John Spratt

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

The known encounter velocity (6.1 kms(-1)) and particle incidence angle (perpendicular) between the Starchist spacecraft and the dust emanating from the nucleus of comet Wild-2 fall within a range that allows simulation in laboratory light-gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre-existing major- and trace-element composition of the foil, geometrical issues for energy dispersive X-ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.

Chemical Composition of Nyerereite and Gregoryite from Natrocarbonatites of Oldoinyo Lengai Volcano, Tanzania

Alkali carbonates nyerereite, ideally Na2Ca(CO3)2 and gregoryite, ideally Na2CO3, are the major minerals in natrocarbonatite lavas from Oldoinyo Lengai volcano, northern Tanzania. They occur as pheno- and microphenocrysts in groundmass consisting of fluorite and sylvite; nyerereite typically forms prismatic crystals and gregoryite occurs as round, oval crystals. Both minerals are characterized by relatively high contents of various minor elements. Raman spectroscopy data indicate the presence of sulfur and phosphorous as (SO4)2− and (PO4)3− groups. Microprobe analyses show variable composition of both nyerereite and gregoryite. Nyerereite contains 6.1–8.7 wt % K2O, with subordinate amounts of SrO (1.7–3.3 wt %), BaO (0.3–1.6 wt %), SO3 (0.8–1.5 wt %), P2O5 (0.2–0.8 wt %) and Cl (0.1–0.35 wt %). Gregoryite contains 5.0–11.9 wt % CaO, 3.4–5.8 wt % SO3, 1.3–4.6 wt % P2O5, 0.6–1.0 wt % SrO, 0.1–0.6 wt % BaO and 0.3–0.7 wt % Cl. The content of F is below detection limits in nyerereite and gregoryite. Laser ablation ICP-MS analyses show that REE, Mn, Mg, Rb and Li are typical trace elements in these minerals. Nyerereite is enriched in REE (up to 1080 ppm) and Rb (up to 140 ppm), while gregoryite contains more Mg (up to 367 ppm) and Li (up to 241 ppm) as compared with nyerereite.

Crystal chemistry of layered Pb oxychloride minerals with PbO-related structures: Part I. Crystal structure of hereroite, [Pb32O20(O,□)](AsO4)2[(Si,As,V,Mo)O4]2Cl10

Abstract The crystal structure of hereroite, a new complex lead oxychloride mineral from the Kombat Mine, Grootfontein, Namibia, has been solved by direct methods and refined to R1 = 0.054 for 6931 unique observed reflections. The mineral is monoclinic C2/c, a = 23.139(4), b = 22.684(4), c = 12.389(2) Å, β = 102.090(3)°, and V = 6358.8(18) Å3. The structure contains 16 independent Pb sites in strongly asymmetric coordination by O and Cl atoms. There are two tetrahedral sites, from which one (As) is occupied solely by As, whereas the second (T) has the mixed occupancy of [Si0.48As0.29V0.15Mo0.09]. There are in total 21 O sites. The O1-O8 sites belong to the AsO4 and TO4 tetrahedral oxyanions. The other O atoms (O9-O20) are tetrahedrally coordinated by Pb atoms, thus being central for the OPb4 oxocentered tetrahedra. The OPb4 tetrahedra share edges to form the [O21Pb32]22+ layers that can be described as derivatives of the [OPb] layer from the structure of tetragonal PbO (litharge). The [O21Pb32]22+ layer in hereroite can be obtained from the [OPb] layer by removal of blocks of oxocentered tetrahedra, which results in formation of double-square sevenfold and square fourfold cavities. The cavities are occupied by the AsO4 and TO4 tetrahedra, respectively. The topology of the [O21Pb32]22+ layer is complex and can be described as a combination of modules extracted from the layers of OPb4 tetrahedra present in the structures of kombatite and symesite. The topological functions of tetrahedra within the layer are analyzed using the square lattice method, which shows that each symmetry-independent tetrahedron has its own topological function in the layer construction. The structure of hereroite belongs to the 2:1 type of layered Pb oxyhalides and consists of alternating PbO-type layers and Cl sheets oriented parallel to the (010) plane.

