Elena Bonaccorsi

Tobermorites; their real structure and order-disorder (OD) character

Abstract The real structures of clinotobermorite, tobermorite 9 Å, and tobermorite 11 Å were determined through the application of OD approach, which allowed us to explain their peculiar disorder and polytypic features and to derive the main polytypes for each of them. The structural arrangements will be described and discussed for one polytype of each compound: clinotobermorite, triclinic polytype C1,a= 11.274, b = 7.344. c = 11.468 Å, α = 99.18°, β = 97.19°, γ = 90.03°; tobermorite 9 Å, triclinic polytype C1̅,a = 11.156, b = 7.303, c = 9.566 Å, α = 101.08°, β = 92.83°, γ = 89.98°; tobermorite 11 Å, monoclinic polytype B11m, a = 6.735, b = 7.385, c = 22.487 Å, γ = 123.25°. Common structural features are infinite layers, parallel to (001), formed by sevenfold-coordinated calcium polyhedra. Tetrahedral double chains, built up through condensation of “Dreiereinfachketten” of wollastonite- type and running along b, link together adjacent calcium layers in clinotobermorite and tobermorite 11 Å, whereas single tetrahedral chains connect adjacent calcium layers in tobermorite 9 Å. The relatively wide channels of clinotobermorite and tobermorite 11 Å host “zeolitic” calcium cations and water molecules. The present structural results now allow for a sound discussion of the crystal chemical relationships between the various members of the tobermorite group and an explanation of the peculiar thermal behavior of tobermorite 11 Å.

The real structures of clinotobermorite and tobermorite 9 Å: OD character, polytypes, and structural relationships

Clinotobermorite, Ca 5 Si 6 O 17 ·5H 2 O, is a rare mineral, structurally related to tobermorite 11 A. It is characterized by the presence of structural disorder (evidenced by the diffuseness of the reflections with k odd) which, until now, prevented the determination of its real structure. In this paper, a model for the real structure of clinotobermorite is proposed on the basis of OD theory, through examination of the X-ray diffraction pattern of a sample coming from the Wessels mine (South Africa). The proposed model, which assumes the presence of silicate double chains of wollastonite-type, is confirmed by structural refinements carried out for the two polytypes with maximum degree of order (MDO). The MDO 1 polytype of clinotobermorite (monoclinic, space group Cc, a = 11.276(2), b = 7.3427(8), c = 22.642(4) A, β = 97.28(1)°) was refined up to R = 0.15, whereas the two refinements performed on the MDO 2 polytype (triclinic, space group C1, a = 11.274(2), b = 7.3439(7), c = 11.468(2) A, α = 99.18(1), β = 97.19(1), γ = 90.09(1)°) converged to R = 0.12 and R = 0.10, respectively. In clinotobermorite infinite calcium polyhedral layers parallel to (001) are connected through double silicate chains [Si 6 O 17 ] 10- running along b; additional calcium cations and H 2 O molecules are placed in the channels of the resulting framework. By dehydration at 225°C, clinotobermorite transforms topotactically into a new phase, which also displays an OD character. The results of the structural refinement carried out for its triclinic MDO 2 polytype (space group Cc 1, a = 11.156(5), b = 7.303(3), c = 9.566(5) A, α = 101.08(4), β = 92.83(5), γ = 89.98(4)°) indicate that this phase, with crystal chemical formula Ca 5 Si 6 O 16 (OH) 2 , exhibits single chains of wollastonite-type, resulting from decondensation of the double chains. On the basis of the new detailed structural information a possible explanation for the enigmatic thermal behaviour of tobermorite 11 A has been proposed.

Crystal structure refinement of sahlinite, Pb14(AsO4)2O9Cl4

Abstract The crystal structure of sahlinite [Pb14(AsO4)2O9Cl4] from Långban (Sweden) has been refined up to R = 0.071 using single-crystal diffraction data collected at the Elettra synchrotron facility. Sahlinite is monoclinic, space group C2/c, with a = 12.704(4), b = 22.576(5), c = 11.287(4) Å , β = 118.37(3)º. Sahlinite is isostructural with kombatite, its vanadium counterpart. Both are derivatives of the litharge form of PbO. In the structure of sahlinite there are seven independent Pb atoms, which are linked to Cl and/or O atoms, with coordination number ranging from V to VIII. The coordination polyhedra are irregularly shaped, due to the 6s2 lone-pair effect displayed by Pb2+.

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

Abstract The haüynophyre emitted from a parasitic vent of the Vulture stratovolcano is a S- and Cl-rich, leucite-melilite-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 SO4 in black haiiynes 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/Al2O3 ratios, larger FeTOT contents and more compatible trace elements than lazurites. Thin opaque nosean-sodalite 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 variable aSiO₂ and aH₂O of the silicate melts.

