Adriana Maras

The monoclinic I2 structure of bassanite, calcium sulphate hemihydrate (CaSO4 · 0.5H2O)

A structural analysis of CaSO 4 · 0.5H 2 O, a dehydration product of gypsum, has been carried out through the Rietveld method on X-ray powder diffraction data. A dehydrated powder of synthetic gypsum has been charged inside a non-hermetically sealed capillary in order to allow a slow rehydration. The starting material has been identified as γ-anhydrite, space group P 6 2 22, cell parameters a = 6.9691(2) A, c = 6.3033(2) A. The final product of the rehydration of γ-anhydrite is CaSO 4 · 0.5H 2 O, space group I 2 (unique axis b ), cell parameters a = 12.0350(5) A, b = 6.9294(3) A, c = 12.6705(4) A, β = 90.266(3)°. The structure of the hemihydrate is strongly pseudo-trigonal, space group P 3 1 21. The symmetry lowering arises from water molecules ordering inside the channels.

Crystal chemistry and IR spectroscopy of Cl- and SO4-bearing cancrinite-like minerals

Abstract Cl- and SO4-bearing cancrinite-like minerals from Italian localities were studied by means of EPMA, X-ray diffraction, and IR spectroscopy to relate their chemical composition, IR data, and structural features. The framework of the cancrinite-like minerals consists of six-membered rings of alumino silicate tetrahedra stacked along c. Six types of subunits resulting from different stacking sequences can be identified: free channel, cancrinite, sodalite, losod, liottite, and giuseppettite cages. The free channel is possible only with an AB sequence; in fact, the insertion of a C layer interrupts the channel forming the various cages. The occurrence of these structural subunits seems to correspond to the different chemistries of the cancrinite-like minerals, especially their anion content. According to this model, the SO4 group seems to playa major role. With the exception of giuseppettite, it tends to fill completely the available voids within the frameworks.

Mineralogical characterization of the blue pigment of Michelangelo’s fresco “The Last Judgment”

Abstract The blue pigment and the plaster layer of Michelangelo.s fresco .The Last Judgment. have been characterized by means of powder XRD, FT-IR, and Raman spectroscopy, as well as analytical TEM and SAED. The pigment is composed of a mineralogical assemblage that is consistent with a raw lapis lazuli rock. To try to define the provenance of this pigment, we investigated various lapis lazuli samples from different deposits that are currently considered as possible sources of the material. According to our results, the provenance of the pigment is quite dubious but the Pakistani one seems the most probable.

In-situ X-ray transmission powder diffraction study of the kinetics of the light induced alteration of realgar (α-As4S4)

An in-situ X-ray transmission powder diffraction study of the kinetics of the light induced alteration of realgar (As 4 S 4 ) has been carried out. Data indicate that the crystalline final products of the process are pararealgar (different As 4 S 4 polymorph) and arsenolite (As 2 O 3 ). Because of the chemical unbalance the evolution of SO x is hypothesized. In fact there are no evidences of the presence of amorphous As-S alloys or amorphous S. The alteration proceeds via the occurrence of an intermediate term along the β-As 4 S 4 -alacranite series joint with an estimated As 4 S 4 .2 composition. This phase reach a maximum wt. fraction of ca. 30 % in the early stages of the process and subsequently disappears. From the fraction X(t) of realgar and pararealgar transformed vs. time was determined the kinetic law of the digestion of realgar and formation of pararealgar according to the JMAK model. The refined n parameters indicate that the behavior of the two transformations is intermediate between that of a diffusion-controlled and a first-order model. The increased expansion of the realgar cell during the alteration could be possibly due to the requirement that structural coherency between at least two of the three sulfide phases is preserved. In particular the coherency between realgar and β-As 4 S 4 is expected to exist along a realgar * sin β and c β-As 4 S 4 , respectively. According to our data we may hypothesize that the light induced alteration of realgar to pararealgar occurs via an intermediate As 4 S 4+x product because it is able to sustain the presence of As 4 S 5 -type cages. Following the model proposed by Kyono et al. , (2005) the As 4 S 5 cage release the extra S atom to form the As 4 S 4 cage of pararealgar-type promoting the development of a new As 4 S 5 cage in a cyclic process.

The crystal structure of 3-methyluracil from X-ray powder diffraction data

Abstract The crystal structure of 3-methyluracil has been determined ab initio by conventional monochromatic X-ray powder diffraction data. The crystal data are: orthorombic, a =6.6294(1), b =13.1816(3), c =6.53938(9) (A), V =571.45(3) (A 3 ), space group Pbnm , Z =8. The structure was solved by direct methods and the final Rietveld refinement converged to R p =0.0398, R wp =0.0528, R Bragg =0.0294. The crystal structure exhibits endless chains of planar molecules, connected via head-to-tail N–H⋯O hydrogen bonds.

