Paul G. Spry

The pearceite-polybasite group of minerals : Crystal chemistry and new nomenclature rules

Abstract The present paper reports changes to the existing nomenclature for minerals belonging to the pearceite-polybasite group. Thirty-one samples of minerals in this group from different localities, with variable chemical composition, and showing the 111, 221, and 222 unit-cell types, were studied by means of X-ray single-crystal diffraction and electron microprobe. The unit-cell parameters were modeled using a multiple regression method as a function of the Ag, Sb, and Se contents. The determination of the crystal structures for all the members of the group permits them to be considered as a family of polytypes and for all members to be named pearceite or polybasite. The main reason for doubling the unit-cell parameters is linked to the ordering of silver. The distinction between pearceite and polybasite is easily done with an electron microprobe analysis (As/Sb ratio). A hyphenated italic suffix indicating the crystal system and the cell-type symbol should be added, if crystallographic data are available. Given this designation, the old names antimonpearceite and arsenpolybasite are abandoned here and the old names pearceite and polybasite, previously defined on a structural basis (i.e., 111 and 222), are redefined on a chemical basis. The old name pearceite will be replaced by pearceite-Tac, antimonpearceite by polybasite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c, polybasite-221 by polybasite-T2ac, and polybasite-222 by polybasite-M2a2b2c. Since all polytypes are composed of two different layers stacked along [001]: layer A, with general composition [(Ag,Cu)6(As,Sb)2S7]2-, and layer B, with general composition [Ag9CuS4]2+, the chemical formulae of pearceite and polybasite should be written as [Ag9CuS4][(Ag,Cu)6(As,Sb)2S7] and [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7], respectively, instead of (Ag,Cu)16(As,Sb)2S11 and (Ag,Cu)16(Sb,As)2S11, as is currently accepted. The new nomenclature rules were approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.

Empressite, AgTe, from the Empress-Josephine mine, Colorado, U.S.A.: Composition, physical properties, and determination of the crystal structure

Abstract The chemistry and composition of empressite, AgTe, a rare silver telluride mineral, has been mistaken in the mineralogical literature for the silver telluride stützite (Ag5-xTe3). Empressite from the type locality, the Empress-Josephine deposit (Colorado), occurs as euhedral prismatic grains up to 400 μm in length and contains no inclusions or intergrowths with other minerals. It is pale bronze in color and shows a grey-black to black streak. No cleavage is observed in empressite but it shows an uneven to subconchoidal fracture and Vickers hardness (VHN25) of 142 kg/mm2. Empressite is greyish white in color, with strong bireflectance and pleochroism. Reflectance percentages for Rmin and Rmax are 40.1, 45.8 (471.1 nm), 39.6, 44.1 (548.3 nm), 39.4, 43.2 (586.6 nm), and 38.9, 41.8 (652.3 nm), respectively. Empressite is orthorhombic and belongs to space group Pmnb (Pnma as standard), with the following unit-cell parameters: a = 8.882(1), b = 20.100(5), c = 4.614(1) Å, V = 823.7(3) Å3, and Z = 16. Electron microprobe analyses gave the chemical formula Ag1.01Te0.99. The calculated density (from the ideal formula) is 7.59 g/cm3. The crystal structure has been solved and refined to R = 4.45%. It consists of edge-sharing AgTe4 tetrahedra forming sheets parallel to (010). The connectivity between the sheets is provided by Te-Te contacts (<2.9 Å) to complete the framework. The structural study presented here shows that empressite and stützite have different crystal structures.

The mineralogy of the Golden Sunlight gold-silver telluride deposit, Whitehall, Montana, U.S.A.

