Sodalite-group Minerals Research Articles

Mutual Transformations of Polysulfide Chromophore Species in Sodalite-Group Minerals: A DFT Study on S6 Decomposition.

It is known that various polysulfide species determine the color of sodalite-group minerals (haüyne, lazurite, and slyudyankaite), and that heating induces their transformations and color change, but the mechanisms of the transitions are unknown. A prominent example is the decay of cyclic S6 molecule. Using density-functional simulations, we explore its main decay pathways into the most probable final reaction products (the pairs of radical anions S3⋅-+S3⋅- and S2⋅-+S4⋅-). It was found that the most favorable reaction path involves initial capture of one electron by the S6 molecule, which greatly facilitates its decay of S6 and leads to the opening of the S6 cycle, and subsequent decomposition of the thus formed chain radical anion, with a limiting energy barrier of ~0.4 eV. Neutral polysulfide molecules capture one electron with a significant energy reduction. The radical anions Sn⋅- (n=2-6) are the most stable ones among corresponding species with the same n values and different charges. The capture of the second electron by S6⋅- occurs with a huge energy barrier (~2 eV). The results of the DFT calculations are in agreement with experimental data on the products of thermal conversions of extra-framework S-bearing groups in sodalite-group minerals.

Structure Modulations and Symmetry of Lazurite-Related Sodalite-Group Minerals

Lazurite and other lazurite-related minerals (LRMs) containing sulfur in both sulfate and sulfide forms are sodalite-type compounds with various extraframework species, of which the tendency to order leads to structural modulations with a period that is either commensurate or incommensurate with the period of the basic lattice. In this work, the structures of incommensurately modulated monoclinic LRMs are re-examined based on the superstructure of slyudyankaite, formerly known as triclinic lazurite. Similarities and differences between three one-dimensionally modulated LRMs and cubic LRM structures modulated in several directions are discussed. Assumptions are made on how the symmetry of the structure and the composition of the crystal can affect the period of structural modulation.

Infrared spectroscopy as a tool for the analysis of framework topology and extra-framework components in microporous cancrinite- and sodalite-related aluminosilicates

The identification of the framework type in multilayer cancrinite- and sodalite-group minerals and synthetic compounds is predominantly based on the unit-cell dimensions determined from powder or single-crystal X-ray diffraction data, by analogy with previously characterized samples with known crystal structures. However, topological type of the framework cannot be reliably determined in this way because of the different possible ABC staking sequences and different sets of cages with the same number of layers in the repeat unit. To solve this problem, additional criteria are required. The use of infrared (IR) spectroscopy makes it possible to distinguish topologically different types with the same unit-cell parameters. The most important diagnostic range in the IR spectrum (the “finger-print region”, from 510 to 760 cm−1) corresponds to the O–T–O bending vibrations (T = Si, Al). The spectral bands at 705 ± 8, 528 ± 5, 547 ± 4, and 555 ± 3 cm−1 indicate the presence of the sodalite, Losod, liottite, and giuseppettite cages, respectively. The band at 528 ± 5 cm−1 shifts towards ∼518 cm−1 in the case when Losod cage hosts carbonate group. The IR spectrum in the “finger-print region” can be also used to identify a mineral species belonging to two-layer or three-layer minerals with different extra-framework compositions. The wavenumber of the antisymmetric stretching mode of the 12CO2 molecule, which is a common admixed extra-framework constituent in minerals belonging to the cancrinite and sodalite groups, depends on the kind of the host cage or channel: 2340–2343 cm−1 for the sodalite cage, 2338 cm−1 for the Losod cage, and 2351–2353 cm−1 for the liottite cage and wide channel in the cancrinite-type framework.

