Crystal structure of Ca5(HAsO4)2(AsO4)2.9H2O (guérinite)

American Mineralogist, 1981

X-ray diffraction intensities were measured by single-crystal diffractometry (MoKa radiation) on picropharmacolite from Sainte-Marie-aux-Mines (Alsace), and the following structuro parameters were determined: a: 13.547(3), D : 13.500(3), c : 6.710(l)A, a : 99.85(l), f :96.41(2), y : 91.60(l)"; Z :2, space group PT. The structure was solved by direct methods; the mixed isotropic (oxygen atoms) and anisotropic (heavier atoms) refinement converged to R : 0.087 (16l I reflections). As, Ca, and Mg coordination polyhedra sharing edges and vertices form corrugated (100) layers, which are linked by hydrogen bonding only. Four independent water molecules are sandwiched between adjacent layers, and build up [001] hydrogen-bonded chains. The Mg coordination octahedron and the Ca polyhedra show typical bond distances, so that no signifcant Ca/Mg substitution should occur in any cation site. The formula of picropharmacolite can then be written as CaoMg(HrO).,(AsO3OH)2(AsOa)2' 4HrO. A close relationship is observed between this structure and those of the two dimorphs guerinite and ferrarisite, Car(HAsOo)r(AsOo)2.9HrO; in these minerals the layers of polyhedra are also present, but are linked by Ca-O bonds in addition to hydrogen bonds. Cleavage and possible [winning are discussed on structural grounds.

Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, 1969

Pharmacolite, CaH(AsO4). 2HzO, crystallizes in the monoclinic system, space group Ia (C~). Unit-cell parameters are: a0 = 5.9745, b0 = 15"4340, c0 = 6.2797/~, fl= 114 ° 50'. Z= 4, Om= 2"725, Dc = 2-731 g.cm-3.

Acta crystallographica, 1973

Calcium hydrogen arsenate trihydrate (CaHAsO4.3H20) crystallizes in the orthorhombic system, space group Pbca. Unit-cell parameters are: a0 = 11.195 (1), b0 = 10.713 (2), Co = 11" 179 (2) ~; Z = 8. The crystal structure was refined anisotropically to R=0.034, using 1321 non-zero reflexions measured on an automatic three-circle diffractometer (Cu Ke radiation). Corrections for absorption, secondary extinction and anomalous scattering were applied. The hydrogen atoms were located from a difference map, and one of the three water molecules appears very weakly hydrogen bonded. CaHAsO4.3H20 is isostructural with MgHPO4.3H20 (newberyite) and with MnHPO4.3H20; its crystal structure is built up from isolated Ca octahedra sandwiched between As tetrahedra. The resulting layers, parallel to (100), are connected by infinite zigzag chains of hydrogen bonds along [100]. Some common features of the related compounds CaHAsO4 (weilite), CaHAsO4. H20 (haidingerite), and CaHAsO4.2H20 (pharmacolite) are discussed. © Experimental Crystals of TPV suitable for X-ray diffraction measurements were kindly supplied by Dr F. A. Neuge

Acta crystallographica, 1973

Calcium hydrogen arsenate trihydrate (CaHAsO4.3H20) crystallizes in the orthorhombic system, space group Pbca. Unit-cell parameters are: a0 = 11.195 (1), b0 = 10.713 (2), Co = 11" 179 (2) ~; Z = 8. The crystal structure was refined anisotropically to R=0.034, using 1321 non-zero reflexions measured on an automatic three-circle diffractometer (Cu Ke radiation). Corrections for absorption, secondary extinction and anomalous scattering were applied. The hydrogen atoms were located from a difference map, and one of the three water molecules appears very weakly hydrogen bonded. CaHAsO4.3H20 is isostructural with MgHPO4.3H20 (newberyite) and with MnHPO4.3H20; its crystal structure is built up from isolated Ca octahedra sandwiched between As tetrahedra. The resulting layers, parallel to (100), are connected by infinite zigzag chains of hydrogen bonds along [100]. Some common features of the related compounds CaHAsO4 (weilite), CaHAsO4. H20 (haidingerite), and CaHAsO4.2H20 (pharmacolite) are discussed. © Experimental Crystals of TPV suitable for X-ray diffraction measurements were kindly supplied by Dr F. A. Neuge

The Canadian Mineralogist, 2004

Calcium catapleiite, CaZrSi 3 O 9 •2H 2 O, the calcium-dominant analogue of catapleiite, Na 2 ZrSi 3 O 9 •2H 2 O, firstly described as hexagonal, space group P6 3 /mmc, with a 7.32, c 10.15 Å, is actually orthorhombic, space group Pbnn, a 7.378(1), b 12.779(1), c 10.096(1) Å, V 951.89 Å 3 and Z = 4. The crystals display three-fold twinning ("Drilling") about the c axis, with the three individuals rotated 120° one from the other. The crystal structure of calcium catapleiite from the type locality, the Burpala massif, northern Baikal Region, Siberia, Russia, has been determined by direct methods from single-crystal X-ray-diffraction data and refined to a final R = 0.0528 (wR 2 = 0.1604) for 2167 reflections with F o > 6(F o ). It is characterized by a mixed framework of regular [ZrO 6 ] octahedra and three-membered silicate rings [Si 3 O 9 ], very similar to the framework of catapleiite. The H 2 O molecules also are similarly placed in the two minerals. The Ca 2+ cations in calcium catapleiite are distributed over two distinct sites, with 0.8 and 0.2 occupancies, which contrasts with the fully disordered distribution of the Na + cations in catapleiite.

