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Garnet, the archetypal cubic mineral, grows tetragonal.

ABSTRACT: Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth's crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca-Fe³⁺ garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca-Fe²⁺-Mg-Mn) garnet is generally attributed to internal strain of the cubic structure. Here we show that common garnet with a non-cubic (tetragonal) crystal structure is much more widespread than previously thought, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. Indeed, a non-cubic symmetry appears to be typical of common garnet that forms at low temperatures (<450?°C), where it has a characteristic Fe-Ca-rich composition with very low Mg contents. We propose that, in most cases, garnet does not initially grow cubic. Our discovery indicates that the crystal chemistry and thermodynamic properties of garnet at low-temperature need to be re-assessed, with potential consequences for the application of garnet as an investigative tool in a broad range of geological environments.

SUBMITTER: Cesare B

PROVIDER: S-EPMC6789019 | biostudies-literature | 2019 Oct

REPOSITORIES: biostudies-literature

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Garnet, the archetypal cubic mineral, grows tetragonal.

Cesare B B Nestola F F Johnson T T Mugnaioli E E Della Ventura G G Peruzzo L L Bartoli O O Viti C C Erickson T T

Scientific reports 20191011 1

Garnet is the archetypal cubic mineral, occurring in a wide variety of rock types in Earth's crust and upper mantle. Owing to its prevalence, durability and compositional diversity, garnet is used to investigate a broad range of geological processes. Although birefringence is a characteristic feature of rare Ca-Fe<sup>3+</sup> garnet and Ca-rich hydrous garnet, the optical anisotropy that has occasionally been documented in common (that is, anhydrous Ca-Fe<sup>2+</sup>-Mg-Mn) garnet is generally ...[more]

PMID: 31605020

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Project description:Temperature and composition dependences of the I41/amd ? [Formula: see text] phase transition in the Mg x Cu1?-?x Cr2O4 spinel solid solution, due to the melting of the cooperative Jahn-Teller distortion, have been studied by means of single-crystal X-ray diffraction. Crystals with x = 0, 0.10, 0.18, 0.43, 0.46, 0.53, 1 were grown by flux decomposition methods. All crystals have been refined in the tetragonal I41/amd space group except for the Mg end-member, which has cubic symmetry. In Mg x Cu1?-?x Cr2O4 the progressive substitution of the Jahn-Teller, d9 Cu2+ cation with spherical and closed-shell Mg2+ has a substantial effect on the crystal structure, such that there is a gradual reduction of the splitting of a and c unit-cell parameters and flattening of the tetrahedra. Single-crystal diffraction data collected in situ up to T = 1173?K show that the tetragonal-to-cubic transition temperature decreases with increasing Mg content. The strength of the Cu-Cu interaction is, in effect, modulated by varying the Cu/Mg ratio. Structure refinements of diffraction data collected at different temperatures reveal that heating results in a gradual reduction in the tetrahedron compression, which remains significant until near the transition temperature, however, at which point the distortion of the tetrahedra rapidly vanishes. The spontaneous strain arising in the tetragonal phase is large, amounting to 10% shear strain, et, and ??1% volume strain, Vs, in the copper chromite end-member at room temperature. Observed strain relationships are consistent with pseudoproper ferroelastic behaviour ([Formula: see text] ? Vs ? [Formula: see text], where qJT is the order parameter). The I41/amd ? [Formula: see text] phase transition is first order in character for Cu-rich samples and then evolves towards second-order character. Although a third order term is permitted by symmetry in the Landau expansion, this behaviour appears to be more accurately represented by a 246 expansion with a change from negative to positive values of the fourth-order coefficient with progressive dilution of the Jahn-Teller cation.

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Garnet, the archetypal cubic mineral, grows tetragonal.

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Garnet, the archetypal cubic mineral, grows tetragonal.

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