Lead-tellurium oxysalts from Otto Mountain near Baker, California: VII. Chromschieffelinite, Pb10Te6O20(OH)14(CrO4)(H2O)5, the chromate analog of schieffelinite

Abstract Chromschieffelinite, Pb10Te6O20(OH)14(CrO4)(H2O)5, is a new tellurate from Otto Mountain near Baker, California, named as the chromate analog of schieffelinite, Pb10Te6O20(OH)14(SO4)(H2O)5. The new mineral occurs in a single 1 mm vug in a quartz vein. Associated mineral species include: chalcopyrite, chrysocolla, galena, goethite, hematite, khinite, pyrite, and wulfenite. Chromschieffelinite is orthorhombic, space group C2221, a = 9.6646(3), b = 19.4962(8), c = 10.5101(7) Å, V = 1980.33(17) Å3, and Z = 2. Crystals are blocky to tabular on {010} with striations parallel to [001]. The forms observed are {010}, {210}, {120}, {150}, {180}, {212}, and {101}, and crystals reach 0.2 mm in maximum dimension. The color and streak are pale yellow and the luster is adamantine. The Mohs hardness is estimated at 2. The new mineral is brittle with irregular fracture and one perfect cleavage on {010}. The calculated density based on the ideal formula is 5.892 g/cm3. Chromschieffelinite is biaxial (-) with indices of refraction α = 1.930(5), β = 1.960(5), and γ = 1.975(5), measured in white light. The measured 2V is 68(2)°, the dispersion is strong, r < v, and the optical orientation is X = b, Y = c, Z = a. No pleochroism was observed. Electron microprobe analysis provided: PbO 59.42, TeO3 29.08, CrO3 1.86, H2O 6.63 (structure), total 96.99 wt%; the empirical formula (based on 6 Te) is Pb9.65Te6O19.96(OH)14.04(CrO4)0.67(H2O)6.32. The strongest powder X-ray diffraction lines are [dobs in Å (hkl) I]: 9.814 (020) 100, 3.575 (042,202) 41, 3.347 (222) 44, 3.262 (241,060,113) 53, 3.052 (311) 45, 2.9455 (152,133) 55, 2.0396 (115,353) 33, and 1.6500 (multiple) 33. The crystal structures of schieffelinite (R1 = 0.0282) and chromschieffelinite (R1 = 0.0277) contain isolated Te6+O6 octahedra and Te26+O11 corner-sharing dimers, which are linked into a three-dimensional framework via bonds to Pb2+ atoms. The framework has large channels along c, which contain disordered SO4 or CrO4 groups and H2O. The lone-electron pair of each Pb2+ is stereochemically active, resulting in one-sided Pb-O coordination arrangements. The short Pb-O bonds of the Pb2+ coordinations are all to Te6+O6 octahedra, resulting in strongly bonded layers parallel to {010}, which accounts for the perfect {010} cleavage.

A cobaltite-framboidal pyrite association from the Kupferschiefer; possible implications for trace element behaviour during the earliest stages of diagenesis

Abstract Cobaltite-cemented pyrite framboids are reported from the base of the Kupferschiefer in the Lubin- Sieroszowice mining district in southern Poland. In the framboids, cobaltite occurs interstitial to the pyrite crystals. The cobaltite-cemented framboids are confined to within a few cm either side of the boundary between the Weissliegendes sandstone and the Kupferschiefer laminated organic-rich shales. Textural relations and distribution of the cobaltite is interpreted as indicating that the framboids acted as a site of preferred cobaltite precipitation and that the cobaltite precipitated in response to the changing chemical environment at the Weissliegendes/Kupferschiefer contact. It is proposed that the texture may have formed as a result of desorption of Co and/or As which had been adsorbed onto the monosulphide precursors to the framboidal pyrite. Desorption during the transition to pyrite resulted in cobaltite saturation within the framboid and subsequent precipitation. The source of the Co and As was probably oxyhydroxides which exist in the upper oxic part of the Weissliegendes.

