Raman scattering experiments of natural FeS2 pyrite were performed at simultaneous high-pressure and high-temperature conditions up to 675 K and 2100 MPa using a hydrothermal diamond anvil cell combined with micro-Raman spectroscopy. Four out of five Raman active modes [Eg, Ag, Tg(1) and Tg(3)] were resolved at ambient conditions, the remaining Tg(2) [∼377 cm–1] mode was weak and unresolved occurring ∼2 cm–1 from the intense Ag [379 cm–1] mode. The frequency shifts of the Eg [343 cm–1] and Ag [379 cm–1] modes were determined to be quadratic functions of pressure and temperature: ν343 = 343.35 – 0.0178 × ΔT – 8.4E – 6 × (ΔT)2 + 0.00367 × Δp 3.7E–7 × (Δp)2 + 1.0E–6 × ΔT × Δp and ν379 = 379.35 – 0.0295 × ΔT – 9.0E–6 × (ΔT)2 + 0.00460 × Δp – 5.3E–7 × (Δp)2 + 7.0E–7 × ΔT × Δp. The positive pressure dependence of both modes indicates stress-induced contraction of S–S and Fe–S bonds, whereas the negative temperature dependence shows temperature-induced expansion of them. The Raman spectra of pyrite were used to derive its bulk modulus at high temperatures, thermal expansion coefficient at high pressures and anharmonic parameters at high-pressure and high-temperature conditions.

The crystal structure of hohmannite, Fe3+2[O(SO4)2]·8H2O, was studied by means of single-crystal X-ray diffraction (XRD) and vibrational spectroscopy. The previous structural model was confirmed, though new diffraction data allowed the hydrogen-bond system to be described in greater and more accurate detail. Ab initio calculations were performed in order to determine accurate H positions and to support the experimental model obtained from XRD data. Infrared and Raman spectra are presented for the first time for this compound and comments are made on the basis of the crystal structure and the known literature for sulfate minerals.

The members of the sartorite homologous series are complex sulfides Me2+8N–16–2XMe3+16+XMe+XS8N+8 where Me2+ is Pb and Me3+ is As and Sb, whereas Me+ is Ag and/or Tl. This paper presents calculation formulae for the homologue order N and for the separate substitution percentages for Tl + (As,Sb) ↔ 2Pb and Ag + (As,Sb) ↔ 2Pb substitutions. This enables one to evaluate the crystal chemistry and build a systematic classification of the sartorite homologues.

When crustal abundance (A, measured in atomic parts per million) of a chemical element is plotted vs. number of mineral species in which that element is an essential constituent (S), a significantly positive correlation is obtained, but with considerable scatter. Repeated exclusion of outliers at the 90% confidence level and re-fitting leads, after the sixth iteration, to a steady state in which 40 of the 70 elements initially considered define a trend with log S = 1.828 + 0.255 log a (r = 0.96), significantly steeper than the original. Three other methods for reducing the effect of outliers independently reproduce this steeper trend. The 'diversity index' D of an element is defined as the ratio of observed mineral species to those predicted from this trend. D separates elements into three groups. More than half of the elements (40 of 70) have D = 0.5–2.0. Apart from these 'typical' elements, a group of 15 elements (Sc, Cr, Ga, Br, Rb, In, Cs, La, Nd, Sm, Gd, Yb, Hf, Re and Th) form few species of their own due to being dispersed as minor solid solution constituents, and a hitherto unrecognized group of 15 elements are essential components in unusually large numbers of minerals. The anomalously diverse group consists of H, S, Cu, As, Se, Pd, Ag, Sb, Te, Pt, Au, Hg, Pb, Bi and U, with Te and Bi by far the most mineralogically diverse elements (D = 22 and 19, respectively). Possible causes and inhibitors of diversity are discussed, with reference to atomic size, electronegativity and Pearson softness, and particularly outer electronic configurations that result in distinctive stereochemistry. The principal factors encouraging mineral diversity are: (1) Particular outer electronic configurations that lead to a preference for unique coordination geometries, enhancing an element's ability to form distinctive chemical compounds and decreasing its ability to participate in solid solutions. This is particularly noteworthy for elements possessing geometrically flexible 'lone-pair cations' with an s2 outer electronic configuration. (2) Siderophilic or chalcophilic geochemical behaviour and intermediate electronegativity, allowing elements to form minerals that are not oxycompounds or halides. (3) Access to a wide range of oxidation states. The most diverse elements can occur as anions, native elements and in more than one cationic valence state.

