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Ten hausmannite crystals (from Ilfeld and Friedrichrode, Harz, Germany), belonging to the ([Mn.sub.1-x][Zn.sub.x])[Mn.sub.2][O.sub.4] (0 [less than or equal to] x [less than or equal to] 0.26) system (I[4.sub.1]/amd hausmannite structure type), were characterized by chemical (electron microprobe) and structural (single-crystal X-ray diffractometer) analysis. The prevailing trivalent cation is [Mn.sup.3+], with very minor Al (not higher than 0.005 apfu). Among divalent cations, the main substitution involves Zn [right arrow] [Mn.sup.2+]. Cation distribution was obtained by comparing chemical and structural data, and results confirm normal distribution, with [Mn.sup.3+] ordered on the octahedral site. A specific bond distance of 2.030 [Angstrom] was refined for [sup.VI][Mn.sup.3+]-O. Unit-cell parameters a and c range from 5.752 to 5.763 [Angstrom] and from 9.408 to 9.461 [Angstrom], respectively. The smallest values are characteristic of the sample with the highest hetaerolite content. T-O bond distance (2.027-2.041 [Angstrom]) shows a strong positive correlation with unit-cell constants, while the O-T-O angle (103.3-103.7[degrees]) is related only to the oxygen coordinate, z. The two octahedral bond distances show limited variations: the shorter one, M-Os, ranges from 1.927 to 1.930 [Angstrom], and is not significantly correlated with unit-cell parameters. The longer one, M-[O.sub.L], shows a larger variation, from 2.281 to 2.290 [Angstrom], and is positively correlated with c. Regularization of the octahedron with increasing hetaerolite content coincides with an increase in the oxygen coordinate y and a decrease in c and c/a. Of particular interest is the positive linear relation between octahedral elongation and [V.sub.T]. As the octahedral content of all samples is almost constant, given the closeness of [Mn.sup.3+] to stoichiometry, all structural distortions are linked to [sup.VI]Zn [right arrow] [sup.VI][Mn.sup.2+] that reduces the T-O bond distance and causes movement of the structure toward cubic symmetry. This interaction is due to the 'dragging effect' of the tetrahedron on the octahedron. In hausmannite-type structures, besides the main structural distortion produced by the Jahn-Teller effect, a secondary one, without symmetry modification, is determined by the geometrical effects of the tetrahedron on the octahedron.

The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric Mg[Al.sub.2][O.sub.4] spinel was investigated between 600 and 1100 [degrees] C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 [degrees] C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O'Neill and Navrotsky (1983) gave [Alpha] = 23 [ + or -] 2 KJ/mol and [Beta] = 13 [+ or -] 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and MO bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of -0.014 [Angstrom] for T-O and +0.007 [Angstrom] for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 x [10.sup.-6] [degrees][C.sup.-1] and 8.9 x [10.sup.-6] [degrees] [C.sup.-1] were calculated for T and M sites, respectively.

Examines hausmannite crystals belonging to the (Mn[sub1-x]Zn[subx])Mn[sub2]O[sub4] (0 ≤ x ≤ 0.26) system using chemical and structural analysis. Methodology used in the study; Procedure for determination of cation distribution; Description of the dragging effect of the tetrahedral structure on octahedral distortions.

Presents a commentary on the article "Ordering in Spinels--A Monte Carlo Study," by M. Yu. Lavrentiev, J. A. Purton and N. L. Allan. Concern on the statement about the contentious issue on the thermal expansion of spinels; Erroneus comparison of structural data taken in room temperature and those taken in high temperature; View on the statement about the difficulties in preparing stoichiometric samples.