Uwe Kolitsch

PHASE EQUILIBRIA AND CRYSTAL CHEMISTRY IN THE Y2O3-AL2O3-SIO2 SYSTEM

In order to clarify inconsistencies in the literature and to verify assumed ternary solubilities, the phase equilibria in the Y 2 O 3 –Al 2 O 3 –SiO 2 system at 1600, 1400, and 1300 °C were experimentally determined using x-ray diffraction (XRD), scanning electron microscope with attached energy-dispersive detector system (SEM-EDX), and electron probe microanalyzer (EPMA). Six quasibinary phases were observed: Y 4 Al 2 O 9 (YAM), YAlO 3 (YAP), Y 3 Al 5 O 12 (YAG), Y 2 SiO 5 , Y 2 Si 2 O 7 (C and D modifications), and ˜ 3 Al 2 O 3 · 2SiO 2 (mullite). Y 4 Al 2 O 9 forms an extended ternary solid solution with the formula Y 4 Al 2(1- x ) Si 2 x O 9+ x ( x = 0 2 ˜0.31). The lowest ternary eutectic temperature was determined at 1371 ± 5 °C by high-temperature differential scanning calorimetry (DSC). The results were compared with previous data available for the Y 2 O 3 –Al 2 O 3 –SiO 2 system and with data for other RE 2 O 3 –Al 2 O 3 –SiO 2 (RE = rare earth element) systems.

Crystal chemistry of REEXO4 compounds (X = P, As, V). II. Review of REEXO4 compounds and their stability fields

A comprehensive critical review of the phase fields, metastable modifications, solid solution ranges and phase transitions of monazite- and zircon-type REE X O 4 ( X = P, As, V) compounds is given. Monazite-type REEPO 4 compounds are stable for REE = La to Gd and metastable for Tb to Ho; zircon-type members exist for REE = Gd to Lu, and Y, Sc. REEAsO 4 compounds with monazite-type structure exist for REE = La to Nd, while zircon-type compounds are known for REE = Pm to Lu, and Y, Sc; no metastable arsenate members are known. The only stable monazite-type REEVO 4 is LaVO 4 , but metastable members are known for REE = Ce to Nd. Zircon-type REEVO 4 compounds are stable for REE = Ce to Lu, and Y, Sc, and metastable for REE = La. Solid solution series are complete only if minor size differences exist between REE 3+ or X 5+ cations in respective end-members. Phase transitions occur under pressure (zircon → (monazite →) scheelite) and at very low temperatures. The evaluation of the metastable phase fields and of naturally occurring members suggests that metastable modifications of REE X O 4 compounds can occur in nature under certain conditions (formation at temperatures

Alunite supergroup: recommended nomenclature

Abstract Minamiite has been discredited and renamed natroalunite-2c to show a double unit-cell structure and natroalunite can be designated as natroalunite-1c to show a single unit-cell structure. Kintoreite can be designated as kintoreite-1c to show the same single unit-cell structure, and IMA 1993-039 is a new superstructure of kintoreite and can be designated as kintoreite-2c to show a double unit-cell structure. Beaverite has been renamed beaverite-(Cu). The Zn-bearing beaverite of Sato et al. (2008) has been named “beaverite-(Zn)″, but data for the mineral have not been approved by the CNMNC. Orpheite has been discredited as P-rich hinsdalite. Proposal 07-D was approved by the CNMNC.

Natural and synthetic compounds with kröhnkite-type chains: review and classification

Abstract The crystal structure of kröhnkite [Na2Cu(SO4)2·2H2O], which contains infinite chains composed of CuO6 octahedra corner-linked with SO4 tetrahedra, was originally determined 1952 by Dahlman. Since then, a large number of both minerals (e.g., collinsite, fairfieldite, etc.) and synthetic compounds with closely related struc-tures have been investigated, but the structural relation was not always recognised. In the present review we compare the structures of AnM(XO4)2·2H2O (A = mono- or divalent cation, n = 1, 2; M = di- or trivalent cation; X = penta- or hexavalent cation) compounds with kröhnkite-type infinite chains, and propose a structural classification into six types. The rod group symmetries of the chains are compared. Furthermore, the linkage of these chains to double chains (in krausite-type compounds) and sheets (in yavapaiite-, merwinite-, bafertisite-type and similar structures) is discussed, and the occurrence of octahedral-tetrahedral chains topologically identical to those in kröhnkite (referred to as kröhnkite-type) and similar chains (referred to as kröhnkite-like) in several related compounds is pointed out.