Growth twins and deformation twins of sodalite-type microporous compounds

The tendency of sodalite-group minerals and synthetic materials with the sodalite-type structure to form growth twins has long been known. Judging by faceting, the twin components are connected by a 180-degree turn around the [111] axis of the cubic lattice. In this work, the structure of growth twins, including simple prismatic twins elongated in the [111] direction and V-shaped twins, has been studied in detail at the atomic level. It is shown for elongated twins that the boundary between the twin components passes in the (111) plane through the common vertices of TO4 tetrahedra (T = Al, Si). The coordinates of these vertices are connected by pseudosymmetry operators, each of which can be a twinning operator. This can be either a 180-degree turn around the [111] axis or a reflection in any of the planes (2‾ 11), (12‾ 1), or (112‾). Twinning by a plane is accompanied by smaller distortions of the motif at the twin boundary and, therefore, is energetically more favorable. V-shaped twins are regular intergrowths of two simple twins, the prism axes of which are directed along different ⟨111⟩ directions of the cubic lattice. In addition to growth twins, deformation twins of sodalite and related materials are discussed. Their symmetry is usually lower than cubic, and twinning is often combined with modulation of the crystal structure or the formation of a superstructure. Crystal twinning can be a consequence of a phase transition from a high-temperature cubic phase with a symmetry lowering. So far, no examples are known when growth twinning is combined with deformation twinning, but such a possibility is not ruled out.

Crystal Chemistry, Isomorphism, and Thermal Conversions of Extra-Framework Components in Sodalite-Group Minerals

Isomorphic substitutions of extra-framework components in sodalite-group aluminosilicate minerals and their thermal conversions have been investigated using infrared, Raman, electron spin resonance (ESR), as well as ultraviolet, visible and near infrared (UV–Vis–near IR) absorption spectroscopy methods and involving chemical and X-ray diffraction data. Sodalite-related minerals from gem lazurite deposits (haüyne, lazurite, and slyudyankaite) are characterized by wide variations in S-bearing extra-framework components including SO42− and various polysulfide groups (S2●−, S3●−, S4●− radical anions, and S4 and S6 neutral molecules) as well as the presence of CO2 molecules. Heating at 700 °C under reducing conditions results in the transformation of initial S-bearing groups SO42− and S3●− to a mixture of S2−, HS−, S2●−, and S4●− and transformation of CO2 to a mixture of CO32− and C2O42− or HC2O4− anionic groups. Further heating at 800 °C in air results in the decomposition of carbonate and oxalate groups, restoration of the SO42− and S3●− groups, and a sharp transformation of the framework. The HS− anion is stable only under reducing conditions, whereas the S3●− radical anion is the most stable polysulfide group. The HS−-dominant sodalite-group mineral sapozhnikovite forms a wide solid-solution series with sodalite. The conditions required for the formation of HS−- and CO20-bearing sodalite-group minerals are discussed.

Composition, Thermal Expansion and Phase Transitions in Framework Silicates: Revisitation and Review of Natural and Synthetic Analogues of Nepheline-, Feldspar- and Leucite-Mineral Groups

Framework silicates form about 70% of the Earth’s crust, mainly feldspars ~50–60% and quartz ~10–15%. Less-abundant feldspathoids include nepheline-, leucite-, and sodalite-group minerals, rich in structurally challenging properties. This review paper deals with anhydrous feldspar-, nepheline-, and leucite/pollucite groups, emphasising the importance of parallel studies on natural and synthetic samples. Four topics are covered. For decades, petrologists have analysed nephelines and recalculated their compositions as endmember molecules but, by not following rules of stuffed-tridymite crystal chemistry, have not estimated reliably the excess SiO2 present in solid solution. Some materials scientists make similar mistakes, and a new approach is described here. Synthesis studies of analogue feldspars, nephelines, and leucite/pollucites led to collaborative studies, mainly using laboratory and synchrotron X-ray powder diffraction methods at room and elevated temperatures, to study thermal expansion and displacive phase transitions. Such work was recently expanded to address the spontaneous strain relations. Topics covered here include work on nepheline/kalsilite analogues in the system SrAl2O4—BaAl2O4; thermal expansion of (K,Na)Al-, RbAl-, RbGa-, and SrAl-feldspars; and thermal expansion and phase transitions in analogue leucites KGaSi2O6 (tetragonal to cubic) and K2MgSi5O12 (monoclinic to orthorhombic). Results are reviewed in the context of research published in mineralogical and more-widely in physical sciences journals.

