Silvio Menchetti

Light-induced variations in realgar and beta -As 4 S 4 ; X-ray diffraction and Raman studies

Abstract Transformations induced by light in realgar and β-As4S4 have been studied by means of X-ray diffraction (powder and single-crystal) and Raman laser spectroscopy. Both polychromatic and monochromatic light alter realgar and the β phase into pararealgar. By lowering the reaction rate (long-wavelength pass filter, low laser power) an intermediate product, corresponding to the so-called “χ phase,” can be observed during the alteration process from β-As4S4 to pararealgar (polychromatic filtered light and laser light) and that from realgar to pararealgar (only with polychromatic filtered light). The powder pattern of the χ phase is fairly similar to that of β-As4S4 but differs from realgar, and it can be indexed on the basis of an expanded β-phase unit cell. Lattice parameters are as follows: a = 9.758(5), b = 9.522(5), c = 9.074(5) Å, 0 = 100.84(5)°, and a = 9.757(4), b = 9.512(7), c = 9.089(4) Å, 0 = 100.97(3)° for the χ phase obtained from alteration of realgar and β-As4S4, respectively. The formation of the χ phase is preceded by a strong anisotropic increase of the unit-cell volume of realgar and the β phase. The effects of light exposure on lattice parameters were studied using single-crystal X-ray diffraction: In realgar a and c sin β increase linearly with increasing exposure times, whereas b remains substantially unchanged. In the β-As4S4 crystals, b and c sin β increase while a decreases so that, after 1560 min, the unit-cell dimensions match those of the χ phase. Raman spectroscopy, which allows monitoring of the alteration process in real time, indicates the presence of the same As4S4 cage molecule in the χ phase as in realgar and β -As4S4 and confirms the similarity between the structure of χ and that of β. However, a less-ordered crystal structure and a lower molecular symmetry is suggested from analysis of the spectroscopic results.

Spectroscopic investigation and normal mode analysis of As4S4 polymorphs

Abstract Tetra-arsenic tetrasulfide polymorphs have been investigated by IR, Raman and UV-visible spectroscopies. The mineral realgar ( α -As 4 S 4 ) and the high-temperature phase β -As 4 S 4 transform to pararealgar by exposure to monochromatic laser light with a reaction rate depending on the wavelength of the exciting radiation. The photoinduced alteration process has been monitored by Raman spectroscopy, following in real time the spectral changes of the samples undergoing irradiation. A satisfactory assignment of realgar, β -As 4 S 4 and pararealgar was obtained by employing a simplified valence force field which provided a complete set of normal coordinates suitable for describing the vibrational spectra of these As 4 S 4 polymorphs.

