Anna G. Turchkova

Crystal structures of potassium-exchanged forms of catapleiite and hilairite

The crystal structures of potassium-exchanged forms of catapleiite and hilairite of the compositions (K0.49Ca0.42Na0.26)ZrSi3O9 · 2[(H2O)0.8(H3O)0.2] and K0.51ZrSi3O9 · 3[(H2O)0.5K0.27(H3O)0.23], respectively, were studied by X-ray diffraction and IR spectroscopy. Both structures retain the heteropolyhedral frameworks of the parent minerals formed by Zr octahedra and Si tetrahedra. The K cations occupy different positions in these minerals. In K-exchanged catapleiite, K cations are located only in the position occupied by Na in the structure of the parent mineral. In the K-exchanged form of hilairite, K cations are not only involved in the Na position but also partially occupy the H2O position.

Crystal structures of K-and Cs-exchanged forms of zorite

The crystal structures of K-and Cs-exchanged forms of zorite were studied by X-ray diffraction and IR spectroscopy: K4.75Na1.82[Ti(Ti0.79Nb0.20)4Si12O34(O,OH)5.2] × 10.62 H2O (sp. gr. Cmmm, R= 0.0481 for 516 independent reflections) and Cs4.34Na1.90[Ti(Ti0.80Nb0.18)4Si12O34(O,OH)5] × 5.37 H2O (sp. gr. Cmmm, R = 0.0285 for 621 independent reflections). Both structures retain the mixed polyhedral framework of zorite: Na6Ti(Ti,Nb)4(Si6O17)2(O,OH)5 × nH2O, where n ∼ 11. It is shown that the positions of the atoms located in the cavities of the frameworks of these compounds differ from those in the structures of zorite and its synthetic analogs.

Crystal Structure of Pb-Exchanged Form of Zorite

The crystal structure of a Pb-exchanged form of zorite is studied by X-ray diffraction: Pb3.95(Ca0.1Sr0.05)[Ti(Ti0.80Nb0.20)4Si12O38(OH)] · 9.52H2O (sp. gr. Cmmm, R = 0.0530 for 680 independent reflections). The structure retains the mixed polyhedral framework of zorite, Na6[Ti(Ti,Nb)4(Si6O17)2(O,OH)5] · 11H2O. This framework is composed of xonotlite-like [Si6O17] ribbons linked to each other by columns of vertex-sharing (Ti,Nb)O6 octahedra and isolated TiO5 half-octahedra. Lead atoms in the Pb-exchanged form occupy one site, unlike Cs cations in the Cs-exchanged form of zorite, which are strongly disordered and partially occupy eight positions. The position of Pb2+ cations corresponds to the Na(2) position in the zorite structure, the Sr position in the Sr-exchanged form of ETS-4, and the K position in the K-exchanged form and is similar to the position of the water molecule W(3) in the structure of the Cs-exchanged form of zorite.

Crystal chemistry of Rb-, Sr-, Ba-, Ca- and Pb-exchanged forms of natural hilairite

Cation-exchange properties of natural hilairite, Na 2 {ZrSi 3 O 9 }· 3H 2 O, in aqueous salt solutions at 150 °C and the crystal structures of its Rb-, Ca-, Sr-, Ba- and Pb-exchanged forms have been studied on single crystals. All studied samples retain the mixed framework of “parent” hilairite consisting of helical chains [Si 3 O 9 ] ∞ linked by isolated [ZrO 6 ] octahedra. The main differences between the crystal structures of these cation-exchanged forms of hilairite and other representatives of the hilairite group are connected with the number and positions of extra-framework cations and water molecules, framework distortion, unit-cell dimensions and space-group symmetry. The single-crystal structure refinements (X-ray diffraction data) gave the following structural formulas for the studied samples: Rb 1.80 Na 0.20 Zr[Si 3 O 9 ]·0.4H 2 O for Rb-exchanged hilairite ( R 3, a = 10.477(1), c = 15.377(2) A, R 1( F ) = 3.35% for 2771 reflections with F o > 4σ( F o )), Ba 0.96 H 0.08 Zr[Si 3 O 9 ]·3H 2 O ( R 3, a = 20.976(3), c = 7.857(2) A, R 1( F ) = 4.96% for 4921 reflections with F o > 4σ( F o )) and SrZr[Si 3 O 9 ]·1.5H 2 O ( R 3, a = 20.964(3), c = 7.836(2) A, R 1( F ) = 10.23% for 3598 reflections with F o > 4σ( F o )) for Ba- and Sr-exchanged hilairites, respectively, and Ca 0.83 H 0.34 Zr[Si 3 O 9 ]·3H 2 O ( R 3, a = 10.456(1), c = 7.995(2) A, R 1( F ) = 2.63% for 1477 reflections with F o > 4σ( F o )) and Pb 0.82 Na 0.18 H 0.18 Zr[Si 3 O 9 ]·2.9H 2 O ( R 3, a = 10.477(1), c = 7.994(2) A, R 1( F ) = 2.21% for 1165 reflections with F o > 4σ( F o )) for Ca- and Pb-exchanged hilairites, respectively. All samples were twinned by merohedry and, except the Ca-exchanged sample, also racemically. The results of this study strongly indicate that natural cation-exchange processes can affect hilairite-group minerals under late-hydrothermal conditions.

