The experimental study of zeolitic properties of delhayelite, K₄Na₂Ca₂[AlSi₇O₁₉]F₂Cl

Turchkova A.G.*, Pekov I.V.*^,**, Yapaskurt V.O.*, Lykova I.S.*, Chukanov N.V.***

*Faculty of Geology, Moscow State University, Moscow, Russia; **Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia; ***Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia.

annaturchkova@rambler.ru

 

Delhayelite, K₄Na₂Ca₂[AlSi₇O₁₉]F₂Cl, is a mineral intermediate in structural aspect between phyllosilicates and silicates with heteropolyhedral frameworks. The base of its structure are the two-layer tetrahedral blocks [(Al,Si)₄Si₁₂O₃₈] linked by the columns of edge-shared Ca-centered octahedra (Cannillo e.a., 1969; Pekov e.a., 2009). These two main building units form a heteropolyhedral quasi-framework with zeolitic channels of two types: 1) inside the tetrahedral blocks, hosting K⁺ and Cl^-; 2) in the interlayer space, between Ca columns, hosting Na⁺ and F^-. Such ordering of large cations of three types (Ca, Na, K) and halogen anions of two types (F, Cl) is a feature of delhayelite making it unique among minerals. Products of the step-by-step hydrothermal alteration of delhayelite in peralkaline rocks and pegmatites of the Khibiny alkaline complex (Kola Peninsula, Russia) are fivegite, K₄Ca₂[AlSi₇O₁₇(O_2-xOH_x)][(H₂O)_2-xOH_x]Cl (x = 0-2), and hydrodelhayelite, KCa₂[AlSi₇O₁₇(OH)₂]·(6-х)H₂O. Both these minerals preserve the delhayelite-type CaAlSiO motif of the structure (Pekov e.a., in press). As it was found by us in the result of the experiments in aqueous solutions, delhayelite has strong zeolitic properties.

Grains (0.5–3 mm) of very fresh delhayelite from Kukisvumchorr apatite deposit, Khibiny, were placed in 1N aqueous solutions of (NH₄)₂SO₄, Na₂SO₄, K₂SO₄, RbNO₃, CsNO₃, SrCl₂, BaCl₂, Pb(NO₃)₂ and CuSO₄ at 80-90ºC and 1 atm for 3 hours and in distilled H₂O under the same conditions for 500 hours. Samples after the experiments were examined by SEM, EMPA and IR spectroscopy. Delhayelite remains unaltered in solutions of Na, K, Sr and Pb salts but shows cation-exchange reactions with NH₄⁺, Rb⁺, Cs⁺ and Ba²⁺. Contents of K, Na, Cl and F in the mineral decrease in these experiments. In acidic solution (CuSO₄), delhayelite was partially dissolved but no Cu absorption obsreved. In distilled H₂O, partial leaching of Na and F from the mineral was detected, with formation of a fivegite-like product. The processes of ion leaching and ion exchange in delhayelite are accompanied by its hydration. The IR spectra show that H-bearing groups in the products of these reactions are different: there are H₂O molecules and strongly H-bonded weak-acidic hydroxyl anions, probably forming silanol groups Si-OH, as it was earlier found in fivegite (Pekov e.a., in press). The CaAlSiO quasi-framework in all delhayelite-like phases obtained in the experiments becomes stable.

This work was supported by grants nos. 08-07-00077-a, 09-05-00143-a, 09-05-12001-ofi_m and 10-05-91333-NNIO_a of RFBR and grants of President of Russian Federation nos. NSh-4034.2010.5 and NSh-3848.2010.5.

 

References:

Cannillo E., Rossi G., Ungaretti L. The crystal structure of delhayelite // Rend. Soc. Ital. Mineral. Petrol. 1969. Vol. 26. P. 63-75.

Pekov I.V., Zubkova N.V., Chukanov N.V., Sharygin V.V., Pushcharovsky D.Yu. Crystal chemistry of delhayelite and hydrodelhayelite // Doklady Earth Sciences. 2009. Vol. 428. No. 7. P. 1216-1221.

Pekov I.V., Zubkova N.V., Chukanov N.V., Zadov A.E., Pushcharovsky D.Yu. Fivegite, K₄Ca₂[AlSi₇O₁₇(O_2-xOH_x)][(H₂O)_2-xOH_x]Cl, a new mineral from the Khibiny alkaline complex, Kola Peninsula, Russia // Zapiski RMO, in press.