SummaryThe Golden Sunlight gold-silver telluride deposit, hosted primarily within the Mineral Hill breccia pipe (MHBP), is spatially related to a high-level, Late Cretaceous multiple intrusive, alkaline to subalkaline porphyry system. Base metal veins and manganese (rhodochrosite) mineralization occur up to 2km from the MHBP and form part of a regional mineral zonation pattern genetically related to a low-grade porphyry molybdenum system. Proterozoic rocks of the LaHood Formation and the informally named Bull Mountain Group host the MHBP and contain stratabound sulphides/ sulphosalts (up to 50% pyrite with minor to trace amounts of chalcopyrite, tennantite, pyrrhotite, sphalerite, galena, and molybdenite). Four periods of hypogene mineralization occur in the breccia pipe. Stages I and IV constitute ,≈99% of the mineralization; native gold (4–11 wt.% Ag), calaverite, tetradymite, tellurobismuthite, Se-bearing Bi sulphosalts (aikinite, lindströmite, krupkaite, gladite, bismuthinite, and ?benjaminite), tennantite (Zn, Fe, Te, and Bi varieties), coloradoite, melonite, galena (up to 6.7 wt.% Bi and 6.4 wt.% Se), stannite, chalcocite, and the rare mineral buckhornite are included in stage Ib. Minor amounts of base metals are present in stage II. Gold-silver tellurides (krennerite, petzite, sylvanite, and possibly the rare “x-phase”) developed in stage III whereas barite, fluorite, dolomite, magnesite, trace kaolinite, and sericite formed during stage IV. Utilizing the mineral assemblages in stage Ib, calculated values of logf Te2 and logf S2 range from -10.5 to -9.7, and -12.6 to -5.5, respectively.Ore forming components (e.g., Au, Ag, Te, Cu, Bi, Mo, and much of the S) were likely derived from the Late Cretaceous intrusive system with possible contributions from the Proterozoic host rocks.ZusammenfassungDie Golden Sunlight Gold-Silber-Tellurid-Lagerstätte, die hauptsächlich im Brekzienschlot von Mineral Hill (Mineral Hill breccia pipe, MHBP) eingelagert ist, steht räumlich mit einem erzreichen, multi-intrusiven, alkalischen bis sub-alkalischen Porphyritsystem aus der Oberkreide in Beziehung. Erzadern und Mn-Mineralisation (Rhodochrosit) finden sich bis zu 2 km vom MHBP entfernt und sind Bestandteil einer regionalen Vererzung die genetisch zu einem erzarmen Mo-hältigen Porphyritsystem in Beziehung steht. Proterozoische Gesteine aus der LaHood-Formation und der inoffiziell benannten Bull Mountain Group umgeben den MHBP und enthalten schichtgebundene Sulfide und Sulfosalze (bis zu 50% Pyrit mit Neben- bis Spurenmengen von Kupferkies, Tennantit, Pyrrhotin, Zinkblende, Bleiglanz und Molybdänit).[▭Der Brekzienschlot zeigt vier Phasen hypogener Mineralisation. Stufen I und IV enthalten ≈ 99% der Mineralisation: gediegen Gold (4–11 Gew.% Ag), Calaverit, Tetradymit, Tellurobismuthit, Se-hältige Bi-Sulfosalze (Aikinit, Lindströmit, Krupkait, Gladit, Bismuthinit und ?Benjaminit) Tennantit (Zn-, Fe-, Te- und Bi-Varietäten), Coloradoit, Melonit, Bleiglanz (mit bis zu 6.7 Gew.% Bi und 6.4 Gew.% Se), Zinnstein, Chalcocit, sowie das seltene Mineral Buckhornit treten in Stufe Ib auf. Geringere Mengen von Buntmetallen kommen in Stufe II vor. Gold-Silber-Telluride (Krennerit, Petzit, Sylvanit und möglicherweise die seltene “X-Phase”) sind in Stufe III ausgebildet und in Stufe IV wurden Baryt, Flusspat, Dolomit, Magnesit, Spuren von Kaolin und Serizit gebildet. Unter Verwendung der Mineralassoziationen der Stufe Ib lassen sich Werte von logf Te2 zwischen - 10.5 und - 9.7 und von logf S2 zwischen - 12.6 und - 5.5 errechnen.[▭Die erzbildenden Komponenten (z.B. Au, Ag, Te, Cu, Bi, Mo und der Grossteil von S) stammen wahrscheinlich vom Intrusivsystem aus der Oberkreide, möglicherweise mit Beiträgen der proterozoischen Umgebung.[/ p]

A sulphur isotope study of the Broken Hill deposit, New South Wales, Australia

Sulphur isotopic compositions of sulphides within garnet-rich rocks and high-grade ore from the Broken Hill deposit, New South Wales, Australia, have been determined and show a range of values of −3.3 to +6.7 per mil. Thermochemical considerations, including the spread of values of δ34S, suggest that the deposit was derived from a mixed source of sulphur in which seawater, reduced by inorganic processes, mixed with magmatic sulphur or that sulphate from contemporaneous seawater was reduced biogenically at low temperatures. Thermochemical considerations also suggest that pyrrhotite formed by desulphidation of pyrite so that the original Fe-S-O assemblage was pyrite ± magnetite.Δ34S measurements show a broad range which is considered to be due to isotopic reequilibration during retrograde metamorphism and analytical and sampling technique. These data should not be used to indicate original temperatures of deposition or metamorphic temperatures associated with the various metamorphic events.