Natural Ion Exchange in Microporous Minerals: Different Aspects and Implications

The ion exchange phenomenon is well-known for crystalline materials including minerals. It has been confirmed experimentally that many microporous minerals representing different chemical classes are capable of cation exchange with salt solutions, including dilute ones, even under room conditions. No doubt that microporous minerals show ion-exchange properties also in nature. Numerous works are devoted to the experimental study and practical use of ion-exchange properties of minerals, especially microporous aluminosilicates (zeolites, clay minerals, etc.), whereas reactions and products of natural ion exchange were almost non-studied to recent time.

Hilarionite, Fe23+(SO4)(AsO4)(OH) · 6H2O, a new supergene mineral from Lavrion, Greece

A new mineral, hilarionite, ideally Fe23+ (SO4)(AsO4)(OH) · 6H2O, has been found in the Hilarion Mine, Agios Konstantinos, Kamariza, Lavrion district, Attiki Prefecture, Greece. It was formed in the oxidation zone of a sulfide-rich orebody in association with goethite, gypsum, bukovskyite, jarosite, melanterite, chalcanthite, allophane, and azurite. Hilarionite occurs as light green (typically with an olive or grayish tint) to light yellowish green spherulites (up to 1 mm in size) and bunches of prismatic to acicular “individuals” up to 0.5 mm long that are in fact near-parallel or divergent aggregates of very thin, curved fibers up to 0.3 mm long and usually lesser than 2 μm thick. The luster is silky to vitreous. The Mohs’ hardness is ca. 2. Hilarionite is ductile, its “individuals” are flexible and inelastic; fracture is uneven or splintery. D(meas) = 2.40(5), D(calc) = 2.486 g/cm3. IR spectrum shows the presence of arsenate and sulfate groups and H2O molecules in significant amounts. The Mössbauer spectrum indicates the presence of Fe3+ at two six-fold coordinated sites and the absence of Fe2+. Hilarionite is optically biaxial (+), α = 1.575(2), γ = 1.64(2), 2V is large. The chemical composition (electron microprobe, average of 7 point analyses; H2O determined by modified Penfield method) is as follows, wt %: 0.03 MnO, 0.18 CuO, 0.17 ZnO, 33.83 Fe2O3, 0.22 P2O5, 18.92 As2O5, 22.19 SO3, 26.3 H2O, total is 101.82%. The empirical formula calculated on the basis of 15 O is: (Fe1.903+Cu0.01Zn0.01)Σ1.92[(SO4)1.24(AsO4)0.74(PO4)0.01]Σ1.99(OH)1.01 · 6.03H2O. The X-ray powder diffraction data show close structural relationship of hilarionite and kaňkite, Fe23+(AsO4)2 · 7H2O. Hilarionite is monoclinic, space group C2/m, Cm or C2, a = 18.53(4), b = 17.43(3), c = 7.56(1) Å, β = 94.06(15)°, V = 2436(3) Å3, Z = 8. The strongest reflections in the X-ray powder diffraction pattern (d, Å-I[hkl]) are: 12.66–100[110],

Delhayelite: Ion Leaching and Ion Exchange

7.60 - 6[00\bar 1]

Crystal chemistry of a Ba-dominant analogue of hydrodelhayelite and natural ion-exchange transformations in double- and triple-layer phyllosilicates in post-volcanic systems of the Eifel region, Germany

, 5.00–10[22l],