Spectroscopic and Crystal-Chemical Features of Sodalite-Group Minerals from Gem Lazurite Deposits

Five samples of differently colored sodalite-group minerals from gem lazurite deposits were studied by means of electron microprobe and wet chemical analyses, infrared, Raman, electron spin resonance (ESR) and UV-Visible spectroscopy, and X-ray diffraction. Various extra-framework components (SO42−, S2− and Cl− anions, S3•−, S2•− and SO3•− radical anions, H2O, CO2, COS, cis- as well as trans- or gauche-S4 neutral molecules have been identified. It is shown that S3•− and S4 are the main blue and purple chromophores, respectively, whereas the S2•− yellow chromophore and SO3•− blue chromophore play a subordinate role. X-ray diffraction patterns of all samples of sodalite-group minerals from lazurite deposits studied in this work contain superstructure reflections which indicate different kinds of incommensurate modulation of the structures.

Extra-Framework Content in Sodalite-Group Minerals: Complexity and New Aspects of Its Study Using Infrared and Raman Spectroscopy

Nine samples of carbonate-free sodalite-group minerals, including those with abnormally high contents of polysulfide groups, fluoride anion and carbon dioxide molecules as well as synthetic fluoraluminate sodalite-type compound Na8(Si7Al5O24)(AlF6)3–·5H2O, have been studied by means of electron microprobe analyses, infrared and Raman spectroscopy; the CO2 content was determined using the selective sorption of gaseous ignition products. This article describes a semi-quantitative method for estimating the content of carbon dioxide molecules in these minerals, based on IR spectroscopy data. The data obtained demonstrate the existence of a sulfide sodalite-group mineral with the idealized formula Na7(Si6Al6O24)(S3−)·H2O, which differs significantly from the formula Na6Ca2(Si6Al6O24)S2–2 accepted for lazurite. According to single-crystal X-ray structural analysis, in the F-rich sodalite-group mineral from the Eifel paleovolcanic region, Germany with the idealized formula Na7(Si6Al6O24)F−·nH2O fluorine occurs as an isolated F− anion, unlike synthetic F-rich sodalite-type compounds.

Defects in sodalite-group minerals determined from X-ray-induced luminescence

The luminescence spectra of a suite of natural sodium framework silicates including four different sodalite variants and tugtupite have been collected during X-ray irradiation as a function of temperature between 20 and 673 K. The origin of the emission bands observed in these samples is attributed to F-centres (360 nm), paramagnetic oxygen defects (400 and 450 nm), S2 − ions (620 nm) and tetrahedral Fe3+ (730 nm). Luminescence in the yellow (550 nm) is tentatively attributed to Mn2+, and red luminescence in Cr-rich pink sodalite is possibly from Cr3+ activation. Sudden reduction in luminescence intensities of emission centres was observed for all minerals in the 60–120 K range. Since it is common to all the sodalite-group minerals, we infer it is a feature of the aluminosilicate framework. Sodalite luminescence has responses from substitutions on the framework (e.g. paramagnetic oxygen defects, Fe3+) which give sodalite properties akin to other framework silicates such as feldspar and quartz. However, the presence of the sodalite cage containing anions (such as F-centres, S2 − ions) imparts additional properties akin to alkali halides. The possibility of coupling between Fe3+ and S2 − is discussed. The overall luminescence behaviour of sodalite group can be understood in terms of competition between these centre types.

Sodalite-Group Minerals From the Somma-Vesuvius Volcano (Naples, Italy): A Combined EPMA, SIMS, and FTIR Crystal-Chemical Study

This work is focused on the crystal-chemical characterization of the cationic and anionic components of sodalite-group minerals (SGM) occurring in various igneous-to-metamorphic rocks and ejecta from the alkaline-potassic Somma-Vesuvius volcano (southern Italy). A combination of different analytical methods, i.e. , XRD, SEM, EPMA, SIMS, FTIR, and μ-FTIR-FPA imaging, have been used. Minor and trace volatile elements (H, C, and F) have been quantified for the first time (to the authors’ knowledge) by SIMS, and in situ ion probe data were integrated with information derived from FTIR spectroscopy. The studied samples correspond to sodalite sensu stricto , nosean and hauyne. SIMS measurements for H, F, and C (quantified as H2O, F, and CO2 wt.%, respectively) show contents 0.02–5.0 wt.% H2O, 0.01–0.14 wt.% F, and 0.08–2.95 wt.% CO2. Within the single crystals, the F and, to a lesser extent, CO2 concentrations are homogeneous, whereas, in terms of H2O, they span from relatively homogenous to strongly zoned. Single-crystal FTIR spectra of SGM can be grouped into sodalites and noseans/hauynes, according to the occurrence of the 12CO2 absorption at 2340 cm–1. The absorption due to H2O and/or OH groups occurs as a very broad band extending from 3700 cm–1 to 3000 cm–1. In all samples FTIR data show the presence of CO32–. The collected data allow us to fully characterize SGM from Somma-Vesuvius, providing also some constraints regarding their genesis. Present data can contribute to a more detailed understanding of the crystal chemistry of SGM, their geological environments of formation, and possibly of technological characteristics of their synthetic analogues.