The crystal structure of pararealgar, AS4S4

Gillulyite, TI2(AsH ,SbH )8S13,is monoclinic with space group P2/n and a = 9.584(3), b = 5.679(2), c = 21.501(6) A, ~ = 100.07(2)°, V = 1152.2(7) A3, and Z = 2, The average structure was determined using direct methods and refined to a final residual of 0.046 using 930 reflections. The structure consists of TI-As-Sand As-S-bearing sheets, each present statistically 50% of the time. Four sheets, linked together by Tl-S, As-S, and S-S bonds, are stacked parallel to (00 I), yielding the 21-A c-axis repeat of the average unit cell. The Tll atom within the TI-As-S sheet is coordinated by six S nearest neighbors, which form a distorted trigonal prism. The partially occupied Tl2 position between the sheets is split symmetrically about the twofold axis with a separation of 1.28 A and equal but partial occupancies of25%. The disordering of the Tl2 position and extreme distortion of its coordination polyhedron is probably the result of the Tl+ 6s2 lone pair effect. The mean bond distances for the AsS3pyramids range from 2.263 to 2.319 A. However, four of five pyramids have a fourth S neighbor at distances ranging from 2.943 to 3.001 A, which may be rationalized in terms of the AsH 4s210ne pair effect. Edge sharing among the AsS3pyramids results in an As-As distance of 2.562 A, comparable with that observed in elemental As. The presence of S-S and As-As bonding suggests that gillulyite might more appropriately be classified as a sulfide rather than a sulfosalt. The new mineral gillulyite, TI2(As,Sb)8SI3" one of several unusual Tl minerals found in the Mercur gold deposit, Utah, was reported by Wilson et al. (1991) to be a new thallium arsenic sulfosalt. The details of the occurrence, composition, optical and physical properties, and powder difITaction and unit-cell data are presented in this earlier work. This paper presents the results of a structural investigation of this unusual mineral. tions were too weak to be measured and suggest minor ordering in the b direction that was not investigated. Systematic absences indicate the space group to be Pn or P2/n. Intensity statistics support the latter choice, which was confirmed by the successful solution and refinement of the structure. The severe effects of absorption were corrected by the empirical "II-scan method. Direct methods produced solutions that showed what appeared to be AsS3 groups in a linked pattern, but the expected large peaks representing the Tl atoms were absent; thus, these solutions were considered to be false. Only after many STRUCTIJRE DETERMINATION AND REFINEMENT attempts with various direct and heavy atom methods After considerable searching, an irregular fragment of were we able to decipher the highly disordered average gillulyite was found that produced relatively narrow and structure described below, with its partially occupied As, symmetrical diffraction maxima. A hemisphere of data S, and Tl positions. In hindsight, we realize that the direct was collected and averaged to yield a more accurate data methods program Mithril (Gilmore, 1984) had produced set. A scan speed of 6°/min (in w) was used. Weak refleccorrect locations for most of the atoms early in the structions [/<IO.OO[] were rescanned (maximum of two reture determination process, but we were initially unwillscans) and the counts accumulated to improve accuracy. ing to accept them. The remaining atomic sites were found Experimental details are given in Table l. by difference-Fourier syntheses. Examination of a series of precession photographs conFull-matrix, least-squares refinement was performed to firmed the cell parameters and revealed a weak zone of minimize ~ w(IFaI IFe I)2. All computer programs used diffuse and split reflections in the b* direction, which efwere from Texsan (Molecular Structure Corporation, fectively doubles the length of the b axis. These reflec1985). Atomic scattering factors and anomalous disper0003-004X/95/0304-0394

Monoclinic members of the epidote group: effects of the AI ⇌ Fe3+ ⇌ Fe2+ substitution and of the entry of REE3+

02.00 394 INTRODUCTION

Crystal chemistry of clinopyroxene from alkaline undersaturated rocks of the Monte Vulture Volcano, Italy

SummaryMinerals of the clinozoisite-epidote series (with Fe3+ ranging from 0.24 to 1.14 a.p.fu.) and allanites (with REE up to 0.93 a.p.f.u.) were studied from a crystal chemical point of view. Structure refinements and microprobe analyses were performed on twentythree single crystals; the determined site populations were correlated with structuralgeometrical parameters. Important modifications affect the M3 octahedron, which is the preferential site for the entry of Fe3+ and Fe2+ substituting for Al. M1 can accomodate iron too (up to 0.18 a.p.f.u.) and, in allanites, small amounts of divalent cations. Minor geometrical variations occur in the A1 and A2 polyhedra, as well as in the M2 octahedron. Tetrahedra are quite constant in shape and dimensions; however, significant variations of their mutual orientation in the Si2O7 group are observed as a function of the total iron content (p value) and the amount of REE. These two chemical variables also affect the donor-acceptor distance. The relationships between unit cell parameters and chemical composition are underlined.ZusammenfassungMinerale der Klinozoisit-Epidot-Reihe (mit Fe3+ von 0,24 bis 1,14 pro Formeleinheit) und Allanite (mit bis zu 0,93 Seltene Erden pro Formeleinheit) wurden von kristallchemischen Gesichtspunkten her studiert. Von 23 Einkristallen wurden Strukturverfeinerungen und Mikrosondenanalysen durchgeführt; die ermittelten Punktlagenbesetzungen wurden mit strukturgeometrischen Parametern korreliert. Wichtige Variationen betreffen das Oktaeder um M3, welches die bevorzugte Punktlage des Ersatzes von Al durch Fe3+ und Fe2+ ist. M1 kann ebenfalls Eisen aufnehmen (bis zu 0,18 pro Formeleinheit) und ferner, in Allaniten, auch kleine Mengen zweiwertiger Kationen. Kleinere Variationen treten in den Polyedern um A1 und A2 auf, ebenso im Oktaeder um M2. Die Tetraeder sind in Gestalt und Dimensionen sehr konstant; als Funktion des Gesamtgehaltes an Eisen (p-Wert) und des Betrages an Seltenen Erden werden indessen signifikante Änderungen ihrer gegenseitigen Orientierung in der Si2O7-Gruppe beobachtet. Diese beiden chemischen Variablen beeinflussen auch die LÄnge der Wasserstoffbrücke. Die Beziehungen zwischen den Gitterkonstanten und der chemischen Zusammensetzung werden betont.