Reduction of Concrete Expansion by Ettringite Using Crystallization Inhibition Techniques

Many researchers have proposed that secondary or delayed ettringite is responsible for serious, premature deterioration of concrete highways. The current research project was designed to determine experimentally if secondary ettringite formation in concrete can be reduced by treating the concrete with commercial crystallization-inhibitor chemicals. The hypothesis is that if ettringite is reduced, a concomitant reduction of concrete expansion and cracking will occur. If ettringite formation and concrete deterioration are simultaneously reduced, then the case for ettringite-induced expansion/cracking is strengthened. Our experiments used four commercial inhibitors—two phosphonates, a polyacrylic acid, and a phosphate ester. Concrete blocks were subjected to continuous-immersion, wet/dry cycling, and freeze/thaw cycling in sodium sulfate solutions and in sulfate solutions containing an inhibitor. The two phosphonate inhibitors were effective in reducing ettringite nucleation and growth in the concrete. Two other non-phosphonate inhibitors were somewhat effective, although less so than the two phosphonates. Reduction of new ettringite formation in concrete blocks also reduced expansion and cracking of the blocks. This relationship clearly links concrete expansion with ettringite formation. Secondary ettringite nucleation and growth must cause concrete expansion, because the only effect of these inhibitor chemicals is to reduce crystal nucleation and growth. These inhibitors cannot be responsible in any other way for reduction in expansion.

The distribution and recovery of gold in the Golden Sunlight gold-silver telluride deposit, Montana, U.S.A.

Abstract The gold balance in an ore deposit where the ore is treated by cyanide is the sum of the ‘visible gold’ that is amenable to cyanidation and ‘visible gold’ and the ‘invisible gold’, which are not amenable to cyanidation. Petrographic analyses, electron and ion microprobe as well as scanning electron microscope studies of ore from the Golden Sunlight deposit, Montana, suggest that periods of relatively poor gold recoveries are primarily due to the presence of inclusions, <25 μm in size, of native gold, petzite, calaverite, buckhornite and krennerite. These are encapsulated in cyanide insoluble grains of pyrite, chalcopyrite and tennantite and are present in the tailings. This contribution probably accounts for 3-25% of the unrecoverable gold processed during the life of the mine. Minor amounts (6-7%) of ‘invisible gold’, as indicated by ion microprobe studies and the presence of up to 5% ‘visible gold’ in buckhornite, which is rare in nature, appears to account for the remainder of the gold budget.

Two new occurrences of benleonardite, a rare silver-tellurium sulphosalt, and a possible new occurrence of cervelleite

Abstract Benleonardite, ideally Agg(Sb,As)Te253, occurs in ore specimens from the Mayflower and Gies epithermal gold−silver telluride deposits in Montana commonly spatially associated with hessite and tetrahedrite. In these deposits, it is Cu-bearing (up to 2.7 wt.%) and exhibits a slight deficiency in Ag+Cu coupled with a slight excess in S. A cervelleite-like mineral coexists with benleonardite at Mayflower and is unusual in composition in that it is Se-bearing suggesting the possibility of solid solution with aguilarite (Ag4SeS).

The chemistry and origin of zincian spinel associated with the Aggeneys Cu-Pb-Zn-Ag deposits, Namaqualand, South Africa

