KOLSKYITE, A NEW TS-BLOCK MINERAL: PREDICTION AND REALITY

Kolskyite, ideally (Ca◊)Na₂Ti₄(Si₂O₇)₂O₄(H₂O)₇ (◊=vacancy), is a new mineral discovered in the Khibiny alkaline massif, Kola Peninsula, Russia (IMA2013-005). Kolskyite is a new TS-block mineral (TS = titanium silicate) (Sokolova, 2006). The TS block consists of HOH sheets where H = heteropolyhedral and O = octahedral, and it is the main structural unit in the TS-block structures. There are four Groups of TS-block structures where Ti equals 1, 2, 3 and 4 apfu. The crystal structure of kolskyite belongs to Group IV where Ti = 4 apfu, a = 5.387(1), b = 7.091(1), c = 15.473(3) Å, α = 96.580(4), β = 93.948(4), γ = 89.818(4)^o, V = 585.8(3) ų, space group P-1 , Z = 1, and is a combination of TS and I (intermediate) blocks. In the crystal structure of kolskyite, TS and I blocks have following compositions: Na₂Ti₄(Si₂O₇)₂O₄(H₂O)₂ and [(Ca◊)(H₂O)₅], respectively.

The discovery of kolskyite is extremely important for us as it proves that we can predict crystal structures of TS-block minerals: the crystal structure of kolskyite was predicted by Sokolova & Cámara (2010). Sokolova & Cámara (2010) defined two types of TS-block structures: basic and derivative structures. A basic structure has only one type of TS block and only one type of I block; in a derivative structure, there is more than one type of TS block and there is more than one type of I block. A derivative structure is related to a number of basic structures of the same group. Hence we can predict a derivative structure as a combination of structural fragments of basic structures. Using inverse prediction, the basiccrystal structure of kolskyite was predicted as a component of known derivative structures.

References:

Sokolova, E. (2006) From structure topology to chemical composition: I. Structural hierarchy and stereochemistry in titanium disilicate minerals. Canadian Mineralogist, 44, 1273-1330.

Sokolova, E. & Cámara, F. (2010) From chemical composition to structure topology in Ti silicates. 20^th General Meeting of the International Mineralogical Association, Abstracts, Budapest, Hungary.