Crystal-chemistry of clinopyroxenes from potassic and ultrapotassic rocks in central Italy: implications on their genesis

Abstract The Monte Vulture is a Late Pleistocene stratovolcano, composed of highly undersaturated alkaline potassic to ultrapotassic rocks belonging to the Roman Magmatic Province. These rocks are notably richer in Na2O if compared to similar rocks of the Roman Province. Two distinct magmatic Series have been recognized: (1) feldspar-bearing series, ranging from basanite to phonolite, and a volumetrically subordinate (2) feldspar-free series, consisting of melilitite, melafoidite, and hauynophyre. The clinopyroxene compositions of the feldspar-bearing series ranges from diopside to ferro-salite (hedenbergite), and shows, from basanite to phonotephrite, increasing FeOtot, Al2O3, and TiO2 and decreasing of MgO contents. Clinopyroxene in basanites and tephrites has generally high Fe3+ contents, which is typical for clinopyroxene from the Roman Province. Clinopyroxene of the feldspar-free series shows a more restricted variation in MgO, and has often very high Al2O3, FeOtot, and TiO2 contents. In all the crystals examined the Al3+ content is high and is present mostly on the T site and for a minor part on the M1 site. The M1–O2 distance shows a good correlation with the RM13+ content; clinopyroxene from basanites (feldspar-bearing series) has the lowest RM13+ and that from melilitite and hauynophyre (feldspar-free series) the highest RM13+ contents. Clinopyroxene crystals from feldspar-free rocks have smaller M1 and larger T polyhedral volumes when compared to those in olivine–melilitites and melilitites (kamafugites) from Umbria, but they show similar polyhedral volumes as clinopyroxene crystals from leucite-bearing rocks. Although clinopyroxene from feldspar-free rocks has small M1 volumes due to the high R3+ contents, M1 volumes of clinopyroxene from melilitites are larger than expected because of the higher (Fe3+/Al3+)M1 values. This larger M1 volume of clinopyroxene in melilitites causes a shortening of and a lengthening of the M2–O3 distance. Strong compositional and structural similarities between clinopyroxene of Monte Vulture and Leucite-bearing rocks of the Roman Province (plagioclase-bearing High Potassium Series=HKS) indicate a common petrogenetic affinity. On the other hand, differences between clinopyroxene in feldspar-free rocks from Monte Vulture and that in kamafugites (i.e, olivine melilitites, kalsilitites) from Central Italy, suggest significant magma dissimilarities between these two groups.

Solid solution between piemontite and androsite-(La), a new mineral of the epidote group from Andros Island, Greece

The clinopyroxenes mentioned have been investigated by single crystal X-ray diffraction combined with electron microprobe analysis. The aim of this study was to characterize the crystal-chemical variations of clinopyroxenes in order to delineate the intracrystalline constraints which are characteristic of specific magmatic environments. Clinopyroxenes (cpx) crystallized from peralkaline ultrapotassic melt with kamafugitic and lamproitic affinities are characterized by high Si contents, which are insensitive to variations in silica abundance and silica saturation of the melt. The high Si occupancy in clinopyroxenes from kamafugitic magma is coupled to large M1 (i.e. Mg and Fe2+) and M2 (high Ca occupancy) sites, whereas in clinopyroxenes from magmas with lamproitic affinity, high Si content is combined with large M1 but small M2 sites. Clinopyroxenes from Romantype alkaline potassic and ultrapostassic rocks are characterized by an expanded tetrahedron (high IVA1 content) and small M1 site which is combined with small M2 polyhedron in clinopyroxenes from the potassic rocks and large M2 site in those from the ultrapotassic rocks.