The crystal structure of the organic mineral refikite has been determined. The mineral was found in joints in bark and wood from pine trees in the 'V Borkách' peat deposit near the town of Krásno, Slavkovský les Mountains, western Bohemia, Czech Republic. It forms white to light-yellow polycrystalline crusts or randomly intergrown, transparent, colourless, very thin, acicular crystals up to 0.2–0.5 mm long. Sometimes, colourless-to-white elongated prismatic crystals up to 1–1.5 mm in size were encountered. The mineral is soft (Mohs hardness ∼1) and very brittle, with an uneven fracture. No visible cleavage was discerned. Crystals have a greasy-to-glassy lustre; fine crystal aggregates have a pearly lustre. Refikite, empirical formula C20H34O2 or C19H33COOH, is a derivative of abietic acid. It is orthorhombic, space group P21212, with a = 22.6520(7), b = 10.3328(3), c = 7.6711(2) Å, V = 1795.49(9) Å3, Z = 4. Refikite comprises two closely related compounds based on perhydrophenanthrene. The major component has two axial methyl groups, one terminal carboxylic group and one terminal propan-2-yl (isopropyl) group joined to the three fused rings in the same fashion as in abietic acid. However, the fused ring system is fully reduced (contains single bonds only). In the minor component, the terminal propan-2-yl group is replaced by a propen-2-yl (methylvinyl) group. The crystal structure is stabilized by strong O···H–O hydrogen bonds. High-resolution mass spectroscopy (HRMS) confirmed a molecular mass of 306 and the formula C20H34O2. Hydrogen-1 and carbon-13 nuclear magnetic resonance (NMR) spectroscopy showed the presence of four methyl groups in the major component; infrared (IR), Raman and NMR spectra are consistent with the structure. The HRMS, IR and Raman spectroscopy methods confirmed the presence of a minor component containing the propen-2-yl group replacing the propan-2-yl group. This is also reflected in a shortened C15–C17 single bond metric of 1.468(6) Å shown by single-crystal X-ray analysis. The trivial name of the major component of refikite is tetrahydroabietic acid or abietan-18-oic acid. This work represents the first proof of the existence of abietic acid derivatives as naturally occurring species.

Two Pbca orthopyroxene samples, donpeacorite (DP N.1) and enstatite (B22 N.60) with chemical formulae Mn0.54Ca0.03Mg1.43Si2O6 (XMn = 0.27) and Fe0.54Ca0.03Mg1.43Si2O6 (XFe = 0.27), respectively, were investigated by single-crystal X-ray diffraction at high-temperature conditions.

The nearly identical XFe and XMn make the two samples the perfect candidates to investigate the effect of the compositional change at the M2 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: V0 = 853.35(4) Å3, αV,303K = 2.31(24) × 10–5 K–1 and V0 = 845.40(6) Å3, αV,303K = 2.51(25) × 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 αb > αc > αa at room T, to αc > αb > αa 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 (Pe).

Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, is a new mineral of the tourmaline supergroup. It occurs in an aplitic dyke of a LCT-type pegmatite body from Grotta d'Oggi, San Piero in Campo, Elba Island, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, schorl, fluor-elbaite and tsilaisite. Crystals are greenish yellow with a vitreous lustre, sub-conchoidal fracture and white streak. Fluor-tsilaisite has a Mohs hardness of ∼7 and a calculated density of 3.134 g/cm3. In plane-polarized light, fluor-tsilaisite is pleochroic (O = pale greenish yellow and E = very pale greenish yellow), uniaxial negative. Fluor-tsilaisite is rhombohedral, space group R3m, a = 15.9398(6), c = 7.1363(3) Å, V = 1570.25(11) Å3, Z = 3. The crystal structure of fluor-tsilaisite was refined to R1 = 3.36% using 3496 unique reflections collected with MoKα X-ray intensity data. Crystal-chemical analysis resulted in the empirical formula: X(Na0.69〈0.29Ca0.02)Σ1.00Y(Mn2+1.29Al1.21Li0.56Ti0.03)Σ6.00ZAl6T(Si5.98Al0.03)Σ6.00B2.92O27V(OH)3W[F0.39(OH)0.25O0.36]Σ1.00.

Comparisons were performed between fluor-tsilaisite and a tsilaisitic tourmaline from the same locality as the holotype specimen. This latter tourmaline sample was selected for this study due to its remarkable composition (MnO = 11.63 wt.%), the largest Mn content found in tourmaline so far.

Fluor-tsilaisite is related to tsilaisite through the substitution WF ↔ W(OH) and with fluor-elbaite through the substitution Y(Al + Li) ↔ 2YMn2+, and appears to be a stepwise intermediate during tourmaline evolution from tsilaisite to fluor-elbaite.

The high-temperature- and moisture-dependent behaviour of synthetic kanemite (NaSi2O4(OH)·3H2O or SKS-10) has been studied by in situ powder X-ray diffraction. Heating experiments in the range between ambient temperatures and 250°C confirm earlier investigations that the dehydration of kanemite occurs in two steps. According to our results the two different reactions start at ∼30 and 75°C. The dehydration products have the following compositions: NaSi2O4(OH)·H2O (monohydrate) and NaSi2O4(OH), respectively. The crystal structures of both phases have been solved at ambient conditions ab initio from laboratory powder diffraction data using samples that have been carefully dehydrated at 60 and 150°C, respectively, and refined subsequently by the Rietveld method. Basic crystallographic data are as follows: NaSi2O4(OH)·H2O: orthorhombic, space group Pna21, a = 7.2019(1), b = 15.3252(2), c = 4.8869(1) Å, V = 539.37(1) Å3, Z = 4; NaSi2O4(OH): monoclinic, space group P21, a = 6.3873(1), b = 4.8876(1), c = 7.1936(1) Å, β = 93.36(1)°, V = 224.19(1) Å3, Z = 2. Both compounds belong to the group of single-layer silicates based on Si2O4 (OH) sheets. The sodium cations are located between the tetrahedral sheets and are surrounded by oxygen atoms from silicate anions and/or water molecules. Depending on the dehydration step the coordination numbers of the alkali ions vary between six (kanemite) and five (NaSi2O4(OH)). Kanemite and its two dehydration products show structural similarities which are discussed in detail. Moisture-dependent diffraction studies at ambient temperatures indicate that kanemite is stable between 10% and at least 90% relative humidity. Below the lower threshold a transformation to the monohydrate phase was observed. Dehydration and rehydration as a function of humidity is reversible. However, this process is combined with a significant loss of crystallinity of the samples.

