Thomas Armbruster

Recommended nomenclature for zeolite minerals: report of the subcommittee on zeolites of the International Mineralogical Association, Commission on New Minerals and Mineral Names

Abstract This report embodies recommendations on zeolite nomenclature approved by the International Mineralogical Association Commission on New Minerals and Mineral Names. In a working definition of a zeolite mineral used for this review, interrupted tetrahedral framework structures are accepted where other zeolitic properties prevail, and complete substitution by elements other than Si and Al is allowed. Separate species are recognized in topologically distinctive compositional series in which different extra-framework cations are the most abundant in atomic proportions. To name these, the appropriate chemical symbol is attached by a hyphen to the series name as a suffix except for the names harmotome, pollucite and wairakite in the phillipsite and analcime series. Differences in space- group symmetry and in order-disorder relationships in zeolites having the same topologically distinctive framework do not in general provide adequate grounds for recognition of separate species. Zeolite species are not to be distinguished solely on Si : Al ratio except for heulandite (Si : Al < 4.0) and clinoptilolite (Si : Al ≥ 4.0). Dehydration, partial hydration, and over-hydration are not sufficient grounds for the recognition of separate species of zeolites. Use of the term ‘ideal formula’ should be avoided in referring to a simplified or averaged formula of a zeolite. Newly recognized species in compositional series are as follows: brewsterite-Sr, -Ba; chabazite-Ca, - Na, -K; clinoptilolite-K, -Na, -Ca; dachiardite-Ca, -Na; erionite-Na, -K, -Ca; faujasite-Na, -Ca, -Mg; ferrierite-Mg, -K5 -Na; gmelinite-Na, -Ca, -K; heulandite-Ca, -Na, -K5 -Sr; levyne-Ca, -Na; paulingite- K, -Ca; phillipsite-Na, -Ca, -K; stilbite-Ca, -Na. Key references, type locality, origin of name, chemical data, IZA structure-type symbols, space-group symmetry, unit-cell dimensions, and comments on structure are listed for 13 compositional series, 82 accepted zeolite mineral species, and three of doubtful status. Herschelite, leonhardite, svetlozarite, and wellsite are discredited as mineral species names. Obsolete and discredited names are listed.

Clinoptilolite-heulandite: applications and basic research

Structural peculiarities of clinoptilolite and heulandite are reviewed. Special attention is given to partial Si, Al ordering within the tetrahedral framework structure. There is strong evidence that the Si, Al ordering pattern depends on the size, charge, and placing of the original extraframework cations. Even if exchanged to homoionic forms clinoptilolite and heulandite may display different properties depending on the degree and type of Si, Al ordering. In some cation-exchanged heulandites symmetry lowering from the topological symmetry C2/m to Cm or C1 has been observed due to partial Si, Al ordering and low symmetry site preference of extraframework cations. Major applications of clinoptilolite are reviewed. In the field of pollution abatement not only the natural product but also surface modified clinoptilolites gain importance.

Mn 3 Al 2 Si 3 O 12 spessartine and Ca 3 Al 2 Si 3 O 12 grossular garnet; structural dynamic and thermodynamic properties

Abstract The structures of synthetic Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet have been refined using single-crystal X-ray diffraction methods at 100 K, 293 K, and 500-550 K. The divalent X-site cations, located in large dodecahedral sites, show measurable anisotropic dynamic disorder in contrast to the rigid vibrational behavior of the SiO4 tetrahedra and AlO6 octahedra. The amplitudes of vibration of Mn2+ in spessartine are similar to those of Fe2+ of almandine, in the plane of the longer X-O(4) bonds, and both are about twice that of Ca2+ in grossular, despite the lighter mass of the latter. Heat capacities measured between 300 and 1000 K on synthetic poly crystalline spessartine and two natural nearly end-member spessartine crystals are similar to those of almandine. In addition, the IR active modes of spessartine at low frequencies are very similar to those of almandine suggesting that their heat capacities are also similar at lower temperatures. The low-energy phonon spectra of pyrope and grossular are probably considerably distinct from the two transition metal-containing garnets as suggested by their different low frequency IR active modes, reflecting the different bonding properties for Mg and Ca in garnet. The large pressure-temperature stability field of spessartine, relative to the other aluminosilicate garnets, does not appear to be due to any sort of intrinsic entropy stabilization.

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.

