Paola Bonazzi

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

Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7

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.

A crystallographic review of arsenic sulfides : effects of chemical variations and changes induced by exposure to light

The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X2T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P\bar 421m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) A, modulation vectors q1 = 0.2815 (3)(a* + b*), q2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.

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

Crystal data for natural and synthetic arsenic sulfides are reported and discussed. Most of them [α- and β-dimorphite, realgar, β-As4S4 phase, pararealgar, Kutoglu’s As4S4(II) phase, alacranite, uzonite, orthorhombic As4S5 phase] have a crystal structure consisting of a packing of cage-like, covalently bonded As4Sn (n = 3, 4 and 5) molecules held together by weak interactions of van der Waals character. Their structures are compared in terms of molecular packing and molecular parameters. The layered structural arrangement of orpiment, As2S3, is described and the effects of the incorporation of Se replacing for S is discussed. The structures of wakabayashilite and getchel lite, which contain mixed (As, Sb) coordination polyhedra, are also described to outline the geometric effects of the Sb → As substitution. The results of recent studies dealing with the effects of the exposure of realgar or other arsenic sulfides to visible light are reported and discussed. Their interest in the study of arsenical pigments and their preservation in artwork is outlined with some examples of application.

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

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.

Single-crystal diffraction and transmission electron microscopy studies of “silicified” pyrochlore from Narssârssuk, Julianehaab district, Greenland

Abstract A single-crystal X-ray diffraction study of synthetic pseuowollastonite (CaSiO3) shows that this crystal has monoclinic C2/c symmetry, with a = 6.8394(5), b = 11.8704(9), c = 19.6313(7) Å, P = 90.667(6)°, and V= 1593.7(2) Å3. Basic features of the C2/c structure are similar to those previously determined with a C1̄ symmetry but fewer sites exist for the monoclinic space group: five symmetrically nonequaivalent Ca sites, three Si sites, and nine O sites.

Hardystonite from Franklin Furnace: A natural modulated melilite

Abstract Pyrochlore-group minerals that exhibit high Si contents are fairly common in geochemically evolved parageneses. However, the role of Si in the structure of these minerals is unclear. Different explanations have been invoked to clarify the way in which Si is incorporated in natural pyrochlores. These include the presence of impurities, the presence of Si in an amorphous or dispersed state, and its presence as an essential part of the structure. This paper reports an integrated XREF, SEM, EMPA, and TEM study on pyrochlore samples with high SiO2 content (up to 11.51 wt%) from Narssârssuk, Julianehaab district (Greenland). TEM observations reveal that Si-poor areas have strong and sharp diffraction peaks, whereas the Si-rich areas showed weaker spots with the diffuse diffraction halo typical of a metamict material. No evidence of crystalline phases other than pyrochlore was observed. Two single crystals having the unit-cell parameter a = 10.4200(7) and 10.3738(7) Å, respectively, were analyzed by X-ray diffraction and the structure was refined to Robs = 2.62 and 4.35%. On the basis of both the refined site scattering and the octahedral bond distance and the results of the TEM investigation, only a fraction (~30.50%) of the Si detected by EMPA is incorporated in the structure. A comparison with structural data of Si-free pyrochlores reported in the literature supports this assumption and allows a linear multiple regression to model the effect of the substitution of (Nb,Ta) by Ti and Si. The remaining 50.70% of the total silicon detected is incorporated in the radiation-damaged portions of pyrochlore.