CHARGE DENSITY IN NATURAL DATOLITE AND ITS COMPARISON WITH OTHER SILICATE MINERALS

Belokoneva E.L., Gorjunova A. N.

Moscow State University, geological faculty, department crystallography Vorobjevi Gori, Moscow 199899 Russia

The crystal structures of natural silicates, alumosilicates, borosilicates are not simple for precise X-ray and charge density investigation because they have usual complicate formula, the unit cell volume ~1000-2000 ų, low (orthorhombic or monoclinic) symmetry and very often isomorphic substitution. Newetheless this group is promising and attract the attention of scientists. As example, the electrostatic properties of alumosilicate natrolite and borosilicate danburite with zeolite character were studied recently in [1,2]. It is still actual to understand the peculiarities of chemical bonds and principles of classification [3] as far as to investigate structure-properties relationship in pyroelectric tourmaline or antoferroelectric sphene on the charge density level [4,5].

Natural monoclinic borosilicate datolite CaBSiO₄(OH)₂, a=9.646(6), b=7.620(4), c=4.839(3) Å, =90.14(5)^o, sp.gr. P2₁/a, was studied with the same X-ray diffraction technique as other silicates [3,4,5,6]. The independent atomic model was refined in the high angle region (sin q/l>0.75) using 1469 reflections, R_hkl=0.0128, R_whkl=0.0133, S=1.000. The -maps were used for localization of Na-impurity (0.21 at.%!) and of Fe-impurity (0.42 at.%!), which presence and content was confirmed by electron microprobe analysis and explains weak apple-green color of our sample. The final structural formula is (Ca_0.986Na_0.014)Fe_0.012H_0.988[BSiO₅]. The trace Fe-atoms occupy the octahedral hole in the structure, which is empty in datolite without impurity and is fully occupied in gadolinite and homilite - minerals from the datolite group.

The datolite crystal structure is first example of silicate (borosilicate) in which the charge density distribution was studied in the layer tetrahedral structural unit [BSiO₅], which is relative to [Si₂O₅] layer unit in apophyllite KCa₄[(Si₄O₁₀)₂]8H₂O. The hydrogen bond in this mineral, as it was found from dr-map, is bifurcated and connects the apical corner of one B-tetrahedron with two oxygens from opposite edge of other B-tetrahedron via octahedral hole, where small amount of Fe-impurity is located. The direction of bonds is parallel to theB,Si- layer and provides the layer additional hardness.

Both Si- and B-atoms demonstrate sp³ s- Si-O and B-O bonding orbitals. The values of -peaks on Si-O-bonds are well compared with other silicates: 0.25-0.40 e/ų, the values of dr-peaks on B-O-bonds are a bit lower in agreement with the electronic structure of the elements. There is direct correlation in our experimental dr-maps between displacement of peak from middle position on bond line, what is accompanied by increasing the bond polarity, and the difference in the electronegativity of the couples of the atoms in the row B-O, Si-O, Be-O using Poling scale. Thus, the chemical bonds may be described as covalent with the ionic (polar) component.

In the isolated [SiO₄] tetrahedron of topaze the bond peculiarities are determined by close packing effects and second coordinaton sphera. The condensation of [SiO₄]-tetrahedra in the rings leads to the accumulation of dr on bridge oxygens [3], expressed especially in dioptase - pure ring silicate in contrast to beryl and cordierite, which can be classified as framework beryllosilicate and alumosilicate. The most remarkable feature of dr-maps in datolite is the presence of bridges of between two s-bonds B-O and Si-O inside the B- and Si-tetrahedra, which are parallel to the tetrahedral edges and the layer surface and demonstrate consolidation of layers thanks "bridge" density. We can conclude that the classical crystal chemical classification describe the structure on mackrolevel, at the same moment the charge density and chemical bond peculiarities determine the separation of structural units on electronic level. The atomic charges will be discussed.

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