Bonazziite is a new mineral from Khaidarkan deposit, Kyrgyzstan and represents the natural analogue of the β-form of the well known As4S4 compound. It occurs as rare crystals up to 100 μm across associated with realgar, sulfur, wakabayashilite, alacránite, non-stoichiometric As4S4+x sulfides and stibnite in a calcite matrix. In thick section, bonazziite is opaque with a resinous lustre and a dark-orange streak. It is brittle; the Vickers hardness (VHN15) is 70 kg/mm2 (range: 60–76) (Mohs hardness of ∼2½). In plane-polarized incident light, bonazziite is strongly bireflectant and pleochroic from orange to light red. The mineral shows orange to red internal reflections. Between crossed polars, the mineral is strongly anisotropic with greyish to light-blue rotation tints. Reflectance percentages in air for Rmin and Rmax are 19.9, 22.2 (471.1 nm), 19.1, 21.3 (548.3 nm), 18.8, 19.7 (586.6 nm) and 17.8, 18.9 (652.3 nm), respectively. Bonazziite is monoclinic, space group C2/c, with a = 9.956(1), b = 9.308(1), c = 8.869(1) Å, β = 102.55(2)° and V = 802.3(2) Å3, Z = 4. The crystal structure [R1 = 0.0263 for 735 reflections with Fo > 4σ(Fo)] is based on the As4S4 cage-like molecule, in which each As atom links one As and two S atoms. The As4S4 molecule is identical to that found in the structure of realgar. The six strongest powder diffraction lines [d in Å (I/I0) (hkl)] are: 5.74 (100) (1̄11); 4.10 (60) (021); 3.92 (50) (1̄12); 3.12 (60) (022, 310); 2.95 (50) (221, 202); 2.86 (80) (2̄22, 1̄31). a mean of six electron microprobe analyses gave the formula As3.95S4.05, on the basis of eight atoms. The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA No. 2013-141) and named for Paola Bonazzi, in recognition of her seminal contributions to the study of arsenic sulfides and their alteration induced by exposure to light.

The new mineral popovite, Cu5O2(AsO4)2, was found in the sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with ericlaxmanite, kozyrevskite, urusovite, lammerite, lammerite-β, johillerite, bradaczekite, tenorite, hematite, aphthitalite, anhydrite, langbeinite, calciolangbeinite, As-bearing orthoclase, etc. Popovite occurs as prismatic or tabular crystals and as grains up to 0.2 mm in size forming clusters up to 1.5 mm in size and as crusts on basalt scoria or on aphthitalite incrustations. Popovite is transparent with a vitreous to greasy lustre. Its colour is olive green to dark olive-green, but fine-grained varieties are light yellow-green. The mineral is brittle, with Mohs' hardness ∼3½. Cleavage was not observed and the fracture is uneven. Dcalc is 5.30 g cm–3. Popovite is optically biaxial (+), α = 1.84(1), β ≈ 1.86, γ = 1.96(1), 2Vmeas = 50(20)°. The Raman spectrum is given. Chemical data (wt.%, electron-microprobe) are CuO 63.28, ZnO 0.56, V2O50.12, As2O5 35.80, SO3 0.27, total 100.03. The empirical formula, based on 10 O a.p.f.u., is (Cu4.99Zn0.04)Σ5.03(As1.95S0.02V0.01)Σ1.98O10. Popovite is triclinic, P1̄, a = 5.1450(3), b = 6.2557(3), c = 6.2766(4) Å, α = 100.064(5), β = 96.351(5), γ = 95.100(5)°, V = 196.47(1) Å3 and Z = 1. The strongest reflections in the powder X-ray diffraction pattern [d, Å (I)(hkl)] are 3.715(36)(110, 101), 3.465(43)(11̄1), 2.968(90)(01̄2), 2.927(100)(111), 2.782(31)(1̄02), 2.768(67)(1̄20), 2.513(55)(1̄2̄1) and 2.462(67)(2̄01). Popovite has a novel structure type. Its crystal structure, solved from single-crystal X-ray diffraction data (R = 0.0459), is based on (010) layers forming an interrupted framework. The layer consists of Cu(1)O6 octahedra with very strong Jahn-Teller distortion and Cu(2)O5 and Cu(3)O5 polyhedra. The linkage between the layers is reinforced by isolated AsO4 tetrahedra. Popovite is named in honour of the Russian mineralogists Vladimir Anatol'evich Popov (b. 1941) and Valentina Ivanovna Popova (b. 1941), a husband and wife research team working in the Institute of Mineralogy of the Urals Branch of the Russian Academy of Sciences, Miass, Russia.

