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Three single crystals of CaTi 2 O 4  (CT)-type, CaFe 2 O 4  (CF)-type, and new low-density CaFe 2 O 4  (LD-CF) related MgAl 2 O 4  were synthesized at 27 GPa and 2500 °C and also CT-type MgAl 2 O 4  at 45 GPa and 1727 °C using conventional and advanced multi-anvil technologies, respectively.The structures of CT-type and LD-CF related MgAl 2 O 4  were analyzed by single-crystal X‑ray diffraction. The lattice parameters of the CT-type phases synthesized at 27 and 45 GPa were  a  = 2.7903(4),  b  = 9.2132(10), and  c  = 9.3968(12) Å, and  a  = 2.7982(6),  b  = 9.2532(15), and  c  = 9.4461(16) Å, respectively, ( Z  = 4, space group:  Cmcm ) at ambient conditions. This phase has an AlO 6  octahedral site and an MgO 8  bicapped trigonal prism with two longer cation-oxygen bonds. The LD-CF related phase has a novel structure with orthorhombic symmetry (space group:  Pnma ), and lattice parameters of  a  = 9.207(2),  b  = 3.0118(6), and  c  = 9.739(2) Å ( Z  = 4). The structural framework comprises tunnel-shaped spaces constructed by edge- and corner-sharing of AlO 6  and a 4+1 AlO 5  trigonal bipyramid, in which MgO 5  trigonal bipyramids are accommodated. The CF-type MgAl 2 O 4  also has the same space group of  Pnma  but a slightly different atomic arrangement, with Mg and Al coordination numbers of 8 and 6, respectively. The LD-CF related phase has the lowest density of 3.50 g/cm 3  among MgAl 2 O 4  polymorphs, despite its high-pressure synthesis from the spinel-type phase (3.58 g/cm 3 ), indicating that the LD-CF related phase formed via back-transformation from a high-pressure phase during the recovery. Combined with the previously determined phase relations, the phase transition between CF-and CT-type MgAl 2 O 4  is expected to have a steep Clapeyron slope. Therefore, CT-type phase may be stable in basaltic- and continental-crust compositions at higher temperatures than the average mantle geotherm in the wide pressure range of the lower mantle. The LD-CF related phase could be found in shocked meteorites and used for estimating shock conditions.

High-pressure syntheses and crystal structure analyses of a new low-density CaFe 2 O 4 -related and CaTi 2 O 4 -type MgAl 2 O 4 phases