K and M4 (= Cu + Li) cations, as well as hydrogen bonding. The M4 site is split, with Cu and Li occupying two different sites. Lavinskyite exhibits more amphibole-like structural features than plancheite, as a consequence of K in the large cavity between the two back-to-back double silicate chains.

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.

Terrywallaceite, AgPb(Sb,Bi)3S6, isotypic with gustavite, a new mineral from Mina Herminia, Julcani Mining District, Huancavelica, Peru

Abstract A new mineral species, terrywallaceite, ideally AgPb(Sb,Bi)3S6, has been found in Mina Herminia, Julcani Mining District, Huancavelica, Peru. It is associated with tetrahedrite, gustavite, barite, and pyrite. Terrywallaceite crystals are lath-shaped, metallic-black, with striations parallel to the elongated direction (the c axis). The mineral is opaque with black streak and metallic luster. It is brittle and has a Mohs hardness of ~4; cleavage is good on {010} and no parting was observed. Twinning is pervasive on (100). The calculated density is 6.005 g/cm3. Optically, terrywallaceite is grayish white in polished thin section, with weak bireflectance, weak pleochroism (white to pale gray), and weak anisotropy (gray with bluish tint to bluish black in air). An electron microprobe analysis yielded an empirical formula, based on 6 (S+As) apfu, Ag1.02Pb0.87(Sb1.53Bi1.47)Σ=3.00(S5.94As0.06)Σ=6.00. Terrywallaceite is a member of the lillianite group and isostructural with P21/c gustavite. Its unit-cell parameters are a = 6.9764(4), b = 19.3507(10), c = 8.3870(4) Å, b = 107.519(2)°, and V = 1079.7(1) Å3. The structure of terrywallaceite contains six symmetrically-nonequivalent S sites and five cation sites [Ag, Pb, M1 (=0.82Bi + 0.18Sb), M2 (=0.60Bi + 0.40Sb), and M3 (=0.95Sb + 0.05Bi)]. The pronounced preference of Sb for the M3 site over M2 and M1 in terrywallaceite is consistent with the site occupancy data reported for Sb-bearing gustavite, and suggests an alternative ideal formula for terrywallaceite of AgPb(Sb,Bi)(Bi,Sb)2S6, instead of AgPb(Sb,Bi)3S6

Redetermination of despujolsite, Ca3Mn4+(SO4)2(OH)6·3H2O

The crystal structure of despujolsite [tricalcium manganese bis(sulfate) hexahydroxide trihydrate], the Ca/Mn member of the fleischerite group, ideally Ca3Mn4+(SO4)2(OH)6·3H2O, was previously determined based on X-ray diffraction intensity data from photographs, without H-atom positions located [Gaudefroy et al. (1968 ▶). Bull. Soc. Fr. Minéral. Crystallogr. 91, 43–50]. The current study redetermines the structure of despujolsite from a natural specimen, with all H atoms located and with higher precision. The structure of despujolsite is characterized by layers of CaO8 polyhedra (m.. symmetry) interconnected by Mn(OH)6 octahedra (32. symmetry) and SO4 tetrahedra (3.. symmetry) along [001]. The average Ca—O, Mn—O and S—O bond lengths are 2.489, 1.915, and 1.472 Å, respectively. There are two distinct hydrogen bonds that stabilize the structural set-up. This work represents the first description of hydrogen bonds in the fleischerite group of minerals.

A new formula and crystal structure for nickelskutterudite, (Ni,Co,Fe)As3, and occupancy of the icosahedral cation site in the skutterudite group

Abstract We propose a new formula for the mineral nickelskutterudite, based on our observation that either (or both) Co or Fe3+ are essential structure constituents. The crystal structure of nickelskutterudite, (Ni,Co,Fe)As3, cubic, Im3¯,

Yangite, PbMnSi3O8·H2O, a new mineral species with double wollastonite silicate chains, from the Kombat mine, Namibia

Im\overline 3,