Upper Cretaceous to Pleistocene melilitic volcanic rocks of the Bohemian Massif: petrology and mineral chemistry

Abstract Upper Cretaceous to Pleistocene volcanic rocks of the Bohemian Massif represent the easternmost part of the Central European Volcanic Province. These alkaline volcanic series include rare melilitic rocks occurring as dykes, sills, scoria cones and flows. They occur in three volcanic periods: (i) the Late Cretaceous to Paleocene period (80–59 Ma) in northern Bohemia including adjacent territories of Saxony and Lusatia, (ii) the Mid Eocene to Late Miocene (32.3–5.9 Ma) period disseminated in the Ohře Rift, the Cheb–Domažlice Graben, Vogtland, and Silesia and (iii) the Early to Late Pleistocene period (1.0–0.26 Ma) in western Bohemia. Melilitic magmas of the Eocene to Miocene and Pleistocene periods show a primitive mantle source [(143Nd/144Nd)t=0.51280–0.51287; (87Sr/86Sr)t=0.7034–0.7038)] while those of the Upper Cretaceous to Paleocene period display a broad scatter of Sr–Nd ratios. The (143Nd/144Nd)t ratios (0.51272–0.51282) of the Upper Cretaceous to Paleocene rocks suggest a partly heterogeneous mantle source, and their (87Sr/86Sr)t ratios (0.7033–0.7049) point to an additional late- to post-magmatic hydrothermal contribution. Major rock-forming minerals include forsterite, diopside, melilite, nepheline, sodalite group minerals, phlogopite, Cr- and Ti-bearing spinels. Crystallization pressures and temperatures of clinopyroxene vary widely between ~1 to 2 GPa and between 1000 to 1200 °C, respectively. Nepheline crystallized at about 500 to 770 °C. Geochemical and isotopic similarities of these rocks occurring from the Upper Cretaceous to Pleistocene suggest that they had similar mantle sources and similar processes of magma development by partial melting of a heterogeneous carbonatized mantle source.

The sulfur speciation in S-bearing minerals: New constraints by a combination of electron microprobe analysis and DFT calculations with special reference to sodalite-group minerals

In this work, we present an improved method for the semi-quantitative determination of sulfur species in S-bearing minerals by electron microprobe analysis. For calibration, we analyzed several sulfate and sulfide standard minerals such as baryte, celestine, chalcopyrite, and pyrite, and correlated the results with theoretical calculations retrieved from density functional theory (DFT). We applied this method to natural sodalite-group minerals from various localities. In addition, we applied the more common Raman spectroscopy to some samples and show that this method cannot be applied to sodalite-group minerals to determine their sulfur speciation. We show that even though sodalite-group minerals have a complex crystal structure and are sensitive to the electron beam, electron microprobe analysis is a reliable tool for the analysis of their sulfur speciation. The natural sodalite-group minerals show systematic variations in sulfur speciation. These variations can be correlated with the independently determined oxidation state of the parental magmas thus making S-bearing sodalitegroup minerals good redox proxies, although we show that the electron microprobe analysis of the sulfur speciation is matrix-dependent, and the sulfur speciation itself depends on crystal chemistry and structure, and not only on fO2.

Haüyne: mutual cations/anionic groups arrangement and thermal expansion mechanism

