Crystal structures of shlykovite and cryptophyllite: comparative crystal chemistry of phyllosilicate minerals of the mountainite family

Abstract

We solved the crystal structures of the phyllosilicates shlykovite, KCaSi 4 O 9 (OH)Á3H 2 O, and cryptophyllite, K 2 CaSi 4 O 10 Á5H 2 O, two minerals recently discovered in the Khibiny alkaline complex (Kola Peninsula, Russia). Both minerals are monoclinic with close values of the a, b and b parameters but significantly different c parameters: shlykovite a ¼ 6.4897(4), b ¼ 6.9969(5), c ¼ 26.714(2) Å , b ¼ 94.597(8) , V ¼ 1209.12(15) Å 3 , sp. gr. P2 1 /c, Z ¼ 4; cryptophyllite a ¼ 6.4934(14), b ¼ 6.9919(5), c ¼ 32.087(3) Å , b ¼ 94.680(12) , V ¼ 1451.9(4) Å 3 , sp. gr. P2 1 /n, Z ¼ 4. The structures were solved from synchrotron diffraction data collected on the same intergrown crystal and refined anisotropically to R(F) ¼ 0.0960 for 1147 unique reflections with I. 2s(I) (shlykovite) and R(F) ¼ 0.0856 for 1667 unique reflections with I. 2s(I) (cryptophyllite). Shlykovite and cryptophyllite are representatives of two new, closely related structure types. The main structural units of both minerals are TOT blocks consisting of tetrahedral Si layers (T) and an octahedral component (O), sandwiched between them. Each T-layer consists of 4-and 8-membered rings of SiO 4-tetrahedra and can be considered as a half of a double layer described in the structures of the members of the rhodesite mero-plesiotype series. A topologically closely related Si layer also forming T-fragments of TOT blocks was recently discovered in mountainite, KNa 2 Ca 2 [Si 8 O 19 (OH)]Á6H 2 O. The O-fragment of the TOT blocks in shlykovite and cryptophyllite is formed by columns of edge-sharing Ca-centred octahedra CaO 5 (H 2 O). In both minerals K cations are located in the voids of the Si layer. The content of the interlayer space is different in each mineral: only H 2 O molecules in shlykovite, K atoms and H 2 O molecules in cryptophyllite. Mountainite, shlykovite and cryptophyllite are closely related in configuration of the TOT blocks, their main structure unit, and could be used to define the mountainite structural family.

K2CaSi4O10: A novel phase in the ternary system K2O–CaO–SiO2 and member of the litidionite group of crystal structures

“…Nevertheless, the silicate anions in both structures share the same principal one-dimensional chain-like building element (see Figure 1C). In cryptophyllite, 25 an infinite number of these chains are linked into a two-dimensional layer (see Figure 6), while in K 2 CaSi 4 O 10 the condensation process stops after forming a single tubular chain.…”

“…Obviously, it would be interesting to study the water release of cryptophyllite in order to shed some light on its dehydration pathway which may lead to K 2 CaSi 4 O 10 as the ultimate product. However, this may not be feasible from an experimental point of view, as natural cryptophyllite is usually intensively intergrown with other minerals such as shlykovite (KCaSi 4 O 9 (OH) 9 3 H 2 O) 25 making a separation of the different phases almost impossible. The discovery and structural elucidation of previously unknown K 2 CaSi 4 O 10 further enlarges the number of well described potassium calcium silicates.…”

“…In a certain sense, our search for new phases was this time “mineral‐inspired.” Actually, there are a larger number of hydrous K–Ca‐silicates that have been described in the literature occurring in rather unusual petrological settings. One of the members of this family is the mineral cryptophyllite (K 2 CaSi 4 O 10 × 5H 2 O) whose crystal structure has been studied by Zubkova et al Since cryptophyllite contains exclusively structural water molecules and no hydroxyl or silanol groups we wanted to test the hypothesis if the anhydrous analogue containing K 2 O, CaO and SiO 2 in the ratio 1:1:4 could be prepared by ceramic methods. The present paper reports the results of our investigation, in fact proving the existence of K 2 CaSi 4 O 10 as a previously unknown phase in the system K 2 O–CaO–SiO 2 .…”

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

“…Minerals of the günterblassite group and the rhodesite mero-plesiotype series are related in term of crystal structure and constitute, together with their single-layer relatives belonging to the mountainite family (Zubkova et al 2010), a specific series Bconnecting^phyllosilicates with microporous tectosilicates, i.e., zeolites. The relationship between single-, double-and triple-layer tetrahedral blocks in these minerals is shown in Fig.…”

“…The double layers are formed by two single layers consisting of four-and eightmembered tetrahedral rings connected with each other via common vertices of T(2)-centered tetrahedra [T(2) = (Al,Si)]. Each single layer is topologically very close to the silicate layer in shlykovite, KCa[Si 4 O 9 (OH)]⋅3H 2 O, and cryptophyllite, K 2 Ca[Si 4 O 10 ]⋅5H 2 O, representatives of the mountainite family(Zubkova et al 2010;Pekov et al 2012). The distribution of Si and Al among three crystallographically non-equivalent sites in BDAH is confirmed by the values of T-O distances (…”