CO2 in minerals of the cancrinite-sodalite group: pitiglianoite

This paper presents spectroscopic evidence for the presence of CO 2 molecules within the structural channels of a member of the cancrinite-sodalite group of feldspathoids. The study was carried out using single-crystal micro-FTIR spectroscopy on a sample of pitiglianoite from Monte Cavalluccio (Bonaccorsi & Orlandi, 1996). A sharp and prominent band is observed in the medium-infrared range at 2351 cm -1 , which can be assigned to carbon dioxide molecules in the studied sample. Polarized-light FTIR spectra collected on a 112-μm-thick [001] section show maximum absorption with E ⊥ c , suggesting that the linear CO 2 molecules are oriented perpendicular to the crystallographic c axis, as in beryl (Wood & Nassau, 1967) or cordierite (Armbruster & Bloss, 1980). In situ high- T FTIR data collection up to 550°C shows that the CO 2 band decreases in intensity and broadens with increasing T ; release of carbon dioxide from the structure starts at T > 250°C and is complete at ∼ 450°C. The same kind of behaviour is observed for structural H 2 O; upon heating pitiglianoite becomes virtually anhydrous at T ∼ 500°C. These data suggest that the extraframework volatile composition of cancrinite-type minerals is strongly T dependent, hence the analysis of pitiglianoite may provide geothermometric information on the genetic environment (typically late-stage volcanic) of these minerals.

Inside baleen: Exceptional microstructure preservation in a late Miocene whale skeleton from Peru

Exceptionally preserved delicate baleen microstructures have been found in association with the skeleton of a late Miocene balaenopteroid whale in a dolomite concretion of the Pisco Formation, Peru. Microanalytical data (scanning electron microscopy, electron probe microanalysis, X-ray diffraction) on fossil baleen are provided and the results are discussed in terms of their taphonomic and paleoecological implications. Baleen fossilization modes at this site include molding of plates and tubules, and phosphatization. A rapid formation of the concretion was fundamental for fossilization. We suggest that the whale foundered in a soft sediment chemically favorable to rapid dolomite precipitation, allowing the preservation of delicate structures. Morphometric considerations on the baleen plates and bristles coupled with the reconstructed calcification of the latter permit speculation on the trophic preferences of this balaenopteroid whale: the densely spaced plates and the fine and calcified bristles provide evidence for feeding on small-sized plankton, as does the modern sei whale.

Marinellite, a new feldspathoid of the cancrinite-sodalite group

Marinellite, [(Na,K) 42 Ca 6 ](Si 36 Al 36 O144)(SO 4 ) 8 Cl 2 .6H 2 O, cell parameters a = 12.880(2) A, c = 31.761(6) A, is a new feldspathoid belonging to the cancrinite-sodalite group. The crystal structure of a twinned crystal was preliminary refined in space group P 31 c , but space group P 62 c could also be possible. It was found near Sacrofano, Latium, Italy, associated with giuseppettite, sanidine, nepheline, hauyne, biotite, and kalsilite. It is anhedral, transparent, colourless with vitreous lustre, white streak and Mohs9 hardness of 5.5. The mineral does not fluoresce, is brittle, has conchoidal fracture, and presents poor cleavage on {001}. D meas is 2.405(5) g/cm 3 , D calc is 2.40 g/cm 3 . Optically, marinellite is uniaxial positive, non-pleochroic, ω = 1.495(1), ϵ = 1.497(1). The strongest five reflections in the X-ray powder diffraction pattern are [d in A (I) ( hkl )]: 3.725 (100) (214), 3.513 (80) (215), 4.20 (42) (210), 3.089 (40) (217), 2.150 (40) (330). The electron microprobe analysis gives K 2 O 7.94, Na 2 O 14.95, CaO 5.14, Al 2 O 3 27.80, SiO 2 32.73, SO 3 9.84, Cl 0.87, (H 2 O 0.93), sum 100.20 wt %, less O = Cl 0.20, (total 100.00 wt %); H 2 O calculated by difference. The corresponding empirical formula, based on 72 (Si + Al), is (Na 31.86 K 11.13 Ca 6.06 )Σ=49.05(Si 35.98 Al 36.02 )Σ=72O 144.60 (SO 4 ) 8.12 C1 1.62 3.41H 2 O. The crystal structure of marinellite may be described as formed by the stacking along c of 12 layers containing six-membered rings of tetrahedra: the stacking sequence is ABCBCBACBCBC…, where A, B, and C represent the positions of the rings within the layers. Its structure consists of two liottite cages superimposed along [0, 0, z ] and of columns of cancrinite and sodalite cages along [1/3, 2/3, z ] and [2/3, 1/3, z ]. Sulphate groups, surrounded by sodium, calcium and potassium cations, occupy the liottite cages. Chlorine anions and sulphate groups occupy the sodalite cages, whereas H 2 O molecules are located within the cancrinite cages, bonded to Na cations. This structural model was refined in the space group P 31 c , conventional R = 0.098 for 2155 reflections. The structural relationships between marinellite and tounkite are discussed.