Phase relationships in the system Gd2O3-Al2O3-SiO2

Abstract Previously unknown phase equilibria in the system Gd2O3-Al2O3-SiO2 at 1400 and 1300 °C were determined using X-ray diffraction, SEM-EDX and EPMA. The following six quasibinary phases are observed: Gd4Al2O9, GdAlO3, Gd2SiO5, Gd9.33(SiO4)6O2, Gd2Si2O7 and 3Al2O3·2SiO2 (mullite). Additionally, there are two extended ternary solid solutions, Gd9.33+2x(Si1−xAlxO4)6O2 (with x=0−∼0.33) and Gd4Al2(1−x)Si2xO9+x (with x=0−∼0.3), that are based on the quasibinary phases Gd9.33(SiO4)6O2 and Gd4Al2O9, respectively. No quasiternary compound was detected. The ternary eutectic temperature was measured at 1306±5 °C by high-temperature STA and DSC. The results are compared with the scarce data available for other RE2O3-Al2O3-SiO2 (RE=rare earth element) systems.

The crystal structures of phenacite-type Li2(MoO4), and scheelite-type LiY(MoO4)2 and LiNd(MoO4)2

Abstract The crystal structures of flux-grown Li2(MoO4), LiY(MoO4)2 and LiNd(MoO4)2 were solved and refined from single-crystal intensity data collected with a four-circle diffractometer (MoKαX-radiation, CCD area detector, room temperature). Li2(MoO4) is isostructural with phenacite, Be2(SiO4) and has space group R3̅, a = 14.330(2), c = 9.584(2) Å, Z = 18 (R1 = 1.6%). The structure consists of a three-dimensional network of corner-linked, slightly distorted LiO4 and fairly regular MoO4 tetrahedra. Average Li–O and Mo–O distances are 1.965 Å (Li1) and 1.967 Å (Li2), and 1.764 Å (Mo). The atomic arrangement is characterised by a narrow open channel along the three-fold axis. Comparisons are drawn with other phenacite-type compounds. Low-temperature single-crystal X-ray studies of Li2(MoO4) in the range from 103 to 293 K gave no evidence of a phase transition, thus contradicting recent literature reports. Both LiY(MoO4)2 and LiNd(MoO4)2 crystallise in the scheelite-type structure, with space group I41/a and Z = 2 (R1 = 1.7 and 2.7 %, re-spectively). The first has a = 5.148(1), c = 11.173(2) Å, V = 296.11(10) Å3, the latter a = 5.243(1), c = 11.440(2) Å, V = 314.47(10) Å3. Both Li and Y/Nd are completely disordered on a jointly occupied site. Mean (Y,Li)– and (Nd,Li)–O distances are 2.40 and 2.47 Å, respectively. In both compounds the unique MoO4 tetrahedron has four identical Mo–O bonds with lengths of 1.779 Å. The stoichiometry of LiRE(MoO4)2 (RE = rare-earth element) compounds is discussed and the relation to structure types of other MRE(XO4)2 (M = alkali metal, X = Mo, W) compounds is briefly addressed.

Synthesis and structural study of five new trisilicates, BaREE2Si3O10 (REE = Gd, Er, Yb, Sc) and SrY2Si3O10, including a review on the geometry of the Si3O10 unit