Zincian spinel or gahnite [(Zn,Fe,Mg)Al2O4] occurs in metamorphosed sulphide-rich rocks, garnet quartzites, quartz-magnetite rocks, aluminous metasediments, barite-magnetite rocks, quartz veins, and pegmatites associated with the Aggeneys base metal deposits, Namaqualand, South Africa. Zincian spinel in, sulphide-bearing rocks, is considered to have formed predominantly by desulphurization reactions involving a member of the system Fe-S-O and sphalerite with sillimanite or garnet. Gahnite in sulphide-free garnet quartzites, quartz-magnetite rocks and barite-magnetite rocks probably formed from Zn and Al that were hydrothermally derived whereas gahnite in aluminous metasediments was derived from the metamorphism of metalliferous shales, in which Zn may originally have been linked to organic material. Gahnite is Zn-rich in sulphide-bearing rock, but is Fe-rich in sulphide-free garnet quartzites and quartz-magnetite rocks. Although Zn-rich spinels represent guides to ore in the Aggeneys area and elsewhere in the Namaqualand Metamorphic Complex, Fe-rich spinels should not be discounted because Zn-rich and Fe-rich spinels occur within metres of sulphides at Aggeneys.

Structural role of copper in the minerals of the pearceite-polybasite group: the case of the new minerals cupropearceite and cupropolybasite

Abstract The pearceite-polybasite group of minerals, general formula [M6T2S7][Ag9CuS4] with M = Ag, Cu; and T = As, Sb, show a crystal structure which can be described as the succession, along the c axis, of two pseudo-layer modules: a [M6T2S7]2- A module layer and a [Ag9CuS4]2+ B module layer. Copper is present in one structural position of the B module layer and replaces Ag in the only fully occupied M position of the A module layer. When the Cu content is >4.00 a.p.f.u., the structural position of the A module layer becomes Cu-dominant and, consequently, the mineral deserves its own name. In this paper we report the crystal-chemical characterization of two Cu-rich members exhibiting the 111 unitcell type (corresponding to the Tac polytype). One sample (space group P3̅m1, a 7.3218(8), c 11.8877(13) Å , V 551.90(10) Å3, Z = 1) having As >Sb and with the structural position of the A module layer dominated by Cu, has been named cupropearceite and the other sample (space group P3̅m1, a 7.3277(3), c 11.7752(6) Å , V 547.56(8) Å3, Z = 1) having Sb >As has been named cupropolybasite. Both the new minerals and mineral names have been approved by the IMA-CNMNC.

Methods for the determination of stable Te isotopes of minerals in the system Au–Ag–Te by MC-ICP-MS

The measurement of stable isotopes in ore- and ore-related minerals can provide insight into the geochemistry and formation of metal-bearing ore systems. Currently, there are few high-precision studies of the natural variability of stable tellurium isotopes, most of which are focused on meteorites and sulfides and are related to cosmogenesis; there are no modern studies on the variability of tellurium isotopes within native tellurium and tellurides from ore-forming systems. Tellurium is an element of interest due to its common association with gold in geologic systems, as well as its rarity in the Earths crust and increasing industrial demand for applications such as photovoltaics. This study presents a method by which tellurium can be sampled from Au–Ag tellurides and native tellurium, isolated by ion exchange chromatography, and analyzed for isotopic composition by multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). Using a micromill, a sufficient mass of telluride or native Te sample can be extracted from coexisting ore and gangue minerals from ∼100 μm wide by ∼50 μm deep drilled holes. Acid digestion of micromilled samples and subsequent ion exchange chromatography isolated Te from matrix metals. The chromatography procedure has Te yields of 96% and produces no net fractionation of Te isotopes. MC-ICP-MS analyses were performed using two techniques, both of which employed the doping of samples using Cd to correct for instrumental mass bias. The first method was the introduction of 100 ppb Te-bearing solutions (with 100 ppb Cd) using a desolvating nebulizer (Aridus II). The second method involved solution nebulization of 2 ppm solutions of Te (with 1 ppm Cd) into the plasma of the MC-ICP-MS. Although the wet and dry methods produce statistically identical delta values, precision is increased using the wet method. The average uncertainty (two standard deviations of the mean) using the Aridus II is ±0.20‰ for 130/125Te (dry method), whereas that for the wet method is ±0.08‰. Repeated analyses of the Te standard over a period of ∼15 months by solution nebulization yielded an external precision of ±0.10‰ for 130/125Te. Natural, hypogene tellurides (calaverite, hessite, krennerite, and sylvanite) and native tellurium samples (n = 32) have a range of 1.64‰ in the isotope composition 130/125Te, demonstrating resolvable, disparate isotope ratios between samples from different areas and between samples from the same locality (e.g., Cripple Creek, Colorado). Fractionation of Te isotopes was caused by mass-dependent geological processes.