Tetragonal Mg2.12Fe3.17Ge2.56O12 is a new germanate forming at 1 atm pressure in the MgO-Fe2O3-GeO2 system. It is an analogue of the high-pressure silicate mineral TAPP occurring as inclusions in diamonds of lower-mantle origin. Its crystal structure was determined by X-ray diffraction using single-crystals grown from a potassium molybdate flux. It crystallizes in the I4̅2d space group with a = 6.8153(4) Å, c = 18.669(2) Å, Z = 4. The refinement of its cation distribution, VIII(Mg0.52Fe0.33)VI(Mg1.60Fe2.40)IV(Ge2.56Fe0.44)O12, shows the presence of a partially filled (85%) dodecahedral site characterized by an unusual geometry with two very different bond lengths [2.169 Å (×4) and 2.609 Å (×4)]. The structure determinations of the Ca2+- and Y3+-substituted phases show that the larger cations are completely partitioned in the dodecahedral site, increasing its occupancy slightly (up to 92% in the case of Ca2+).

Cubic Mg2.35Y2.00Fe0.97Ge2.59O12 is a new garnet phase forming at 1 atm pressure in the MgO-Y2O3-Fe2O3-GeO2 system. Its crystal structure and cation distribution were also determined by single-crystal X-ray diffraction: VIII(MgY2)VI(Mg1.35Fe0.56)IV(Ge2.59Fe0.41)O12, Iad space group, a = 12.232(1) Å, Z = 8. In spite of similarities in their chemical compositions, the tetragonal and garnet phases are structurally distinct, with different ratios of dodecahedral to octahedral sites, viz., VIIIAVIB4IVT3O12 and VIIIA3VIB2IVT3O12 respectively. As a consequence, the tetragonal phase forms in systems containing smaller cations, such as Mg2+, Fe3+, Ni2+, Co2+, whereas larger cations, such as Y3+, show a strong preference for the garnet phase.

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