Crystal chemistry and cation ordering in pseudobrookite and armalcolite from Spanish lamproites

The roedderite-chayesite series from Spanish lamproites: crystal-chemical characterization

“…Thus, the crystallization of the roedderite-type minerals, with Na/K substitution at the C site and/ or very low Na occupancy at the B site, record the inversion in the activity of Na and K in the residual magmatic fractions, from which crystallize K-rich phases such as dalyite, end-products of the Cancarix magma fractionation. The increase in Fe 3 + at the 72 tetrahedron with continued crystallization supports the notion of an increase in the activity of oxygen in the very late stages of crystallization, as suggested by the crystallization of Fe3+-bearing pseudobrookite (Brigatti et al, 1993) and by the increase in Fe 3+ in the late clinopyroxene and amphibole (Contini et al, 1993). The increase in oxygen fugacity was accompanied by low PH20, as suggested by the absence of HzO in the studied minerals.…”

Crystal Chemistry of Cation Order–Disorder in Pseudobrookite-Type MgTi2O5

“…1); on the other, each M2 shares four octahedral edges with others to form M2 octahedral layers parallel to (010); these layers are linked together along b by M1 through edge sharing as well (6). Previous studies on armalcolite (Fe Mg Ti O ) show that M1 is preferentially occupied by Fe and Mg, and M2 by Ti, and that the cation distribution in these compounds is temperature dependent: samples quenched from higher temperatures are substantially more disordered than those quenched from lower temperatures (3)(4)(5)(7)(8)(9)(10)(11). Wechsler and Navrotsky (4) further found that the unit-cell parameters of MgTi O depend strongly on the cation ordering state or quenching temperature: a, c, and » increase with increasing quenching temperature, whereas b decreases.…”

“…However, due to the lack of single-crystal X-ray diffraction data, this model was calibrated only on the basis of structural data and site occupancies on armalcolite reported by Smyth (8) and Wechsler (3). Despite the sophistication of the model proposed by Brown and Navrotsky (12), caution should be exercised when it is applied to the end members, because of the difficulty in accurately determining site occupancies of three cations (Fe, Mg, and Ti) between two sites in Fe Mg Ti O (3,7,8,11). We have undertaken a singlecrystal X-ray diffraction study on five synthetic MgTi O crystals annealed at 600, 700, 800, 1000, and 1400°C to characterize variations of the unit-cell parameters and crystal structure of MgTi O with cation order-disorder.…”

Nonstoichiometric Li-Pseudobrookite(ss) in the Li2O-Fe2O3-TiO2System

“…The results are presented in Table 4 in the form of mean atomic number (m.a.n.) of the M1 site (16) The results in Table 4 show that the main structural changes accompanying substitution of Li#Ti for Fe in the pseudobrookite(ss) are parallel displacements along [100] of M1 and O1 which both lie on the (001) mirror planes at z"$1/4, and displacement of O3 along [001]. These displacements are illustrated in a [010] projection of the structure in Fig.…”