The paper reports a crystal chemical investigation of a hauyne sample, \( ({\rm Na}_{4.4}{\rm K}_{1.1}{\rm Ca}_{2.1}) _{\Sigma_{7.6}} [{\rm Si}_6{\rm Al}_6{\rm O}_{24}]\)(SO4)1.6(S3)0.3(CO2)0.1, from Sacrofano, Italy, conciliating spectroscopic, chemical, and room temperature (RT) X-ray single-crystal diffraction data. The RT structure refinement provides a detailed description of the extra-framework cations and anionic groups mutual arrangement. The occurrence of minor amounts of the S3− poly-sulphur radical, responsible of the light-blue colour of the sample, revealed by Raman spectroscopy, has been shown to play an important role in modifying the cation partition among the three sites: M1, M2, and M3. Moreover, traces of enclathrated CO2 were revealed by FTIR. Besides, the thermal behaviour has been investigated, up to 1,098 K, by in situ HT-XRPD. Results indicate relevant differences with respect to reference data. The sample at RT shows weak satellite reflections that disappear at 673 K. As expected, the geometry of the rigid TO4 tetrahedra is independent from temperature, whereas the framework expands by decreasing the angle of rotation of both the SiO4 (φSi) and AlO4 (φAl) tetrahedra as a function of temperature. The quadric dependence of both φSi and φAl from temperature ends at 973 K, in correspondence of angles of rotation of ca. 3°, indicating the attainment of a nearly full expansion of the framework. At temperatures exceeding such value, only weak tetrahedral expansion acts. As a result of such thermal expansion mechanism, the dependence of the a-parameter from temperature shows a change of regime at 973 K. Such thermal expansion model is similar, albeit non-identical, to that of several cancrinite–sodalite group minerals.

Sodalite-group minerals from the Somma Vesuvius volcanic complex, Italy: a case study of K-feldspar-rich xenoliths

AbstractSodalites and sulfatic sodalites in holocrystalline K-feldspar-rich ejecta from the Somma Vesuvius volcanic complex, Italy, have been characterized by combined chemical, structural and spectroscopic analyses. Sodalite has a relatively homogeneous chemical composition, with the ubiquitous presence of CO32– and H2O/OH–. The sulfatic sodalites are isomorphous and have cage structures which can contain a wide variety of anions, molecular species and cations. Molecular CO2 and H2O/OH– are present in sulfatic sodalites, instead of the CO32– anions that are present in sodalite sensu stricto. The cell dimensions of the all of the studied sodalite samples are very similar, the sulfatic sodalite cell dimensions are more variable. Structure refinement of a distinctly green sample confirms the P3n space group. Metasomatic magma-derived fluids that are rich in H2O, CO2, Cl and S are believed to be responsible for the genesis of the relatively late-stage phases which form the ejecta containing the sodalite-group minerals.

Sulfur-bearing Magmatic Accessory Minerals

Research Article| January 01, 2011 Sulfur-bearing Magmatic Accessory Minerals Fleurice Parat; Fleurice Parat Géosciences Montpellier - Université Montpellier 2, Place E. Bataillon, 34095 Montpellier cedex 5. France and Mineralogie-Geochemie, Institut für Geowissenschaften, Albertstr. 23b, 79104 Freiburg, Germany, Fleurice.Parat@gm.univ-montp2.fr Search for other works by this author on: GSW Google Scholar François Holtz; François Holtz Institut für Mineralogie, Callinstr. 3, Leibniz Universität, Hannover 30167, Germany Search for other works by this author on: GSW Google Scholar Martin J. Streck Martin J. Streck Department of Geology, Portland State University, Portland, Oregon 97207-0751, U.S.A. Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2011) 73 (1): 285–314. https://doi.org/10.2138/rmg.2011.73.10 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Fleurice Parat, François Holtz, Martin J. Streck; Sulfur-bearing Magmatic Accessory Minerals. Reviews in Mineralogy and Geochemistry 2011;; 73 (1): 285–314. doi: https://doi.org/10.2138/rmg.2011.73.10 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Almost all magmas contain sulfide- or sulfate-bearing phases. In most natural samples the sulfur-bearing phase is a sulfide, which typically is pyrrhotite (Fe1−xS) or pyrite (FeS2) although chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8), sphalerite (ZnS) or molybdenite (MoS2) may be present as well. Sulfate minerals are rare at magmatic conditions, and anhydrite (CaSO4) is the most common. Other magmatic SO4-bearing minerals include the sodalite group minerals (haüyne simplified formula: (Na,Ca)4–8(Al6Si6(O,S)24)(SO4,Cl)1–2), scapolite minerals (silvialite:... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Isomorphic substitutions in afghanite: Identification of the mineral

