ALBERT

Description: This decade marks the centenary of the discovery of X-ray diffraction. The development of mineralogy as a scientific discipline in which the properties of minerals are understood in terms of their atomic-scale structures has paralleled the development of diffraction crystallography. As diffraction crystallography revealed more precise details of mineral structures, more subtle questions about mineral properties could be addressed and a deeper understanding of the relationship between the two could be attained. We review the developments in X-ray single-crystal diffraction crystallography over the last century and show how its power to provide fundamental information about the structures of minerals has evolved with the improvements in data quality and the increased technological capacity to handle the data. We show that modern laboratory X-ray diffraction data are of the quality such that mineralogical results are no longer limited by the data quality, but by the physical validity of the refinement models used to interpret the data.

Description: The new mineral species braccoite, ideally Na $${\mathrm{Mn}}_{5}^{2+}$$ [Si 5 AsO 17 (OH)](OH), has been discovered in the Valletta mine dumps, in Maira Valley, Cuneo province, Piedmont, Italy. Its origin is probably related to the reaction between ore minerals and hydrothermal fluids. It occurs as subhedral crystals in brown-red coloured thin masses, with a pale-yellow streak and vitreous to resinous lustre. Braccoite is associated with tiragalloite, for which new data are provided, as well as gamagarite, hematite, manganberzeliite, palenzonaite, quartz, saneroite, tokyoite, unidentified Mn oxides, organic compounds, and Mn arsenates and silicates under study. Braccoite is biaxial positive with refractive indices α = 1.749(1), β = 1.750(1), = 1.760(1). It is triclinic, space group P 1I, with a = 9.7354(4), b = 9.9572(3), c = 9.0657(3) Å, α = 92.691(2), β = 117.057(4), = 105.323(3)°, V = 740.37(4) Å 3 and Z = 2. Its calculated density is 3.56 g/cm 3 . The ten strongest diffraction lines of the observed powder X-ray diffraction (XRD) pattern are [ d in Å, ( I ), ( hkl )]: 3.055 (69)(22I1), 3.042 (43)(102), 3.012 (65)(32I1I), 2.985 (55)(23I1I), 2.825 (100)(213I), 2.708 (92)(220), 2.627 (43)(23I2I), 2.381 (58)(41I1I), 2.226 (25)(214I) and 1.680 (433I)(36). Chemical analyses by wavelength-dispersive spectroscopy electron microprobe gave (wt.%): Na 2 O 4.06, CaO 0.05, MnO 41.76, MgO 0.96, Al 2 O 3 0.04, CuO 0.02, SiO 2 39.73, As 2 O 5 6.87, V 2 O 5 1.43, SO 3 0.01 and F 0.04. H 2 O 2.20 was calculated on the basis of 2OH groups p.f.u. Raman spectroscopy confirmed the presence of (SiO 4 ) 4– , (AsO 4 ) 3– and OH groups. The empirical formula, calculated on the basis of cations-(Na,K) = 11 p.f.u., in agreement with the results of the crystal structure, is Na 1.06 ( $${\mathrm{Mn}}_{4.46}^{2+}$$ $${\mathrm{Mn}}_{0.32}^{3+}$$ Mg 0.19 $${\mathrm{V}}_{0.01}^{3+}$$ Al 0.01 Ca 0.01 )[Si 5 (As 0.48 Si 0.37 $${\mathrm{V}}_{0.15}^{5+}$$ )O 17 (OH)](OH 0.98 F 0.02 ); the simplified formula is Na(Mn,Mg,Al,Ca) 5 [Si 5 (As,V,Si)O 17 (OH)](OH,F). Single-crystal XRD allowed the structure to be solved by direct methods and revealed that braccoite is the As-dominant analogue of saneroite. The structure model was refined on the basis of 4389 observed reflections to R 1 = 3.47%. Braccoite is named in honour of Dr Roberto Bracco (b. 1959), a systematic minerals collector with a special interest in manganese minerals. The new mineral was approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA 2013-093).

Description: In this work a single crystal of synthetic hercynite, FeAl 2 O 4 , was investigated by X-ray diffraction up to 7.5 GPa and at room temperature, in order to determine its pressure–volume equation of state. The unit-cell volume decreases non-linearly with a reduction of 3.4% (i.e. 18.43 Å 3 ). The pressure–volume data were fitted to a third-order Birch-Murnaghan equation of state providing the following coefficients: V 0 = 542.58(3) Å 3 , K T0 = 193.9(1.7) GPa, K ' = 6.0(5). These results are consistent with previous investigations of Cr and Al spinels measured with the same experimental approach but the K T0 differs significantly from the experimental determination carried out more than 40 years ago by Wang and Simmons ( 1972 ) by the pulse echo overlap method. Our new results were used to redetermine the FeAl 2 O 4(hercynite) = FeO (wüstite) + Al 2 O 3(corundum) equilibrium in P–T space and obtain geobarometric information for Cr-Al spinels found as inclusions in diamond.