Zincalstibite, a new mineral, and cualstibite: Crystal chemical and structural relationships

Abstract Zincalstibite, a new mineral occurring within the cavities of marble of the Apuan Alps, Tuscany, Italy, has chemical formula Zn2AlSb(OH)12, space group P3̄, a = 5.321(1), c = 9.786(2) Å. It is associated with sub-millimeter tufts of white crystals of mimetite and sub-millimetric stalactite aggregates of opal and an amorphous copper-silicate phase (possibly crisocolla). The crystals are trigonal prismatic, with forms {110}, {001}, elongated [001], generally less than 10 × 10 × 40 ÷ 50 μm, with few larger crystals. They are colorless, transparent, with vitreous luster, white streak and {001} cleavage. The stronger reflections are [(hkl), d (Å), Irel]: (002), 4.904, 100; (100), 4.620, 35; (101), 4.179, 57; (103, 110), 2.669, 31; (112, 112̄), 2.343, 88; (114, 114̄), 1.805, 57. Zincalstibite is structurally related to cualstibite, as evidenced by the structural determinations and refinements we present for both the minerals. They are built up by layers of isolated Sb(OH)6 octahedra alternating along c with trioctahedral layers, which contain Zn and Al cations and Cu and Al cations in zincalstibite and cualstibite, respectively. In cualstibite the ordering of Al and Cu within these trioctahedral layers results in a supercell, with acual = 9.150(2) Å ≈ √3 azinc. The name of zincalstibite is related to its chemical composition and points to its relationships with cualstibite. Both the mineral and its name were approved by the IMA Commission for New Minerals and Mineral Names (IMA 1998-033).

Thermal behaviour of davyne-group minerals

The thermal behaviour of microsommite (MC), davyne from Vesuvius (DV) and from Zabargad (DZ) was determined by X-ray single crystal data obtained employing a microfurnace connected to a four-circle diffractometer. Upon heating, the a parameter increased linearly, with similar thermal expansion rates for the three samples: the mean linear expansion coefficients, αa, were 10.2(3)·10-6, 13.4(7)·10-6, 15.1(8)·10-1 K-1 for MC, DV and DZ respectively.At about 473 K both MC and DZ showed a discontinuity in the expansion of the c parameter. The mean linear expansion coefficient, αc, changed abruptly from 16(4)·10-6 K-1 for both minerals below the discontinuity to 2(1)·10-6 and 3(1)·10-6 K-1 for MC and DZ, respectively, above the discontinuity. In DV, however, the αc coefficient was constant between 293 und 827 K and equal to 1(2)·10-6 K-1.The substructure of MC was refined under room conditions (R=0.055 with 758 independent reflections) in space group P63 and at 943 K in space group P63 (R=0.062, with 750 independent reflections) and in space group P63/m (R=0.065 with 394 independent reflections). By comparing the structural refinements the discontinuity could be related to the tilting of the tetrahedra connected along c: for T <473 K the temperature increase induced the stretching of such chains through tetrahedral tilting. At 473 K the chains were completely stretched and a purely displacive phase transition occured, with symmetry change from space group P63 to P63/m. The further limited increase of the c parameter for T> 473 K was due to the small tetrahedral expansion. In DV, where the completely stretched structure was already realized under room conditions, no phase transition occured.

Fukalite: An example of an OD structure with two-dimensional disorder

Abstract The real crystal structure of fukalite, Ca4Si2O6(OH)2(CO3), was solved by means of the application of order-disorder (OD) theory and was refined through synchrotron radiation diffraction data from a single crystal. The examined sample came from the Gumeshevsk skarn copper porphyry deposit in the Central Urals, Russia. The selected crystal displays diffraction patterns characterized by strong reflections, which pointed to an orthorhombic sub-structure (the “family structure” in the OD terminology), and additional weaker reflections that correspond to a monoclinic real structure. The refined cell parameters are a = 7.573(3), b = 23.364(5), c = 11.544(4) Å, β = 109.15(1)°, space group P21/c. This unit cell corresponds to one of the six possible maximum degree of order (MDO) polytypes, as obtained by applying the OD procedure. The derivation of the six MDO polytypes is presented in the Appendix1. The intensity data were collected at the Elettra synchrotron facility (Trieste, Italy); the structure refinement converged to R = 0.0342 for 1848 reflections with I > 2σ(I) and 0.0352 for all 1958 data. The structure of fukalite may be described as formed by distinct structural modules: a calcium polyhedral framework, formed by tobermorite-type polyhedral layers alternating along b with tilleyitetype zigzag polyhedral layers; silicate chains with repeat every fifth tetrahedron, running along a and linked to the calcium polyhedral layers on opposite sides; and finally rows of CO3 groups parallel to (100) and stacked along a.