Cavansite, Ca(VO)(Si4O10)•4H2O, is a novel layer silicate, dimorphous with pentagonite. The earlier determination of its structure did not yield the location of the hydrogen atoms or clear resolution of the oxygen atoms of the H2O molecules. In this study, structures are reported from data collected at 100(2) and 296(2) K, and they clearly resolve and allow refinement of the position of the H atoms. The 100 K and 296 K structures were refined to R 1 values of 0.0141 and 0.0389, respectively. Cavansite is orthorhombic with space-group symmetry Pcmn , with a 9.7518(4), b 13.6854(6), and c 9.5965(4) A at 100 K, and a 9.7614(7), b 13.6666(10), and c 9.6049(7) A at 296 K. Cavansite consists of two layers, a layer of silicate tetrahedra and a layer formed of irregular CaO4(OH2)4 polyhedra and VO5 square-based pyramids that alternate in ribbons parallel to [100] in (010); the layers join laterally at the four corners of the base of the V pyramid. The silicate layer consists of four- and eight-membered rings of silicate tetrahedra with apices pointing along [010] or [010]. The four oxygen atoms in the Ca polyhedron that are not linked to the V square-based pyramids are part of H2O molecules. Adjacent ribbons in (010) are linked by hydrogen bonding.

The arrangement of atoms in hummerite, KMg(V5O14).8H(2)O, has been redetermined to locate the H-atom positions, essential for analysis of the bonding between the structural unit and the interstitial complex. The mineral crystallizes in space group P (1) over bar, with a 8.8178(4), b 10.7236(5), c 11.0707(5) Angstrom, alpha 65.798(1), beta 74.057(1), and gamma 71.853(1)degrees. The structure was solved and the hydrogen atoms located using 20,335 reflections measured on a CCD-detector diffractometer with MoKalpha X-radiation; the final refinement converged to an R of 0.021. The structural unit in hummerite is the [V10O28](6-) decavanadate group, similar to that in synthetic vanadium compounds and in pascoite. The symmetry-equivalent structural units are linked by bonding to potassium and hydrogen atoms in the interstitial complex, {(K2Mg2)-K-[10](H2O)(16)}(6+). The Mg atom occurs in octahedral coordination with six (H2O) groups in the interstitial complex. The K atom occurs in an irregular [10]-coordination, with five bonds to (H2O) groups of the interstitial complex and five bonds to oxygen atoms of the decavanadate structural unit. An extensive network of hydrogen bonds further links the structural unit and interstitial unit. Evaluation of the hydrogen bonding between these two components of the structure shows that the valence-matching principle is satisfied, and thus the atom arrangement in hummerite defines a stable structure.

Nous avons etabli de nouveau l'agencement des atomes dans la hummerite, KMg(V 5 O 1 4 )8H 2 O, afin de determiner la position des atomes d'hydrogene, donnees essentielles dans une analyse des liaisons entre l'unite structurale de base et le complexe interstitiel. Le mineral cristallise dans le groupe spatial P1, avec a 8.8178(4), b 10.7236(5), c 11.0707(5) A, a 65.798(1), β 74.057(1), et γ 71.853(1)°. Nous en avons affine la structure et localise les atomes d'hydrogene au moyen de 20,335 reflexions mesurees avec un diffractometre muni d'un detecteur de type CCD (rayonnement MoKa). L'affinement final a converge sur un residu R de 0.021. L'unite structurale de la hummerite est le groupe decavanadate [V 1 0 O 2 8 ] 6 - , semblable a celui des composes synthetiques de vanadium et de la pascoite. Les unites structurales equivalentes selon la symetrie sont liees au potassium et a l'hydrogene du complexe interstitiel, { 1 0 1 K 2 Mg 2 (H 2 O) 1 6 } 6 + . L'atome de Mg possede une coordinence octaedrique, avec six groupes (H 2 O) dans un complexe interstitiel. L'atome K, a coordinence [10], est situe dans un polyedre irregulier, avec cinq liaisons a des groupes (H 2 O) du complexe interstitiel et cinq liaisons a des atomes d'oxygene du group decavanadate de l'unite structurale. Un reseau important de liaisons hydrogene assure le reste des liaisons entre le complexe structural et le complexe interstitiel. Une evaluation des liaisons hydrogene entre ces deux composantes de la structure montre que le principe de la concordance des valences est satisfait, et que donc l'agencement des atomes dans la hummerite mene a une structure stable.

Dickthomssenite, Mg(V 2 O 6 )·7H 2 O, is a new mineral species from the Firefly–Pigmay uranium–vanadium mine, San Juan County, Utah. The phase crystallizes as platy light golden brown crystals up to 1.5 mm in length, with white streak and vitreous luster. Its Mohs hardness is 2½, its calculated density, 2.037(1), and its observed density, between 1.96 and 2.09 g/cm 3 . Dickthomssenite displays a perfect {100} cleavage. In 589.3 nm light, the mineral is translucent, α 1.6124(3), β 1.6740, and γ 1.7104(4), 2 V meas 74(1)°, 2 V calc 73°, b = Z, c ∧ Y = 17°. Electron-microprobe analyses yielded (in wt.%) MgO 15.38%, FeO 0.46%, V 2 O 5 73.92%, H 2 O (difference) 10.24%, yielding (Mg 0.94 Fe 0.02 )V 2 O 6 ·1.4H 2 O. Dickthomssenite dehydrates rapidly under the electron beam; the crystal-structure determination reveals that there are seven H 2 O groups, giving the ideal formula Mg(V 2 O 6 )·7H 2 O. Dickthomssenite is monoclinic, C 2/ c , a 38.954(2), b 7.2010(4), c 16.3465(9) A, β 97.602(1)°, V 4544.0(4) A 3 , Z = 16. The strongest three measured X-ray lines [ d in A(I)( hkl )] are: 9.704(100)(400), 5.843(100)(402), and 3.139(90)(1202). The phase is formed of (100) planes of chains of V 2 O 6 divanadate edge-sharing polyhedra containing [2 + 3]-coordinated vanadium atoms, linked in (100) by corner-sharing with Mg octahedra. These (100) planes are linked along [100] by layers of Mg(H 2 O) 6 octahedra and non-bonded H 2 O molecules. Dickthomssenite is named after Richard W. Thomssen (b. 1933), consulting geologist from Dayton, Nevada, U.S.A.

Esperanzaite, ideally NaCazA12(As5+0.i)[As5+03 (OH)] (OH)2FJH20), Z =2, is a new mineral from the Mina h Esperarq Durango State, Mexico. The mineral occurs as blue-green botryoidal crystalline masses on rhyolite, with separate spheres up to 1.5 mm Y Deceased in diameter. Mobs hardness is 4.5, specific gravity 3.240h, and 3.36( 3)C.IC. Optical properties were measured in 589 nm light. Esperanzaite is biaxial (-), .Y= Y = Z= colorless, a 1.580(1), ~ 1.588( 1), and y 1.593(1 ); 2V0hs is 74(1 ~ and 2 }'CUIC is 76.3. Dispersion is medium, r 80, 001, 020; 4.796,80,011. Esperanzaite is monoclinic, u 9.687(5), b 10.7379(6), c 5.5523(7)& ~ 105.32( 1 ), space group P21/nz. The atomic arrangement of esperanzaite was solved by Direct Methods and Fourier analysis (R= 0.03 1). The Fundamental Building Block is formed of stacks of heteropolyhedral tetramers; the tetramers are formed of two arsenate tetrahedral and two Al octahedra, comer-linked in 4-member rings. The Fundamental Building Blocks are linked by irregular lda~j and Ca@ polyhedra.