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Two new occurrences of willemite coexisting with franklinite are described from the Bergslagen region. At the Garpenberg Norra Pb-Zn mine (Hedemora, Dalarna) willemite and franklinite occur in silicate skarn rocks in association with clino-amphibole, phlogopite, quartz, galena, sphalerite, fluorite and barite. At the Hasselhojden metacarbonate quarry (Hallefors, Orebro) the two minerals appear with mainly sphalerite in talc-bearing dolomite marble. Based on electron-microprobe (EMP) analyses, willemite from Garpenberg Norra contains 0.4–2.1 MnO, 0.2–1.5 MgO and 0.1–1.2 Fe2O3, whereas at Hasselhojden the mineral has 0.4 Fe2O3 as the main impurity (all in wt.%). Mossbauer spectroscopy and EMP data show that Fe is dominantly triva-lent and to some extent substitutes for Si in these willemite samples. Franklinite from Hasselhojden contains up to 7.6 wt.% V2O5, incorporated according to the substitution 2 (Zn, Fe)2+ + V5+ = 3 Fe3+.

The compositional dependence of the intracrystalline cation distribution was investigated in four synthetic spinels belonging to binary solid solutions. Spinel single crystals were flux-grown in the (Mg,Zn)(Al,Fe 3+ ) 2 O 4 system, and cation distribution was determined by means of single-crystal X-ray structural refinement, electron-microprobe analysis and Mossbauer spectroscopy. Experimental data were processed and a detailed topochemical model was obtained for each sample: IV (Mg 0.76 Al 0.24 ) VI (Al 1.76 Mg 0.24 )O 4 ; IV (Zn 0.65 Mg 0.22 Al 0.13 ) VI (Al 1.87 Mg 0.13 )O 4 ; IV (Mg 0.73 Al 0.18 Fe 3+ 0.09 ) VI (Al 1.62 Mg 0.27 Fe 3+ 0.11 )O 4 and IV (Mg 0.62 Zn 0.15 Al 0.15 Fe 3+ 0.08 ) VI (Al 1.61 Mg 0.23 Fe 3+ 0.16 )O 4 . Zinc was found to occupy only the tetrahedrally coordinated site and its presence strongly influenced intersite cation partitioning. In Zn-bearing crystals both intracrystalline exchanges Mg ⟷ Al and Mg ⟷ Fe 3+ turned out to be limited. Zinc in octahedral coordination, which is rare in natural spinels, was shown to be favoured by high Fe 3+ contents and high equilibration temperatures. The cation distributions determined experimentally were found to be in close agreement with those calculated using the general thermodynamic model for spinel solid solutions by O9Neill & Navrotsky (1984).

More than 100 polished sections of magnetite, chromite, franklinite and jacobsite from about 50 localities were examined by ore microscopic methods. During sample preparation, two final polishing procedures were applied: (a) a 0.1 μm diamond suspension on «Microcloth», (b) an alkaline silica solution on «Microcloth». With the exception of {100} and {111} sections, procedure (a) always led to weak optical anisotropy, while with procedure (b), with the exception of five zoned magnetites, sections remained isotropic. Electron channeling pattern (ECP) images on a scanning electron microscope (SEM) showed a strongly damaged surface for a), and a perfect, undisturbed surface structure for b). This convincingly indicates that anomalous optical anisotropy effects in the spinel group are usually caused by surface deformation caused by mechanical polishing procedures. The relation-ship between surface deformation and face symmetry leads to the isotropic appearance of deformed {100} and {111} sections and to the anisotropic appearance of low-symmetric {hkl} sections