Canadian Mineralogist, 2010

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Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry

CaHAsO4 (weilite) crystallizes in the triclinic system, space group PT. Unit-cell parameters are: a0 = 7.0591 (8), b0=6.8906 (9), c0=7"2006 (16)/~, ~=97°26 ' (1'), ,8=103033 , (1'), ~,=87°45 ' (1'); Z=4. Weilite is isostructural with monetite (CaHPO4); the present X-ray study was based on the structure of the latter as a starting model. 96% (I 475) of the reflexions accessible to Cu K0c radiation were measured on a single-crystal diffractometer; corrections for absorption and anomalous scattering (As and Ca atoms) were applied. The structure presents layers, parallel to the (001) plane, in which the two Ca atoms are crystallographically independent and coordinate respectively seven and eight oxygen atoms; the A.s-tetrahedra are on the layer surface. A plausible system of hydrogen bonds is proposed on the basis of the oxygen atom positions and contacts.

Zeitschrift für anorganische und allgemeine Chemie, 2008

The existence of three different modifications of anhydrous calcium oxalate is reported since decades. Their crystal structures, however, remained unclear, yet. In the present work the crystal structure of the so‐called β‐modification was revealed by a combination of atomistic computer simulations and Rietveld refinements of the X‐ray powder pattern. No indication for polymorphism was obtained.

Inorganic Chemistry, 2011

Fluor-chlorellestadite solid solutions Ca 10 (SiO 4) 3 (SO 4) 3 Cl 2−x F x , serving as prototype crystalline matrices for the fixation of hazardous fly ash, were synthesized and characterized by powder X-ray and neutron diffraction (PXRD and PND), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The lattice parameters of the ellestadites vary linearly with composition and show the expected shrinkage of unit cell volume as fluorine (IR = 1.33 Å) displaces chlorine (IR = 1.81 Å). FTIR spectra indicate little or no OH − in the solid solutions. All compositions conform to P6 3 / m symmetry where F − is located at the 2a (0, 0, 1 / 4) position, while Cl − is displaced out of the 6h Ca(2) triangle plane and occupies 4e (0, 0, z) split positions with z ranging from 0.336(3) to 0.4315(3). Si/S randomly occupy the 6h tetrahedral site. Ellestadites rich in Cl (x ≤ 1.2) show an overall deficiency in halogens (<2 atom per formula unit), particularly Cl as a result of CaCl 2 volatilization, with charge balance achieved by the creation of Ca vacancies (Ca 2+ + 2Cl − →□ Ca + 2□ Cl) leading to the formula Ca 10−y (SiO 4) 3 (SO 4) 3 Cl 2−x−2y F x. For F-rich compositions the vacancies are found at Ca(2), while for Cl-rich ellestadites, vacancies are at Ca(1). It is likely the loss of CaCl 2 which leads tunnel anion vacancies promotes intertunnel positional disorder, preventing the formation of a P2 1 /b monoclinic dimorph, analogous to that reported for Ca 10 (PO 4) 6 Cl 2. Trends in structure with composition were analyzed using crystal-chemical parameters, whose systematic variations served to validate the quality of the Rietveld refinements. 65 rellestadites are regarded as isostructural, although the halides 66 reside at different locations along the c-axis anion tunnels. In 67 the synthetic fluorellestadite (P6 3 /m) endmember, F − at (0, 0, 68 1/4 and 0, 0, 3 / 4) lies in the center of a Ca(2) triangle on the 69 mirror planes at z = 1/4 and z = 3 / 4. 12 In the corresponding 70 chlorellestadite endmember, Cl − cannot fit into the Ca(2) 71 triangle (ionic radii for VI F − = 1.33 Å and VI Cl − = 1.81 Å 13), 72 and statistically occupies (0, 0, z) sites, ∼1.1 Å above or below 73 the mirror plane. 9 In the case of natural hydroxyellestadite, 74 Sudarsanan 7 refined separate sites for OH − (0, 0, 0.2033), F − 75 (0, 0, 1 / 4), and Cl − (0, 0, 0.3644) in P2 1 /m with site 76 occupancies of 0.806(9), 0.15(6), and 0.092(5) respectively.

Acta crystallographica, 1972

The crystal structure of Ca(H2AsO4)2 (triclinic, PT, with two formula units per cell) has been refined by single-crystal neutron-diffraction analysis to give an R value of 0.025 over 930 independent neutron reflexions. For the non-hydrogen atoms, bond lengths, angles and thermal parameters agree very closely with the results of a recent X-ray analysis. Difference Fourier syntheses, before and after anisotropic leastsquares refinement of all atomic parameters, enabled the four hydrogen atoms to be located unequivocally. In one anion, two half-hydrogens, located at centres of symme:ry, are believed to be engaged in symmetrical hydrogen bonds, 0(5)...0(5')= 2.436 A, 0(8)...0(8')= 2.444 /~. By contrast, another hydrogen bond is relatively long (2.875 + 0.002 ,~) and non-linear [angle 0(7)... H(3)... 0(3')= 150-2 + 0-3°]. The As-O bonds carrying the half hydrogen atoms are 1"677 + 0-002 .~ long, intermediate in length between As-O bonds with full hydrogens (1-727 + 0.002/~) and thase with no hydrogen atom attached (1.651 + 0.002 A) (averaged values).