'Clinobarylite', BaBe2Si2O7, was defined as a monoclinic dimorph of orthorhombic barylite. Subsequently, its crystal structure was also proved to be orthorhombic, differing from barylite in terms of the space group symmetry, Pmn21 instead of Pmnb, and in unit-cell dimensions. Through the order-disorder (OD) theory, the polytypic relationships between 'clinobarylite' and barylite are described. 'Clinobarylite' corresponds to the MDO1 polytype, with unit-cell parameters a = 11.650, b = 4.922, c = 4.674 Å, space group Pmn21; barylite corresponds to the MDO2 polytype, with a = 11.67, b = 9.82, c = 4.69 Å, space group Pmnb. The re-examination of the holotype specimen of 'clinobarylite' confirmed its orthorhombic symmetry. Its crystal structure has been refined starting from the atomic coordinates calculated for the MDO1 polytype and the refinement converged to R1 = 0.0144 for 929 observed reflections [Fo > 4σFo]. Owing to their polytypic relationships, 'clinobarylite' and barylite should be conveniently indicated as barylite-1O and barylite-2O, respectively; the name 'clinobarylite' should be discontinued. This new nomenclature of the barylite polytypes has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 13-E).

The present work analyses the thermal behaviour of alum-(K), KAl(SO4)2·12H2O, by in situ laboratory high-temperature powder X-ray diffraction data from 303 K to melting, which starts at 355 K and is completed, due to kinetics, at 359 K. The calculated a0 linear thermal expansion coefficient is of 14.68(11) × 10–6 K–1 within the investigated thermal range. The k disorder parameter, describing the extension of the orientational disorder of the sulfate group, has been found to decrease from ∼0.70 to ∼0.65 just before melting. It has been demonstrated that the occurrence of the disorder implies the coexistence of K+ ions in both six- and seven-fold coordination. This is necessary for assigning a reasonable bond-valence sum of 0.81 valence units (vu) to the 'average' K+ ion a instead of 0.66 vu, which is obtained in the case of six-fold coordination alone. We can describe the temperature dependence of k from 93–355 K by means of the empirical equation k = 0.798(12) + 2.5(11) × 10–4 T – 1.9(2) × 10–6T2, which includes reference low-temperature data. Bond-valence analysis has shown that, on cooling, an increase of the k disorder parameter and shortening of the K–O2 bond distance act together to maintain constancy in the bond-valence sum at the K site, stabilizing the structure. Therefore, the need for keeping the 'average' K+ ion at a reasonable bond-valence sum appears to be the driving force for the ordering process involving the sulfate group.

The new mineral species braccoite, ideally NaMn2+5[Si5AsO17(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), γ = 1.760(1). It is triclinic, space group P1̄, with a = 9.7354(4), b = 9.9572(3), c = 9.0657(3) Å, α = 92.691(2), β = 117.057(4), γ = 105.323(3)°, V = 740.37(4) Å3 and Z = 2. Its calculated density is 3.56 g/cm3. The ten strongest diffraction lines of the observed powder X-ray diffraction (XRD) pattern are [d in Å, (I), (hkl)]: 3.055 (69)(22̄1), 3.042 (43)(102), 3.012 (65)(32̄1̄), 2.985 (55)(23̄1̄), 2.825 (100)(213̄), 2.708 (92)(220), 2.627 (43)(23̄2̄), 2.381 (58)(41̄1̄), 2.226 (25)(214̄) and 1.680 (433̄)(36). Chemical analyses by wavelength-dispersive spectroscopy electron microprobe gave (wt.%): Na2O 4.06, CaO 0.05, MnO 41.76, MgO 0.96, Al2O3 0.04, CuO 0.02, SiO239.73, As2O5 6.87, V2O5 1.43, SO3 0.01 and F 0.04. H2O 2.20 was calculated on the basis of 2OH groups p.f.u. Raman spectroscopy confirmed the presence of (SiO4)4–, (AsO4)3– and OH groups. The empirical formula, calculated on the basis of Σ cations-(Na,K) = 11 p.f.u., in agreement with the results of the crystal structure, is Na1.06(Mn2+4.46Mn3+0.32Mg0.19V3+0.01Al0.01Ca0.01)[Si5(As0.48Si0.37V5+0.15)O17(OH)](OH0.98F0.02); the simplified formula is Na(Mn,Mg,Al,Ca)5[Si5(As,V,Si)O17(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 R1 = 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).