Temperature dependence of the silver distribution in the crystal structure of natural pearceite, (Ag,Cu)16(As,Sb)2S11.

Abstract A new member of the epidote group, androsite-(La), was found at Andros Island, Cyclades, Greece, as an accessory constituent of metamorphic Mn-rich rocks. Associated minerals are rhodochrosite, braunite, rhodonite, spessartine, and quartz. The mineral is brown-red in color, transparent with vitreous luster and very strong pleochroism. Lattice parameters are a = 8.896(1), b = 5.706(1), c = 10.083(1) Å, β= 113.88(1)°, V = 468.0(1) Å3, Z = 2, space group P21/m. Crystal-structure refinement and chemical analyses yielded the ideal formula (Mn,Ca)REEAlMn3+Mn2+(SiO4)(Si2O7)O(OH). Another Mn-rich sample of the epidote group was found at Varenche Mn-mine, Valle di Saint Barthélemy, Western Alps, Italy; because of its chemical composition and crystallographic features, it can be considered an intermediate member of the piemontite-androsite-(La) series.

Ultrapotassic clinopyroxene from the Kumdy-Kol microdiamond mine, Kokchetav Complex, Kazakhstan: Occurrence, composition and crystal-chemical characterization

The crystal structure of the mineral pearceite, (Ag,Cu)16(As,Sb)2S11, has been solved and refined at 300, 120 and 15 K. At room temperature pearceite crystallizes with trigonal symmetry, space group P3m1; the refinement of the structure leads to a residual factor of R = 0.0464 for 1109 independent observed reflections and 92 variables. The crystal structure consists of sheets stacked along the c axis. The As atoms form isolated (As,Sb)S3 pyramids, which typically occur in sulfosalts, copper cations link two S atoms in a linear coordination, and the silver cations are found in a fully occupied position and in various sites corresponding to the most pronounced probability density function locations (modes) of diffusion-like paths. These positions correspond to low-coordination (2, 3 and 4) sites, in agreement with the preference of silver for such environments. d10 silver-ion distribution has been determined by means of a combination of a Gram-Charlier description of the atomic displacement factors and a split-atom model. To analyse the crystal chemical behaviour of the silver cations as a function of temperature, a structural study was carried out at 120 K (R = 0.0450). The refinement indicates that the mineral exhibits the same structural arrangement as the room-temperature structure (space group P3m1) and shows that the silver cations are still highly disordered. In order to investigate a possible ordering scheme for the silver cations, a data collection at ultra-low temperature (15 K) was performed. The structural skeleton was found to be similar to that of the room-temperature and 120 K atomic structures, but the best solution was achieved with a fully split-atom model of five silver positions, giving an R value of 0.0449 for 651 observed reflections and 78 parameters. Although the silver cation densities condense into better defined modes, the joint probability density function still exhibits a strong overlapping of neighbouring sites.

Al-Fe disorder in synthetic epidotes: A single-crystal X-ray diffraction study

Abstract We report data on the composition and crystal structure of the most K-rich (3.61 wt% K2O) natural clinopyroxene yet discovered. The studied crystal was found as a tiny inclusion in garnet from a garnet-clinopyroxene rock of the Kumdy-Kol microdiamond mine, Kokchetav complex, Northern Kazakhstan. Microprobe analysis yields the formula (Ca0.61Fe0.13Mg0.04Mn0.01K0.17Na0.05) (Al0.61Mg0.39)(Si1.61Al0.39)O6.00. Lattice parameters are: a = 9.773(1), b = 8.926(1), c = 5.269(1) Å, β = 105.75(1)°. The structure was refined up to Rall = 2.42% using 982 independent reflections. Substitution of K for Ca causes significant modification of the average structure. No evidence for an additional M2’ position was found. Crystal-chemical characteristics are compared with published data on both natural and synthetic K-bearing clinopyroxenes.