Struvite-(K), KMgPO4·6H2O, the potassium equivalent of struvite – a new mineral

The new mineral struvite-(K) is the natural potassium equivalent to struvite NH4MgPO4·6H2O. It was discovered independently at two different localities: (1) at the famous sulphosalt locality Lengenbach in Binntal, Switzerland, where it occurs in a dolomitic rock of Triassic age, in close association with various Pb-As-sulphosalts (mainly rathite). It forms extremely fine needles reaching up to a maximum length of about 0.3 mm. The needles are elongated along the crystallographic a -axis, they are completely colourless and transparent. The acicular crystals are generally well developed, and several forms such as {0 0 1}, {0 1 0}, {1 0 1}, {0 1 2}, {1 1 0}, {1 1 1}, {0 1 2} could be identified by optical goniometry. Single-crystal study showed the mineral to be orthorhombic, with space group Pmn 21 (from analogy to struvite), a = 6.903(3), b = 6.174(2), c = 11.146(3) A, V = 475.0(2) A3 (refined from powder data). Qualitative chemical data derived from EDS analyses on an SEM gave major K, Mg, P, and traces of Sb, Fe, and Cu. Attempts to prepare samples for quantitative EMP analyses failed, therefore a structure determination (using a Bruker AXS three-circle diffractometer) and a refinement of K versus NH4 were carried out which prove that the mineral represents the pure K end-member. Optically, the mineral is biaxial positive, with 2 V Z = large, α = 1.490(2), β = 1.493(2) (for 589 nm), γ could not be measured, the optic axes plane (OAP) is perpendicular to the needle axis, therefore β parallels the crystallographic a -axis. At the Lengenbach occurrence, struvite-(K) is a product of the latest stage of hydrothermal activity to supergene alteration. A second occurrence (2) is Rossblei, Schladminger Tauern, Styria, Austria, an abandoned galena mine. The host rock of the Pb-mineralisation is a sericite-schist belonging to the polymetamorphic basis of the Schladminger Tauern. The mineral occurs as pseudomorphosed aggregates of dirty white colour reaching up to several millimetres . The aggregates represent close intergrowths of fine-grained struvite-(K) and newberyite Mg(PO3OH)·7H2O. Cell parameters refined from the powder data, after deduction of newberyite lines, are a =6.878(1), b =6.161(1), c =11.100(1) A, V =470.41(9) A3. No additional physical, optical, morphological data could be derived due to the close intergrowth of the two minerals. Struvite-(K) from Schladming obviously represents a recent alteration product.

Peculiarity and defect structure of the natural and synthetic zeolite mordenite: A single-crystal X-ray study

Abstract Single-crystal X-ray data were collected from a natural fibrous mordenite crystal of composition K2.99Ca1.85Na1.06Al7.89Si40.15O96⋅28H2O and from a platy synthetic mordenite crystal of composition Na6Al6.02Si42.02O96⋅19H2O. Diffraction data were measured with a point detector using a sealed X-ray tube and an image plate using synchrotron radiation, respectively. Both structures exhibit the same defect features visible in difference-Fourier maps. Domains of the entire Cmcm framework structure are reproduced by a non-crystallographic (001) mirror plane at z = 0 and z = 1/2. An identical description is a shift of framework domains 1/2 along the c axis. The concentration of this defect domain is 2.7(2) and 3.1(1)% for the natural and synthetic mordenite crystals, respectively. Reproductions of reciprocal layers from synchrotron image-plate data reveal diffuse scattering for hkl layers with l = 2n + 1. The diffuseness of these layers is not homogeneous but concentrates in the form of halos around selected reflections allowed for C-centering. Diffuse features in electron diffraction patterns of natural and synthetic mordenite have been described before and were interpreted either as evidence of c/2 faults or intergrowth with different mordenite-related structure-types. We have modeled a (100) defect layer that is modified from the mordenite characteristic puckered sheet of six-membered rings and allows coherent intergrowth of identical structural subunits shifted by c/2. These defect domains do not influence or obstruct the 12-membered ring-channels characteristic of this zeolite. The major difference in Si,Al distribution between the two samples is that the natural crystal has Al strongly enriched at T3, which is part of the four-membered rings. We suggest that a synergetic effect between extraframework cations and Si, Al ordering during crystal growth is responsible for Al enrichment in natural mordenite with ca. 2 Ca p.f.u. close to T3.

P-T-X data on P21/c-clinopyroxenes and their displacive phase transitions

Abstract The P21/c clinopyroxene kanoite (ideally MnMgSi2O6) was studied as a function of pressure and temperature using powder X-ray diffraction, differential scanning calorimetry (DSC) and optical methods. The temperature of the P21/c to high-temperature (HT) C2/c transition ranges from 425 °C in endmember MnMgSi2O6 to 125 °C in natural samples with an aegirine component. Compiling pigeonite and clinoenstatite–clinoferrosilite literature data, the temperature of the transformation was found to decrease linearly with M2 cation size. A synchrotron powder diffraction study in a heated diamond-anvil cell (DAC) yielded compression and thermal expansion data for low kanoite of composition Mn1.2Mg0.4Fe0.4Si2O6. The high-pressure (HP) phase transition from P21/c to HPC2/c was reversed at 5.8 GPa at 417 °C. The high-temperature phase transition from P21/c to HTC2/c was rather indistinct and occurred at approximately 530 °C and 0.76 GPa. In a separate experiment, the HT transition was observed optically in a hydrothermal DAC between 0.0 and 0.4 GPa. The in-situP-T data of both experiments yielded an increase in transition temperature with increasing pressure (approx. 149 °C/GPa) and suggest a change in character of the transition from first order to continuous with increasing pressure. The data indicate that the HTC2/c and HPC2/c polymorphs are distinct phases with different stability fields. Since the high-temperature and the high-pressure polymorphs of kanoite were shown to be isotypic with other low-Ca clinopyroxenes such as the (Mg,Fe)SiO3 series, the conclusions we draw from this study are valid for all clinopyroxenes with small (<0.88 Å) M1 and M2 cation sizes. The petrologic implications of these conclusions for the occurrence of “clinoenstatite” in the Alpe Arami peridotite are discussed.