Mineral phases and element composition of the copper hyperaccumulator lichen Lecanora polytropa

Abstract Mineral phases and element localization were investigated in the vivid turquoise-coloured lichen, Lecanora polytropa, sampled from a psammite boulder in a wall supporting mine spoil at the abandoned copper mine, Riddarhyttan Kopparverke, southern Sweden. Normally pale yellowish (usnic acid), the lichen is turquoise coloured internally with bluish inclusions. X-ray mapping shows that Cu occurs on and within the lichen and does not coincide with P or S, suggesting that it is indeed associated with carbon or other elements not detected (or reported) using X-ray mapping. Scanning electron microscopy in back-scatter mode confirmed that the greatest Cu concentrations occur in the form of crystalline aggregates in coloured inclusions below the major internal turquoise layer with smaller Cu contents. X-ray diffraction with a position-sensitive detector (XRD-PSD) confirmed coloured crystalline aggregates consisted of the copper oxalate, moolooite. The study confirms the value of XRD-PSD as a non-destructive tool to characterize small (~50 μm) metal oxalate inclusions obtained from within lichen samples.

Crystal chemistry of layered Pb oxychloride minerals with PbO-related structures: Part II. Crystal structure of vladkrivovichevite, [Pb32O18][Pb4Mn2O]Cl14(BO3)8·2H2O

Abstract The crystal structure of vladkrivovichevite, a new complex lead oxychloride mineral from the Kombat Mine, Grootfontein, Namibia, has been solved by direct methods and refined to R1 = 0.048 for 3801 unique observed reflections. The mineral is orthorhombic, Pmmn, a = 12.759(1), b = 27.169(4), c = 11.515(1) Å, and V = 3992.0(9) Å3. The structure of vladkrivovichevite belongs to a novel type of layered Pb oxychloride structure. The structure contains 12 symmetrically independent Pb sites. All Pb sites have strongly asymmetric coordination. Two B atoms form slightly distorted BO3 triangles. One symmetrically independent Mn atom forms five Mn-O bonds and one Mn-Cl bond by forming MnO5Cl octahedra. The O1, O2, O10, O11, and O12 atoms are tetrahedrally coordinated by four Pb atoms each, forming OPb4 oxocentered tetrahedra. The O7 site has a remarkable octahedral coordination, consisting of four Pb and two Mn atoms. The O1Pb4, O2Pb4, O10Pb4, and O11Pb4 tetrahedra share common edges to produce bands interconnected by O12Pb4 tetrahedra, forming a [O18Pb32]28+ layer. A O7Pb4Mn2 heterometallic oxocentered octahedron serves as the core of the [OPb4Mn2Cl2(BO3)8]16- clusters that link to the [O18Pb32]28+ layer via BO3 triangles. The presence of [OPb4Mn2Cl2(BO3)8]16- clusters is associated with large cross-like vacancies in the [O18Pb32]28+ layer.

Natropharmacoalumite, NaAl4[(OH)4(AsO4)3].4H2O, a new mineral of the pharmacosiderite supergroup and the renaming of aluminopharmacosiderite to pharmacoalumite