Samples of afghanite, (Na,Ca,K)8 (Al6Si6O24)(SO4,Cl,CO3)3 · H2O, from the Malaya Bystraya and Tultui lazurite deposits in the Baikal region have been studied with X-ray diffraction and an electron microprobe. The unit-cell dimensions a and c of the examined afghanite samples range from 12.729 to 12.762 &0A and from 21.385 to 21.415 &0A. Isomorphic substitution is exhibited between Na and K; Ca content is about 2.5 apfu; the Cl/SO4 ratio is close to 1.0 with slight variations. Parameter a was plotted versus K content. The correlation between Na and K is established. Comparative analysis has shown that afghanite from the Baikal region is characterized by a lower K content than that from volcanic complexes in Italy. In the studied samples, K2O ranges from 0.51 to 2.28 wt %, whereas in comparable samples from volcanic complexes of Italy, it varies from 3.44 to 8.26 wt %. Sodalite-group minerals display a similar behavior of potassium. In lazurite from the Baikal region, the K2O content ranges from 0 to 0.95 wt %; the K2O content in hauyne from Italy varies from 0.88 to 5.75 wt %. The wider isomorphic miscibility in the feldspar minerals from volcanic-hosted assemblages in Italy as compared with the same minerals from lazurite deposits of the Baikal region reflects the difference in the geological and physicochemical conditions under which the minerals have been formed. In addition to the X-ray procedure, the Cl/SO4 ratio determined from microprobe data may be used to identify cancrinite-group minerals found at the Baikal lazurite deposits. It is near 1.0 in afghanite; 0.8 in tounkite; and more than 2 in davyne.

H2O and CO2in minerals of the haüyne-sodalite group: an FTIR spectroscopy study

AbstractThis paper reports an infrared spectroscopic study of a set of sodalite-group minerals. The specimens have been identified using a combination of X-ray diffraction and microchemical analysis. As expected, the Si/Al ratio is ~1; the extra framework cation content is characterized by a well-defined Na ⇌ (Ca+K) substitution. The lattice parameters of the studied samples range from sodalite (sample LM11) with a = 8.889(2) Å, to haüyne (sample HR3S) with a = 9.1265(2) Å. The specimens, having the SO2–group as a dominant anion, show a clear correlation between the a cell edge and the K content. Singlecrystal FTIR spectroscopy shows that haüyne and nosean typically contain enclathrated CO2molecules, inadditionto H2O and minor carbonate, while sodalite is virtually CO2-free. Detailed microspectrometric mappings show a non-homogeneous distribution of volatile constituents across the crystals, which may be related to the presence of fractures in the crystals. Because of such zoning, a relatively wide variation is observed when calibrating extinction coefficients on the basis of a bulk analytical method such as CHN elemental analysis.

Complex colour and chemical zoning of sodalite-group phases in a haüynophyre lava from Mt. Vulture, Italy

AbstractThe haüynophyre emitted from a parasitic vent of the Vulture stratovolcano is a S- and Cl-rich, leucitemelilite- bearing lava flow containing an unusually large amount of sodalite-group minerals (>23 vol.%). Mineralogical and chemical study of phenocrysts has led to the identification of black haüynes, blue lazurites and of Cl-rich white or black noseans. X-ray diffraction (XRD) study confirms the occurrence of nosean having a low symmetry (P23). Raman spectra and XRD data show that S is fully oxidized to SO4in black haüynes and in white noseans, while it is partly reduced to form S3–groups in blue lazurites, which also contain H2O molecules. Structural and chemical data strongly question the validity of the Hogarth and Griffin (1976) method widely used to resolve the ratio S6+/S2–in sodalite-group phases from EMPA data. Among euhedral phenocrysts, large lazurites are only faintly zoned. All other phases show variable core-rim chemical zoning and many phenocrysts are partially resorbed and/or colour-zoned. Black haüynes have highly variable S/Cl and slightly lower SiO2/Al2O3ratios, larger FeTOT contents and more compatible trace elements than lazurites. Thin opaque noseansodalite rims surrounding all crystals are interpreted as a result of rapid crystallization driven by exsolution of a S-scavenging fluid phase. We suggest that the extreme complexity of the mineralogical assemblage reflects variableaSiO2andaH2Oof the silicate melts.

