Immiscibility in the Fe3O4-FeCr2O4 Spinel Binary

Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to palaeomagnetic results of the dunites

“…Heating procedures influence much more the magnetic than mineralogical characteristics. When mineralogical data do not show much change, magnetically they are drastic; the T b and T sd of the dominant phase decrease, due to annealing, from well defined at 530 • C to a series of temperatures ranging from about 100 to 350 • C. This behaviour is probably due to a miscibility gap observed in the Fe-Cr series (Cremer 1969;Ziemniak & Castelli 2003). The dominant ferrichromite (Crmagnetite) phase with n of about 0.1-0.05 begins to unmix at temperatures exceeding 550-600 • C and, passing through the solvus temperature during cooling, forms two phases: one with n about 0.25-0.3 at the magnetite-rich end of the series and another close to the chromite-rich end, non-magnetic above r.t..…”

“…Binary mixed Fe–Cr spinels are described as Fe 2+ (Fe 1– n Cr n ) 2 O 4 where 0 < n < 1 denotes the mole fraction of chromite; n = 1 describes pure chromite, whereas magnetite has n = 0. The physical properties and structure of the spinel series depend on the contributions from normal and inverse spinels, as, for example, the Curie temperatures T c and magnetic moments μB shown in Figs 1(a) and (b) (Robbins et al 1971; Ziemniak & Castelli 2003). Both parameters decrease with an increasing amount of Cr.…”

“…Its magnetic structure is ferromagnetic (collinear spins of the same directions in A and B sites), but spins in the octahedral sites are canted against the tetrahedral ones, forming a cone-spiral magnetic spin structure. The mineral is ferromagnetic below its Curie temperature T c equal 71 K (Ziemniak & Castelli 2003). The magnetite end-member, Fe 2+ Fe 3+ 2 O 4 has an inverse spinel structure, where all tetrahedral sites are occupied by trivalent cations but half of the octahedral sites are substituted by divalent cations (e.g.…”

Magnetic Mineralogy and Petrophysical Properties of Ultramafic Rocks: Consequences for Crustal Magnetism

“…Although the chrome‐spinel grains crystallized early in the magmatic history, the exsolution of the more iron‐rich phase occurred later when the rocks cooled through the solvus temperature of the Cr spinel‐magnetite, at a T slightly below 600 °C. The position of the miscibility gap is indicative of the temperature of exsolution; however, in this system it is a function of both the compositions of the Cr‐spinel host and the olivine (Barnes & Roeder, ; Sack & Ghiorso, ; Ziemniak & Castelli, ). Based on the compositions of the olivine in the RUC Fo 84.6 to Fo 76.1 (Grant et al, ) and of the chrome‐spinel (Pastore, McEnroe, ter Maat, et al, ), the initial exsolution would have occurred when the rocks cooled to slightly below ~600 to 550 °C, with variations in T dependent on the composition of coexisting olivine (Sack & Ghiorso, ).…”

“…The high stability and unblocking temperatures of exsolved phases in the hematite‐ilmenite system increase the remanent contribution to the total magnetization (McCammon et al, ; McEnroe & Brown, ; McEnroe et al, , , , , , ). Chrome‐spinel can also be stable at lower‐crustal and upper‐mantle levels (Barnes & Roeder, ; Wasilewski & Mayhew, ; Wasilewski et al, ); however, whether this phase is magnetic is controlled by the composition (Francombe, ; Ziemniak & Castelli, ).…”