Five new mixed-framework trisilicates were synthesised using a high-temperature flux growth technique. Colourless, glassy plates of SrY 2 Si 3 O 10 crystallise in space group P 1, with a = 6.757(1), b = 6.885(1), c = 9.273(2) A, α = 72.42(3), β = 86.37(3), γ = 88.37(3)°, V = 410.38(12) A 3 , Z = 2. The main building units of the new structure type represented by SrY 2 Si 3 O 10 are slightly curved Si 3 O 10 trimers and Y 2 O 11 dimers (composed of YO 6 octahedra sharing an edge with YO 7 polyhedra), which are further edge-connected to adjacent dimers to form twisted zigzag chains parallel to [010]. BaREE 2 Si 3 O 10 (REE = Gd, Er, Yb, Sc) form colourless small prisms, pseudohexagonal plates or isometric crystals, and crystallise in space group P 2 1 / m , with respectively a = 5.435(1) / 5.389(1) / 5.377(1) / 5.273(1), b = 12.241(2) / 12.163(2) / 12.117(2) / 11.918(2), c = 6.932(1) / 6.840(1) / 6.790(1) / 6.591(1) A, β = 106.26(3) / 106.47(3) / 106.50(3) / 107.06(3)°, V = 442.74(13) / 429.94(12) / 424.17(12) / 395.98(12) A 3 , Z = 2. BaREE 2 Si 3 O 10 (REE = Gd, Er, Yb, Sc) are isotypic with BaY 2 Si 3 O 10 . Their topology is characterised by horseshoe-shaped trisilicate (Si 3 O 10 ) groups and zigzag chains of edge-sharing distorted M O 6 octahedra ( M = Gd, Er, Yb, Sc). Correlations between β –, Si–Si–Si angle and unit-cell volume and REE 3+ ionic radii are discussed. The geometries of the Si 3 O 10 and T 3 O 10 groups ( T = Ge, P, As, Al, Ga, V) in non-silicates are briefly reviewed, with special focus on narrow Si–Si–Si angles.

Phase relationships in the systems RE2O3-Al2O3-SiO2 (RE = rare earth element, Y, and Sc)

A literature survey and recent results on phase relationships in the quasi-ternary systems RE2O3-Al2O3-SiO2 are given. The investigated systems exhibit extended ternary solid solutions, RE9.33+2x(Si1_xAlxO4)6O2 (withx up to ~0.33) and/or RE4Al2(1_X)Si2xO9+x (withx up to ~0.3), which are based on the quasi-binary phases RE9.33(SiO4)6O2 and RE4A12O9, respectively. The former is encountered only in systems with laige RE3+ ions (e.g., La3+), whereas the latter is found in systems with small RE3+ ions (e.g., Yb3+); in systems with medium-sized KE3+ ions (e.g., Gd3+) both types exist Quasi-ternary compounds are known only in the La, Ce, and Sc systems. Severe discrepancies in reported ternary eutectic temperatures led to a need for their accurate redeteimination.

Natural and synthetic compounds with kröhnkite-type chains. An update

Abstract The crystal structures of five new triclinic double salt dihydrates have been determined from single-crystal X-ray diffraction data. The following four compounds all contain kröhnkite-[Na2Cu(SO4)2·2 H2O]-type tetrahedral-octahedral chains: the three isotypic synthetic compounds K2Mg(CrO4)2·2 H2O (a = 5.674(1), b = 6.462(1), c = 7.517(2) Å, α = 110.38(3), β = 95.24(3), γ = 109.86(3)°, V = 236.0(1) Å3, R(F) = 0.036, space group P1̅, no. 2, Z = 1), K2Mg(MoO4)2·2 H2O (a = 5.884(1), b = 6.491(1), c = 7.700(1) Å, α = 111.67(2), β = 96.59(2), γ = 108.62(2)°, V = 249.8(1) Å3, R(F) = 0.019, space group P1̅, no. 2, Z = 1) and K2Mn(SeO4)2·2 H2O (a = 5.674(1), b = 6.608(1), c = 7.523(2) Å, α = 110.31(3), β = 95.69(3), γ = 108.35(3)°, V = 244.10(9) Å3, R(F) = 0.024, space group P1̅, no. 2, Z = 1), and the mineral messelite Ca2(Fe2+,Mn2+,Mg)(PO4)2·2 H2O (a = 5.480(1), b = 5.759(1), c = 6.569(1) Å, α = 90.18(3), β = 102.62(3), γ = 108.45(3)°, V = 191.3(1) Å3, R(F) = 0.022, space group P1̅, no. 2, Z = 1). The fifth compound, the synthetic dichromate K2Zn(Cr2O7)2·2 H2O (a = 6.794(1), b = 7.735(2), c = 7.834(2) Å, α = 88.97(3), β = 80.90(3), γ = 64.57(3)°, V = 366.5(2) Å3, R(F) = 0.036, space group P1̅, no. 2, Z = 1) is closely related to the kröhnkite-group; it contains Cr2O7 groups assuming the same bridging role that XO4 groups have in kröhnkite-type oxysalts. The atomic arrangements of the title compounds are described and structural relations with other kröhnkite-type and -like compounds are discussed. In addition, related yavapaiite-type sheet structures of AM′(XO4)2 compounds are briefly reviewed, including the rod group symmetry of the underlying kröhnkite-chain building unit. Twelve different space groups have been found for these sheet structures. The previously reported space group symmetries for α-NH4Fe(CrO4)2 (P21) and CsTa(PO4)2 (P1̅) are obviously incorrect and should be revised to P21/n and C2/m, respectively.