Description: Two Pbca orthopyroxene samples, donpeacorite (DP N.1) and enstatite (B22 N.60) with chemical formulae Mn 0.54 Ca 0.03 Mg 1.43 Si 2 O 6 ( X Mn = 0.27) and Fe 0.54 Ca 0.03 Mg 1.43 Si 2 O 6 ( X Fe = 0.27), respectively, were investigated by single-crystal X-ray diffraction at high-temperature conditions. The nearly identical X Fe and X Mn make the two samples the perfect candidates to investigate the effect of the compositional change at the M 2 site (i.e. Fe-Mn substitution) on the thermal expansion behaviour of orthopyroxenes. Therefore, the unit-cell parameter thermal expansion behaviour of both samples has been investigated in the temperature range between room T and 1073 K. No evidence for phase transitions was found over that range. The two samples have been previously disordered with an ex situ annealing at ~1273 K. The unit-cell parameters and volume thermal expansion data, collected on the disordered samples, have been fitted to a Fei Equation of State (EoS) and the following coefficients obtained: V 0 = 853.35(4) Å 3 , α V,303K = 2.31(24) x 10 –5 K –1 and V 0 = 845.40(6) Å 3 , α V,303K = 2.51(25) x 10 –5 K –1 for DP N.1 and B22 N.60, respectively. While there is no difference in the volume thermal expansion coefficient as a function of composition and the expansion along the b direction is nearly identical for both samples, slight differences have been found along a and c lattice directions. The thermal expansion along the a direction is counterbalanced by that along c being responsible for the changes in lattice expansion scheme from α b 〉 α c 〉 α a at room T , to α c 〉 α b 〉 α a at high T . Therefore, as a result of the different behaviour along a and c, the unit-cell volume thermal expansion for both samples is identical within estimated standard deviations. The negligible effect of the Fe-Mn substitution on the bulk thermal expansion can be applied when dealing with geothermobarometry based on the elastic host-inclusion approach (e.g. Nestola et al. , 2011 ; Howell et al. , 2010 ; Angel et al. , 2014 a , b , 2015 ). In fact, though the compressibility effect is still not known, the nearly identical thermal expansion coefficients will not affect the entrapment pressure ( P e ).

Description: The crystal structure of the rare mineral dalnegroite, Tl5-xPb2x(As,Sb)21-xS34 with x {approx} 1, was determined for a crystal from Lengenbach, Binn Valley, Switzerland. The structure is triclinic, space group P1, with a = 16.218(3), b = 42.546(7), c = 8.558(1) A, {alpha} = 95.70(4), {beta} = 90.18(3), {gamma} = 96.38(4){degrees}, V = 5838.9(9) A3, Z = 4. Refinement of an isotropic model led to an R1 index of 0.0536 for 22226 observed reflections and 980 parameters, and R1 = 0.0590 for all 25266 independent reflections. Although dalnegroite cannot be considered a layered compound, its structure can be usefully described as a regular alternation of two kinds of layers stacked along the b axis, with four layers in the unit cell: (1) a layer 7.8 A thick, at y {approx} 0.15 and 0.65, can be considered as derived from the SnS archetype; (2) a layer 13.6 A thick, at y {approx} 0.35 and 0.85, derived from the PbS archetype. Different chemical compositions, such as Tl:Pb and Sb:As ratios, for different samples belonging to the chabourneite-dalnegroite family could play a central role in controlling different degrees of order, leading to different superstructures.

Description: Fassinaite, ideally Pb2+2(S2O3)(CO3), is a new mineral from the Trentini mine, Mount Naro, Vicenza Province, Veneto, Italy (holotype locality). It is also reported from the Erasmus adit, Schwarzleo District, Leogang, Salzburg, Austria and the Friedrich-Christian mine, Schapbach, Black Forest, Baden-Württemberg, Germany (cotype localities). At the Italian type locality it occurs as acicular [010], colourless crystals up to 200 µm long, closely associated with galena, quartz and anglesite. At the Austrian cotype locality it is associated with cerussite, rare sulphur and very rare phosgenite. At the German cotype locality anglesite is the only associated phase. Fassinaite crystals commonly have flat chisel-shaped terminations. They are transparent with vitreous to adamantine lustre and a white streak. Fassinaite is brittle with an irregular fracture and no discernible cleavage; the estimated Mohs hardness is 1½–2. The calculated density for the type material is 6.084 g cm-3 (on the basis of the empirical formula), whereas the X-ray density is 5.947 g cm-3. In common with other natural lead thiosulphates (i.e. sidpietersite and steverustite) fassinaite has intense internal reflections, which do not allow satisfactory optical data to be collected; the crystals are length-slow and have very high birefringence. The mineral is not fluorescent.Fassinaite is orthorhombic, space group Pnma, with unit-cell parameters (for the holotype material) a = 16.320(2), b = 8.7616(6), c = 4.5809(7) Å, V = 655.0(1) Å3, a:b:c = 1.863:1:0.523, Z = 4. Single-crystal structural studies were carried out on crystals from all three localities: R1(F) values range between 0.0353 and 0.0596. The structure consists of rod-like arrangements of Pb-centred polyhedra that extend along the [010] direction. These ‘rods’ are linked, alternately, by (CO3)2- and (S2O3)2- groups. The (S2O3)2- groups point alternately left and right (in a projection on [001] with [010] set vertical) if the apex occupied by the S2- in the thiosulphate group is defined to be the atom giving the direction. The lead atoms are nine-coordinated by seven oxygen atoms and two sulphur (S2-) atoms. The eight strongest X-ray powder-diffraction lines [d in Å (I/I0) (hkl)] are: 4.410 (39) (101), 4.381 (59) (020), 4.080 (62) (400), 3.504 (75) (301), 3.108 (100) (121), 2.986 (82) (420), 2.952 (49) (221) and 2.736 (60) (321). Electron-microprobe analyses produce an empirical formula Pb2.01(1)(S1.82(2)O3)CO3 (on the basis of six oxygen atoms). The presence of both carbonate and thiosulphate groups was corroborated by Raman spectra, which are discussed in detail. Fassinaite is named after Bruno Fassina (b. 1943), an Italian mineral collector who discovered the mineral in 2009.

Description: A synthetic single crystal of pure orthoenstatite (MgSiO 3 , space group Pbca ) has been investigated at high pressure for structural determinations by in situ single-crystal X-ray diffraction using a diamond-anvil cell. Ten complete intensity data collections were performed up to 9.36 GPa. This study significantly improved the accuracy of structural parameters in comparison to a previous high-pressure structural study, allowing a more detailed examination of structural behavior of orthoenstatite at high pressures and a comparison to other more recent structural studies performed on orthopyroxenes with different compositions. The structural evolution determined in this work confirms the high-pressure evolution found previously for other orthopyroxenes and removes some ambiguities originating from the less accurate published data on the MgSiO 3 structure at high pressure. The structural compression is mostly governed by significant volume decrease of the Mg1 and Mg2 octahedra, affecting in turn the kink of the tetrahedral chains, especially the TB chain of larger SiO 4 tetrahedra. The Mg2 polyhedron undergoes the largest volume variation, 8.7%, due especially to the strong contraction of the longest bond distance (Mg2-O3B), whereas Mg1 polyhedral volume decreases by about 7.4%. The compressional behavior of the tetrahedral sites is quite different from previously published data. The TA and TB tetrahedral volumes decrease by about 2.8 and 1.8%, respectively, and no discontinuities can be observed in the pressure range investigated. Using the data on the pure orthoenstatite as reference, we can confirm the basic influences of element substitutions on the evolution of the crystal structure with pressure.

Description: Debattistiite, ideally Ag9Hg0.5As6S12Te2, is a new mineral (IMA-CNMNC 2011-098) from the Lengenbach quarry in the Binn Valley, Valais, Switzerland. It occurs as very rare tabular euhedral crystals up to 150 μm across in cavities in dolomitic marble, associated with realgar, rutile, trechmannite and hutchinsonite. Debattistiite is opaque with a metallic lustre and a grey streak. It is brittle; the Vickers hardness (VHN25) is 80 kg mm−2 (range: 65–94), corresponding to a Mohs hardness of 2–2½. In reflected light debattistiite is dark grey, highly bireflectant and weakly pleochroic from dark grey to a slightly greenish grey. Between crossed polars it is highly anisotropic with brownish to blue rotation tints. Internal reflections are absent. Reflectance percentages for the four COM wavelengths (Rmin and Rmax) are 27.2, 34.5 (471.1 nm), 25.5, 31.0 (548.3 nm), 22.9, 28.4 (586.6 nm), and 20.1, 25.2 (652.3 nm), respectively.Debattistiite is triclinic, space group P1İ, with a = 7.832(5), b = 8.606(4), c = 10.755(5) Å, α = 95.563(9), β = 95.880(5), γ = 116.79(4)°, V = 635.3(6) Å3 and Z = 1. The crystal structure [R1 = 0.0826 for 795 reflections with I 〉 2σ(I)] consists of corner-sharing AsS3 pyramids forming three-membered distorted rings linked by Ag atoms in triangular or tetrahedral coordination.The five strongest powder-diffraction lines [d in Å (I/I0) (hkl)] are as follows: 10.56 (6) (001); 3.301 (5) (2İ12); 2.991 (4) (21İ2); 2.742 (2İ1İ1) and 2.733 (10) (2İ30). A mean of nine electron microprobe analyses gave: Ag 44.88, Hg 4.49, As 20.77, S 17.72, Te 11.82; total 99.68 wt.%, which corresponds to Ag9.02Hg0.49As6.012S11.98Te2.01, on the basis of 29.5 atoms. The new mineral is named for Luca De Battisti, a systematic mineralogist and expert on the minerals of Lengenbach quarry.