Tugtupite: High-temperature structures obtained from in situ synchrotron diffraction and Rietveld refinements

The structural behavior of tugtupite, (ideally Na 8 [Al 2 Be 2 Si 8 O 24 ]Cl 2 ), a member of the sodalitegroup minerals, at room pressure and from 33 to 982 °C on heating, was determined by using in situ synchrotron X-ray powder diffraction data [λ = 0.91997(4) Å] and Rietveld refinement. The sample was heated at a rate of 9.5 °C/min and X-ray traces were collected at intervals of 16 °C. The unit-cell parameters for tugtupite increase smoothly and non-linearly to 982 °C. The percent volume change between 33 and 982 °C is 2.97(3)%. In tugtupite, large displacement parameters occur for the Na and Cl atoms, and the Na-Cl bond expands with temperature. The [Na 4 ⋅Cl] 3+ clusters expand with increases of the Na-Cl bond length by 0.073(3) Å between 33 and 982 °C. This forces the Na atoms toward the plane of the framework six-membered rings, and causes the framework tetrahedra to rotate. The framework TO 4 (T = Al 3+ , Be 2+ , or Si 4+ ) tetrahedra distort slightly with temperature, but the T-O distances remain nearly constant. This mechanism causes a fairly high-rate of expansion in tugtupite. If the Na atom reaches approximately the plane of the six-membered ring, because of the increase in bonding to the Na atom, the expansion will be retarded, but tugtupite melts before this occurs. Tugtupite melts at 1029 °C. The NaCl component in tugtupite is lost in two main stages; 1.8 wt% NaCl is first lost at about 1007 °C, and 8.2 wt% NaCl is lost in several steps between 1019 and 1442 °C.

Mineral chemistry and petrogenesis of granular ejecta from the Alban Hills volcano (Central Italy)

A suite of lithics (ejecta) collected from the latest erupted pyroclastic products of the Alban Hills volcano (Central Italy) has been studied to determine their mineralogical composition and to investigate their genesis. The ejecta commonly have granular texture and consist of coarse-grained crystals often associated with a fine- to medium-grained matrix. The mineralogical composition is variable and consists of both typical igneous minerals and contact metamorphic phases. Garnet, clinopyroxene K-feldspar are almost ubiquitous, whereas leucite, wollastonite, sodalite-group minerals, phlogopite, nepheline and phillipsite are present in most of the ejecta; minor and accessory phases include cuspidine, amphibole, pyrrhotite, magnetite, apatite, uranpyrochlore, sphene, kalsilite, and melilite; anorthite, zircon and fluorine-bearing Ca, Zr silicate phases, larnite, and baryte are found sporadically. Ca, REE, Th silicophosphates occur in many samples generally disseminated along interstices and fractures of main minerals. Calcite is present as discrete crystals sometimes enclosed in other minerals, as granules in the fine-grained matrix and as late microcrystalline veins. It shows high oxygen and low carbon isotope ratios with δ18O = + 17.96 to + 27.19, and δ13C = −4.74 to −19.57. Clinopyroxene ranges from diopside to compositions strongly enriched with both Ca-Tschermak’s and esseneite components. Feldspars are generally potassic even though Ba and Sr are found in significant concentrations in some samples. K-feldspars from wollastonite-bearing ejecta are often rimmed with elongated felty crystals identified by X-ray diffraction analysis as leucite. These feldspars show a depletion in Si, and enrichment in Al and K from core to rim. Significant compositional variations are also shown by various other phases such as nepheline, apatite, Ca, REE, Th silicophosphate. The occurrence of igneous and contact metamorphic minerals, as well as the chemical variations of clinopyroxenes and feldspars in the investigated ejecta reveal complex genetic processes related to the interaction between potassic magma and wall rocks. The Ca-rich composition of most phases points to a carbonate nature for the wall rocks. Textural evidence suggests that coarse-grained rocks formed at the margin of the magma chamber were invaded by a late, volatile rich potassic liquid which crystallized as a fine-grained matrix and produced disaggregation and reaction of early formed minerals. Fluid phases percolating through the rocks generated infiltration metasomatism and deposited some uncommon phases rich in Ca, REE, Th, U, which are found along cracks and at the margins of early crystallized minerals. Overall, the all spectrum of the minerals found in this study are also typical of carbonatitic rocks. Their presence in the Alban Hills ejecta demonstrates that their genesis can be related to interaction between ultrapotassic melts and carbonate wall rocks, in addition to precipitation from carbonatitic melts.