Lakargiite CaZrO3: A new mineral of the perovskite group from the North Caucasus, Kabardino-Balkaria, Russia

Abstract Lakargiite CaZrO3-the zirconium analog of perovskite [Pbnm, a = 5.556(1), b = 5.715(1), c = 7.960(1) Å, V 252.7(1) Å3, Z = 4]-was discovered as an accessory mineral in high-temperature skarns in carbonate-silicate rocks occurring as xenoliths in ignimbrites of the Upper-Chegem (Verkhniy Chegem) volcanic structure, the North Caucasus, Kabardino-Balkaria, Russia. Lakargiite forms pseudo-cubic crystals up to 30-35 μm in size and aggregates up to 200 μm. Lakargiite is associated with spurrite, larnite, calcio-olivine, calcite, cuspidine, rondorfite, reinhardbraunsite, wadalite, perovskite, and minerals of the ellestadite group. The new perovskite mineral belongs to the ternary solid solution CaZrO3-CaTiO3-CaSnO3 with a maximum CaZrO3 content of ca. 93%, maximum CaTiO3 content of 22%, and maximum CaSnO3 content of 20%. Significant impurities are Sc, Cr, Fe, Ce, La, Hf, Nb, U, and Th. Raman spectra of lakargiite are similar to those of the synthetic phase Ca(Zr,Ti)O3 with strong bands at 352, 437, 446, 554, and 748 cm-1. Lakargiite crystallized under sanidinite-facies conditions of contact metamorphism characterized by very high temperatures and low pressures.

Cordierite I: The coordination of Fe 2+

Abstract The incorporation of Fe2+ was investigated in four natural cordierite samples. 57Fe Mössbauer, single-crystal UV-VIS optical absorption, and X-ray absorption spectroscopies, as well as X-ray single-crystal diffraction were used. Mössbauer, optical, and XAS spectroscopy show that Fe2+ is incorporated on two different structural sites in two Mg-rich samples. Mössbauer measurements give the best quantitative measure of the amounts of Fe2+, but the optical spectra are the most sensitive for determinations at low concentrations and at high-bulk Fe2+ concentrations in cordierite. The spectroscopic data are most consistent with small amounts of Fe2+ (i.e., 0.02 of Fe2+ per formula unit) being located on a tetrahedral site rather than in the center (or off center) of the six-membered tetrahedral rings or in channel cavities. X-ray single-crystal refinements on two Mg-rich cordierites show a very small excess electron density on T11 and not in the channels. A third refinement on a slightly more iron-rich sample shows, in contrast, no excess electron density on T11. We interpret these data as indicating that small amounts of Fe2+ (0.01 to 0.02 atoms per formula unit) replace tetrahedral Al11 in cordierite, where charge balance is achieved by placing Na in the center of the six-membered rings. This substitution is consistent with the known chemistry of natural cordierites and with simple structural energetics. The identification and assignment of small amounts of Fe2+ on T11 requires spectroscopic determination or careful X-ray single-crystal refinements and cannot be achieved from composition data and structural formula calculations.

Ar, N2, and CO2 in the structural cavities of cordierite, an optical and X-ray single-crystal study

Ar, N2 and CO2 were introduced into the structural cavities of channel-evacuated single-crystals of White Well cordierite with the composition:K0.01Na0.03(Mg1.91Fe0.09Mn0.01)Al3.98Si5.01O18. The gas refilling experiments were carried out in conventional hydrothermal bombs at 6–7 kbar and 600–700°C. The increase in the mean refractive indices for gas-treated crystals, as determined with a spindle-stage equipped microscope, was used along with point-dipole calculations to estimate the percentage of occupied structural cavities. The steep increase of the electronic polarizability parallel to the a-axis, which can be derived from the increase of the refractive index nγ (Z∥a) upon introduction of volatiles, indicates that N2 and CO2 are preferentially aligned parallel to the a-axis of cordierite. Single-crystal structure refinements at room temperature confirm these predictions. Additionally, decreased C–O and N–N bond lengths suggest a librational motion with a mean rotary oscillation angle of 35° (N2) and 25° (CO2) about a, where c is the rotation axis. Mean libration angles of 40° (N2) and 28° (CO2) were estimated from the electronic polarizability tensors of CO2 and N2. Site occupancy refinements of the channel position are in good agreement with the optically derived values for the volatile concentrations, both indicating about 70% and 60% filled cavities for Ar- and N2-cordierite, respectively. Chemical analyses and point-dipole calculations confirm that about 45% of the cavities are occupied in the CO2-treated crystal. The structural framework of cordierite is slightly but specifically altered by the various channel occupants.