ORIGINAL PAPER D. Lenaz  H. Skogby  F. Princivalle  U. Ha˚ lenius Structural changes and valence states in the MgCr 2 O 4 –FeCr 2 O 4 solid solution series Received: 10 May 2004 / Accepted: 1 August 2004 Abstract The influence on the structure of Fe 2+ ! Mg substitution was studied in synthetic single crystals belonging to the MgCr 2 O 4 –FeCr 2 O 4 series produced by flux growth at 900–1200 °C in controlled atmosphere. Samples were analyzed by single-crystal X-ray diffrac- tion, electron microprobe analyses, optical absorption-, infrared- and Mo¨ssbauer spectroscopy. The Mo¨ssbauer data show that iron occurs almost exclusively as IV Fe 2+ . Only minor Fe 3+ (<0.005 apfu) was observed in sam- ples with very low total Fe. Optical absorption spectra show that chromium with few exceptions is present as a trivalent cation at the octahedral site. Additional absorption bands attributable to Cr 2+ and Cr 3+ at the tetrahedral site are evident in spectra of end-member magnesiochromite and solid-solution crystals with low ferrous contents. Structural parameters a 0 , u and T–O increase with chromite content, while the M–O bond distance remains nearly constant, with an average value equal to 1.995(1) A ˚ corresponding to the Cr 3+ octahedral bond distance. The ideal trend between cell parameter, T–O bond length and Fe 2+ content (apfu) is described by the following linear relations: a 0 ¼ 8.3325(5) + 0.0443(8)Fe 2+ (A ˚ ) and T– O ¼ 1.9645(6) + 0.033(1)Fe 2+ (A ˚ ) Consequently, Fe 2+ and Mg tetrahedral bond lengths are equal to 1.998(1) A ˚ and 1.965(1) A ˚ , respectively. Keywords MgCr 2 O 4  FeCr 2 O 4 spinels  Single-crystal XRD  EPMA  OAS  Mo¨ssbauer Introduction The spinel structure is based on a nearly ideal cubic close-packed array of oxygen atoms with tetrahedral (T) and octahedral (M) cavities. In common 2-3 spinels, one eighth of the T sites and one half of the M sites are occupied by heterovalent cations A and B in the ratio AB 2 O 4 , where A ¼ (Mg, Fe 2+ , Zn, Mn 2+ ) and B ¼ (Al, Fe 3+ , Cr 3+ ). In general, spinels do not show the ideal- ized configuration, with A cations in T sites and B ca- tions in M sites. Commonly there exists octahedral– tetrahedral disorder of A and B cations depending on thermal history. Consequently, the crystal chemistry of spinels is described by the general formula IV (A 1-i B i ) VI (A i B 2-i )O 4 , where ‘i’ refers to the inversion parameter. There are two ordered configurations stable at low temperature, one with i ¼ 0 (normal spinel; e.g. MgA- l 2 O 4 , FeAl 2 O 4 , MgCr 2 O 4 , FeCr 2 O 4 ) and one with i ¼ 1 (inverse spinel; e.g. MgFe 2 O 4 , FeFe 2 O 4 ). With increas- ing temperature, disorder takes place, i.e. A and B ca- tions undergo increasing intersite exchange over the three cation sites per formula unit. Modifications of T–O and M–O bond distances to accommodate various chemical compositions and/or cation distribution determine variations in the oxygen positional parameter u and the cell edge a 0 (Lavina et al. 2002). In geological environments MgCr 2 O 4 –FeCr 2 O 4 end members are very common components of spinel solid solutions. In particular, Cr is an essential constituent of many spinels in mafic and ultramafic igneous and metamorphic rocks of the crust and the upper mantle. Even if Cr-bearing spinels typically occur as accessory phases, they are widely considered as petrogenetic indi- cators (Irvine 1965, 1967; Evans and Frost 1975; Dick and Bullen 1984; Sack and Ghiorso 1991; Barnes and Roeder 2001), given the general relationships between spinel chemistry, rock type and petrological processes. New knowledge has been achieved through Fe–Mg ex- change reactions involving Cr-spinels, olivine and orth- opyroxene geothermometry (Fujii 1977; Fabries 1979), Phys Chem Minerals (2004) 31: 633–642 Ó Springer-Verlag 2004 DOI 10.1007/s00269-004-0420-0 D. Lenaz (&)  F. Princivalle Dipartimento di Scienze della Terra, via Weiss 8, 34127 Trieste, Italy Tel: +390405582218 Fax: +390405582213 e-mail: lenaz@univ.trieste.it H. Skogby  U. Ha˚lenius Department of Mineralogy, Swedish Museum of Natural History, SE-10405 Stockholm, Sweden