Limitations of Fe2+ and Mn2+ site occupancy in tourmaline: Evidence from Fe2+- and Mn2+-rich tourmaline

Abstract Fe2+- and Mn2+-rich tourmalines were used to test whether Fe2+ and Mn2+ substitute on the Z site of tourmaline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mössbauer and optical spectroscopy. The sample has large amounts of Fe2+ (~2.3 apfu), and substantial amounts of Fe3+ (~1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe3+ (no delocalized electrons) and Ti4+ to the Z site and the amount of Fe2+ and Fe3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): X (Na0.9Ca0.1) Y(Fe2+2.0Al0.4Mn2+0.3Fe3+0.2) Z(Al4.8Fe3+0.8Fe2+0.2Ti4+0.1) T(Si5.9Al0.1)O18 (BO3)3V(OH)3W[O0.5F0.3(OH)0.2] with a = 16.039(1) and c = 7.254(1) Å. This formula is consistent with lack of Fe2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. The formula was further modified by considering two ED doublets to yield: X(Na0.9Ca0.1) Y(Fe2+1.8Al0.5Mn2+0.3Fe3+0.3) Z(Al4.8Fe3+0.7Fe2+0.4Ti4+0.1) T(Si5.9Al0.1)O18 (BO3)3V(OH)3W[O0.5F0.3(OH)0.2]. This formula requires some Fe2+ (~0.3 apfu) at the Z site, apart from that connected with delocalization of a hopping electron. Optical spectra were recorded from this sample as well as from two other Fe2+-rich tourmalines to determine if there is any evidence for Fe2+ at Y and Z sites. If Fe2+ were to occupy two different 6-coordinated sites in significant amounts and if these polyhedra have different geometries or metal-oxygen distances, bands from each site should be observed. However, even in high-quality spectra we see no evidence for such a doubling of the bands. We conclude that there is no ultimate proof for Fe2+ at the Z site, apart from that occupancy connected with delocalization of hopping electrons involving Fe cations at the Y and Z sites. A very Mn-rich tourmaline from a pegmatite on Elba Island, Italy, was characterized by crystal-structure determination, chemical analyses, and optical spectroscopy. The optimized structural formula is X(Na0.6□0.4) Y(Mn2+1.3Al1.2Li0.5) ZAl6TSi6O18 (BO3)3V(OH)3 W[F0.5O0.5], with a = 15.951(2) and c = 7.138(1) Å. Within a 3σ error there is no evidence for Mn occupancy at the Z site by refinement of Al ↔ Mn, and, thus, no final proof for Mn2+ at the Z site, either. Oxidation of these tourmalines at 700-750 °C and 1 bar for 10-72 h converted Fe2+ to Fe3+ and Mn2+ to Mn3+ with concomitant exchange with Al of the Z site. The refined ZFe content in the Fe-rich tourmaline increased by ~40% relative to its initial occupancy. The refined YFe content was smaller and the distance was significantly reduced relative to the unoxidized sample. A similar effect was observed for the oxidized Mn2+-rich tourmaline. Simultaneously, H and F were expelled from both samples as indicated by structural refinements, and H expulsion was indicated by infrared spectroscopy. The final species after oxidizing the Fe2+-rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn2+-rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn2+ to Mn3+. The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase.