PREDICTIVE CRYSTAL-CHEMICAL RELATIONS IN TI-SILICATES
BASED ON THE TS BLOCK
© 2010 E. V. Sokolova
Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetnyi per. 35, Moscow, 119017, Russia Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba
R3T 2N2 Canada Received November 10, 2009
Abstract—In a group of minerals of reasonable complexity in which the structure topology is related but not identical, the general relation between structure topology and chemical composition is not known. This problem is of major significance. The structural hierarchy and stereochemistry are described for 27 titanium disilicate minerals that contain the TS (titanium-silicate) block, a central trioctahedral (O) sheet and two adjacent (H) sheets of [5]-and [6]-coordinated polyhedra and (Si2O7) groups and related delindeite. The TS block is characterized by a planar cell based on translation vectors, ti and t2, with ti ~ 5.5 and t2 ~ 7 A and ti A t2 close to 90°. The general formula
of the TS block is Ap Bp M^ M^ (Si2O7)2X4 + n, where M^ and M^ = cations of the H and O sheets; MH =
= Ti (= Ti + Nb), Zr, Mn2+, Ca; MO = Ti, Zr, Mn2+, Ca, Na; AP and BP are cations at the peripheral (P) sites = = Na, Ca, Ba; X = anions = O, OH, F; n = 0, 2, 4; the core part of the TS block is shown in bold and is invariant. Cations in each sheet of the TS block form a close-packed layer and the three layers are cubic close packed.
There are three topologically distinct TS blocks, depending on the type of linkage of two H sheets and the central O sheet. The H sheets of one TS block attach to the O sheet in the same manner. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, (H2O) groups and oxyanions (PO4)3-, (SO4)2- and (CO3)2-. These structures naturally fall into four groups, based on differences in topology and stereochemistry of the TS block. In Group I, Ti = 1 apfu, Ti occurs in the O sheet, and (Si2O7) groups link to a Na polyhedron of
the O sheet (linkage 1). In Group II, Ti = 2 apfu, Ti occurs in the H sheet, and (Si2O7) groups link to two M2+ octahedra of the O sheet adjacent along t2 (linkage 2). In Group III, Ti = 3 apfu, Ti occurs in the O and H sheets, and (Si2O7) groups link to the Ti octahedron of the O sheet (linkage 1). In Group IV, Ti = 4 apfu (the maximum possible content of Ti in the TS block), Ti occurs in the O and H sheets, and (Si2O7) groups link to two Ti octahedra of the O sheet adjacent along t1 (linkage 3). The stability of the TS block is due to the ability of Ti (Nb) to have an extremely wide range in Ti (Nb)-anion bond lengths, 1.68—2.30 A, which allows the chemical composition of the TS block to vary widely. In crystal structures so far known, only one type of TS block occurs in a structure. The TS block propagates close-packing of cations onto the I block. The general structural principles and the relation between structure topology and chemical composition are described for the TS-block minerals. These principles allow prediction of structural arrangements and possible chemical compositions, and testing whether or not all aspects of the structure and chemical formula of a mineral are correct. Here, I show how these principles work, and review recent results that show the effectiveness of these principles as a predictive technique.
INTRODUCTION
In a group of minerals of reasonable complexity in which the structure topology is related but not identical, the general relation between structure topology and chemical composition is not known. This problem is of major significance. In terms of the relation between structure and chemical composition, it is simple to go from structure topology to chemical composition. How-
Corresponding author: E.V Sokolova. E-mail: elena_sokolova@ umanitoba.ca
The text was submitted by the author in English.
ever, what one wants to do is to go in the reverse direction: from chemical composition to structure topology. Here, I consider first the relation between structure topology and chemical composition for a group of structurally related but topologically distinct titanium disilicate minerals with the view of developing a general relation between structure topology and chemical composition. Once this is established, I will consider the inverse problem: to establish a distinct structure topology for a specific chemical composition.
In this paper, I consider the structure topology and stereochemistry of 27 titanium disilicate minerals with
the TS (titanium-silicate) block, which has a three-layered structure: the central part is a sheet of octahedra (i.e., trioctahedral sheet that is common in micas), and two adjacent sheets containing different polyhedra, including (Si2O7) groups. In a structure, the TS block can alternate with another block, which I call an intermediate (I) block, as it is intercalated between two TS blocks. The TS block can have different topologies, and its chemical composition changes, but the Si—O radical is always the same, the [Si2O7] group. In minerals, Ti4+ is generally [6]-coordinated, and less commonly [5]-coordinated. In many Ti-silicate minerals, Ti -«—► Nb substitution is common (r[[6]Ti4+] = 0.605, r[[6]Nb5+] = 0.64 A; Shannon, 1976). Hence, where I refer to (TiO6) octahedra, I include ({Ti,Nb}O6) and ({Nb,Ti}O6) octahedra.
I consider only minerals of known structure. I do not take into account minerals for which the crystal structure data are not available. The structure data and references for 27 minerals are summarized in Tables 1, 2. Table 1 gives general and structural formulae from Sokolova (2006). Ideal formulae of minerals can be easily obtained from the structural formulae by summing cations. In this paper, I describe a structure hierarchy for Ti disilicate minerals, the stereochemistry of their structures and the general crystal-chemical principles which underlie this structural hierarchy. These principles allow us to make predictions of structural arrangements and possible chemical compositions, and to test whether or not all aspects of the structure and chemical formula of a mineral are correct. In this paper, I will show how these principles work.
PREVIOUS WORK
In the late 1950s and early 1960s, N. V Belov and his coworkers began structure work on Ti-silicate minerals of this group: seidozerite (Simonov and Belov, 1960), bafer-tisite (Guan et al., 1963; Pen and Sheng, 1963), rosen-buschite (Shibaeva et al., 1964) and murmanite (Khalilov et al., 1965a) were the first structures to be done. Khalilov et al. (1965b) used available structural information to compare the structures of murmanite, lomonosovite and epistolite (possible structure), and emphasized the stability of the main structure unit, a TS block. In two books, Belov (1976) and Pyatenko et al. (1976) considered the TS block as a stable fragment characteristic of several Ti-silicate structures. Egorov-Tismenko and Sokolova (1987, 1990) originally developed the crystal chemistry for minerals with the TS block characterized by (Si2O7) groups. Sokolova (1997, 1998) considered 16 minerals as a poly-somatic series seidozerite—nacaphite in which the seidozerite block (= TS block) is a stable structural unit, whereas the nacaphite block varies in size and chemical composition. Furthermore, Sokolova (1997) emphasized an important condition for formation of such a series: adjacent surfaces of two types of blocks must be similar and have nearly identical unit-cell dimensions. Egorov-Tis-menko (1998) redescribed in detail minerals with a TS
block dealt with by Egorov-Tismenko and Sokolova (1990), and emphasized that those minerals are analogues of micas. Ferraris et al. (1996) introduced a new notation, the HOH block, where O denotes a sheet ofoc-tahedra and H denotes a heteropolyhedral silicate sheet, and considered minerals with various HOH blocks as different somatic series. Christiansen et al. (1999) gave detailed descriptions of the structures with HOH blocks, and characterized different stacking sequences. A detailed discussion of the previous work is given in Sokolova (2006), together with a detailed analysis of the structure topology and stereochemistry of all TS-block minerals.
THE TOPOLOGY OF THE TS (TITANIUM-
SILICATE) BLOCK: GENERAL FEATURES
The structures of all minerals considered in this paper have one common fundamental building block or structure unit, a Ti-silicate (TS) block. For the description of its components, I will use the (HOH) notation ofFerraris et al. (1996).
The Osheet
The O sheet is commonly an array of close-packed octahedra with the formula MO X°, where MO and XO are cations and anions ofthe O sheet (Fig. 1a). In all minerals with the TS block, all cation sites of the O sheet are fully occupied, i.e., the O sheet is a trioctahedral sheet analogous to that in micas.
The H sheet
The H sheet consists of various polyhedra: (Si2O7) groups and [6]- (or [5]-) coordinated MH polyhedra in the ratio 1 : 1 (Fig. 1b). The topology of the H sheet dictates two translation vectors, tj and t2, with lengths of these vectors being t1 ~ 5.5 and t2 ~ 7 A and tj A t2 close to 90°. In the H sheet in all structures, (Si2O7) groups are invariably oriented with their Si—Si distance parallel to t2.
The TS block
The TS block has a three-layered structure consisting of a central sheet ofoctahedra (O sheet) and two adjacent heteropolyhedral sheets (H sheets). The O and H sheets link together through common vertices of constituent polyhedra (Fig. 1c). As the topology of the H sheet dictates two translation vectors, t1 and t2, then the O sheet has the same periodicity, and thus the TS block is invariably characterized by two minimal translation vectors, as was noted by Egorov-Tismenko and Sokolova (1987).
The peripheral (P) cation sites
Linkage ofO and H shee
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