Supposed 'merlinoite' crystals from Monte Somma, Vesuvius (Italy) and Fosso Attici, north of Rome (Italy) represent highly twinned coherent intergrowths between merlinoite and phillipsite on a submicroscopic level. The MER (Immm, a ≈ 14.1, b ≈ 14.2, c ≈ 9.9 Å) and PHI (P 21/m, a ≈ 9.9, b ≈ 14.3, c ≈ 8.7 Å, β = 124.8°) frameworks of similar composition are assembled from identical tetrahedral units, though with a different connectivity. Coherent intergrowth and twinning of the two frameworks lead to P42/mnm pseudosymmetry, which is diagnostic of the intergrowth. Under ambient conditions merlinoite has Immm symmetry or I4/mmm if twinned. a low-symmetry model of space group P121/m1 (a ≈ 14.2, b ≈ 14.2, c ≈ 10 Å, β = 90°) allows structure refinement and quantification of the two frameworks.

Upon in situ dehydration to 250°C the evolution of the unit-cell volume of the Monte Somma merlinoite/phillipsite intergrowth displays an intermediate trend between previously studied pure merlinoite from the Khibiny massif (Russia) and Ba-rich phillipsite.

The Monte Somma crystal studied by temperature-dependent single-crystal X-ray diffraction methods also contained a subordinate chabazite inclusion with no coherent structural relationship to the merlinoite/phillipsite framework. Thus, the modification of the chabazite framework on dehydration could also be studied.

The new mineral geschieberite (IMA2014-006), K2(UO2)(SO4)2(H2O)2, was found in the Svornost mine, Jáchymov, Czech Republic, where it occurs as a secondary alteration phase after uraninite in association with adolfpateraite and gypsum. Geschieberite forms crystalline aggregates of bright green colour (when thick) composed of multiply intergrown prismatic crystals elongated on [001] typically reaching 0.1–0.2 mm across; observable forms are {010} and {001}. Crystals are translucent to transparent with a vitreous lustre. The mineral is brittle, with perfect cleavage on {100} and an uneven fracture. It has a greenish-white streak and a probable Mohs hardness of ∼2. The mineral is slightly soluble in cold H2O. The calculated density is 3.259 g cm–3. The mineral exhibits strong yellowish-green fluorescence under both shortwave and longwave UV radiation. Optically, geschieberite is biaxial (–), with β = 1.596(2) and γ = 1.634(4) (measured at 590 nm), with X = a. Electron-microprobe analyses provided Na2O 0.23, K2O 14.29, MgO 2.05, CaO 0.06, UO3 49.51, SO3 27.74, H2O 6.36 (structure), total 100.24 wt.%, yielding the empirical formula (K1.72Mg0.29Na0.04Ca0.01)Σ2.06(U0.98O2)(S0.98O4)2(H2O)2 based on 12 O atoms per formula unit. The Raman spectrum is dominated by the symmetric stretching vibrations of UO22+, SO42– and weaker O–H stretching vibrations. Geschieberite is orthorhombic, Pna21, with a = 13.7778(3), b = 7.2709(4), c = 11.5488(2) Å, V = 1156.92(7) Å3, Z = 4. The eight strongest powder X-ray diffraction lines are [dobs in Å (hkl) Irel]: 6.882 (200) 100, 5.622 (111) 53, 4.589 (211) 12, 4.428 (202) 16, 3.681 (311) 18, 3.403 (013) 12, 3.304 (401,1̄13) 15 and 3.006 (122) 17. The structure, refined to R = 0.028 for 1882 I > 3σ(I) reflections, contains [(UO2)(SO4)2(H2O)]2– sheets that are based on the protasite anion topology. Sheets are stacked perpendicular to a. Potassium atoms and one H2O molecule are located between these sheets, providing an interlayer linkage. The remaining H2O molecule is localized within the structural unit, at the free vertex of the uranyl pentagonal bipyramid; this vertex does not link to sulfate tetrahedra. The mineral is named for one of the most important ore veins in Jáchymov – the Geschieber vein.