Pathological biominerals: Raman and infrared studies of bioapatite deposits in human heart valves

We studied pathological bioapatite from patients undergoing valvular replacement due to severe aortic and mitral stenosis. Three different types of mineralized human cardiac valves were analyzed. We used infrared and Raman spectroscopy to infer the presence of the carbonate group and evaluate the carbonate substitution in bioapatite structure. The Raman spectra showed that the pathological bioapatite is a B-type “carbonateapatite” (CO32- for PO43-) similar to the major mineralized products derived from normal biomineralization processes occurring in the human body. Fourier transform infrared spectra (FT-IR) confirmed the B-type carbonate substitution (CO32- for PO43-) and showed evidence for the partial replacement of [OH] by [CO3] (A-type substitution). The carbonate content of the samples inferred by the spectroscopic measurements is in good agreement with the range of values estimated for biological apatite. On the contrary, the crystal size of the pathological apatite estimated using the percentage area of the component at 1059 cm−1 of the infrared spectrum is in the nanometer range and it is significantly smaller than the crystal size of normal mineralized tissues.

The crystal structure of a “disordered” cancrinite

An X-ray single-crystal structural investigation on a sample of cancrinite of formula Na 7.13 Ca 0.87 (Si 6 Al 6 O 24 ) (SO 4 ) 0.07 (CO 3 ) 1.43 2H 2 O has been carried out. The cancrinite cage content has been found to be similar to that reported in previous structure refinements on natural and synthetic samples. However, differently from reference data, cations located inside the uninterrupted channel are distributed between two independent sites M1 and M2. Moreover carbonate groups are distributed, with partial occupancy, between two different z elevations (Cl-OC1 and C2-OC2). The tetrahedral co-ordination of the sulphate group is obtained by partial substitution of carbon atoms at C1 and C2 by sulphur and oxygen. Two possible orientation with the apical oxygen pointing “down” or “up” are possible.

Crystal chemical and structural characterization of an unusual CO3-bearing sodalite-group mineral

We report the crystal chemical and structural characterization of an unusual CO 3 -bearing sodalite-group mineral. The sample was found in metasomatic ejecta from Mount Vesuvio, Italy. The mineral is characterised by an a cell parameter of 9.0352(2) A and by a formula, recalculated from EMPA, of (Na 5.28 K 1.40 Ca 1.23 Fe 0.01 ) Σ7.92 [Si 5.93 Al 6.07 O 24.01 ][(SO 4 ) 0.77 Cl 0.94 (CO 3 ) 0.30 ] Σ2.01 . The structure refinement, conducted in P 43 n space group, indicates that the cations are distributed among three independent sites M1, M2, and M3, each one with partial occupancy. The Cl anion is located exactly at the centre of the cages (site Cl) along with the carbon atom of the CO 3 group. The sulphur atom of the sulphate group is located in a site slightly displaced from the centre of the cage, similarly to some hauyne samples (site S). The sulphate group shows two different statistical orientations with probabilities of 2/3 and 1/3. Despite the very contrasted dimensions of the different anions/anionic groups involved, the framework is fully ordered with Si-O and Al-O bond distances of 1.6156(6) and 1.7341(6) A respectively, without splitting of the framework oxygen sites (O1 and O2). This behaviour is different from that reported for nosean, lazurite and some hauyne samples.

The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si3B3O18)15− polyanion

Abstract The crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th)15Fe3+(SiO4)3 (Si3B3O18)(BO3)(As5+O4)(As3+O3)x(NaF3)1-xF7·0.2H2O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers. Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO4)6 group and a threefold B3O9 borate ring. Each tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si3B3O1815- polyanion. Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO4 tetrahedron and a BO3 triangle. Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85 Å) As3+O33- and NaF32- polyhedra, the occurrence of which is mutually exclusive and statistically disordered. A 3-dimensional network of M-(O,F)n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections among neighboring layers.

FIB, TEM and LA-ICPMS investigations on melt inclusions in Martian meteorites - Analytical capabilities and geochemical insights

In order to obtain full information coverage on melt inclusions in Martian meteorites (subgroup nakhlites) complementary micro-analytical techniques were used, i.e. focused ion beam, transmission electron microscopy and laser ablation. Using focused ion beam several lamellae for transmission electron microscopy were prepared and secondary electron images of cross-sections could be acquired. Laser ablation-inductively coupled plasma mass spectrometry analyses were performed on selected inclusions to obtain mass-oriented bulk composition of inclusions at depth. The differences in composition between melt inclusions in olivine and augite crystals would suggest a xenocrystic origin for olivine. Furthermore, electron diffraction patterns clearly indicated that the SiO(2)-rich phase in inclusions from augite in meteorites from Northwest Africa site is re-crystallized, whereas it is still vitreous in the inclusions from Nakhla sampling site. Therefore, different post-entrapment evolutions were active for the two nakhlite meteorite sets, the Nakhla and the NWA817 set. Melt inclusions in Nakhla olivine presented alteration veins, which were presumably produced before their landing on Earth. If this is the case, this would indicate a alteration stage already on Mars with all the consequence in terms of climate history. Melt inclusions in Nakhla augite resulted unaffected by any alteration or modification following the entrapment, and therefore represent the best candidate to indicate the pristine magma composition.