Abstract Natropharmacoalumite, ideally NaAl4[(OH)4(AsO4)3]·4H2O, is a new mineral from the Maria Josefa Gold mine, Rodalquilar, Andalusia region, Spain. It occurs as colourless, intergrown cubic crystals with chenevixite, kaolinite, jarosite and indeterminable mixtures of Fe and Sb oxyhydroxides. Individual crystals are up to 0.5 mm on edge, although crystals are more commonly ~0.25 mm across and occur in patchy aggregates several millimetres across. The mineral is transparent with a vitreous to adamantine lustre. It is brittle with an imperfect cleavage, irregular fracture and a white streak. The Mohs hardness is ~2.5 with a calculated density of 2.56 g cm-3 for the empirical formula. Electron microprobe analyses yielded Na2O 2.52%, K2O 1.49%, Al2O3 29.50%, As2O5 48.84% and H2O was calculated in line with the structural analysis as 16.28% totalling 98.63%. The empirical formula, based upon 20.21 oxygen atoms, is [Na0.57K0.22(H3O)0.21]∑1.00Al4.05(As2.97O12)(OH)4·4H2O. The five strongest lines in the X-ray powder diffraction pattern are [dobs(Å), Iobs, (hkl)]: 7.759,100,(100); 4.473,40,(111); 3.870,50,(200); 2.446,9,(301); 2.331,12,(311). Natropharmacoalumite is cubic, space group P4̄3m with a = 7.7280(3) Å, V = 461.53(3) Å3 and Z = 1. The crystal structure was solved by direct methods and refined to R1 = 0.063 for 295 reflections with F>4σ(F). The structure conforms broadly to that of the general pharmacosiderite structure type, with Na as the dominant cation in cavities of strongly distorted Al octahedra and As tetrahedra. A new group nomenclature system for minerals with the pharmacosiderite structure has been established, including the renaming of aluminopharmacosiderite to pharmacoalumite.

Rumseyite, (Pb 2 OF)Cl, the first naturally occurring fluoroxychloride mineral with the parent crystal structure for layered lead oxychlorides

Abstract Rumseyite, ideally [Pb2OF]Cl, is a new mineral species which is associated with calcite, cerussite, diaboleite, hydrocerussite and undifferentiated Mn oxides in a small cavity in ‘hydrocerussite’ from a manganese pod at Merehead quarry, Somerset, England. Rumseyite is tetragonal, I4/mmm, a = 4.065(1), c = 12.631(7) Å, V = 208.7(1) Å3, Z = 2. The mineral is translucent pale orange-brown with a white streak and vitreous lustre. It is brittle with perfect {100} cleavage; Dcalc = 7.71 g cm-3 (for the ideal formula, [Pb2OF]Cl). The mean refractive index in air at 589 nm is 2.15. The six strongest reflections in the X-ray powder-diffraction pattern [dmeas in Å, (Irel), (hkl)] are as follows: 2.923(100)(013), 2.875(68)(110), 3.848(41)(011), 6.306(17)(002), 1.680(14)(123), 2.110(12)(006). The crystal structure of rumseyite is based on alternating [OFPb2] and Cl layers. Rumseyite is related to other layered Pb oxyhalides. Fluorine and oxygen are statistically disordered over one crystallographic site. Rumseyite is named in honour of Michael Scott (Mike) Rumsey (1980- ), Curator and Collections Manager at the NHM (London), who discovered the mineral. The mineral and name have been approved by the IMA Commission on New Mineral Names and Classification (IMA 2011-091). The holotype specimen is in the collections of the Natural History Museum, London (specimen number BM1970,110).

TRACE-ELEMENTS IN PLATINUM-GROUP MINERALS STUDIED USING NUCLEAR MICROSCOPY

Abstract A combination of μ-PIXE and μ-RBS has been used to study concentrations and distributions of trace elements in platinum group minerals from eluvial and alluvial deposits from the western seaboard of North America. Thirteen of the pure element standards, used in the EPMA investigation, were first analysed by μ-PIXE. These analyses were used to establish limits of confidence in the analytical technique. It is shown that using simultaneous RBS to determine the matrix composition and incident charge accurate PIXE analysis is possible without standards. Natural grains of Pt-Fe and Os-Ir-Ru alloys were then analysed and traces of Cu, Fe, Ni, Ti, Ru, Rh, Pd, Cr, etc., were determined at levels ranging from 80 ppm to 5000 ppm. It is shown that while there may be minor variations between localities, the trace element distribution within the grains studied is essentially uniform.