Garnet, the archetypal cubic mineral, grows tetragonal

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

doi:10.1038/s41598-019-51214-9

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.

Highlights

Garnet is one of the most widely occurring minerals in the Earth
Using some spectacular natural samples of garnet-bearing high-P, low-T metamorphosed basalt exhumed from subduction zones and greenschist-facies metamorphosed mudstones from the roots of mountain ranges, and utilising a multi-technique approach including optical microstructural analysis, BSEM, Electron Microprobe Analysis (EMPA), Electron Backscatter diffraction (EBSD), Fourier-Transform Infrared Spectroscopy (FTIR), TEM and single-crystal X-Ray Diffraction (XRD), we show that common garnet in these low-T regional metamorphic rocks[16] initially grows as a tetragonal, not cubic, mineral
Garnet exhibiting optical birefringence is common in samples of blueschist from the Franciscan mélange, California[22] and from Corsica and in phyllites and micaschists from the central and eastern Italian Alps (Figs 1 and 2; Supplementary Fig. S1, Supplementary Videos S1–S4)

Summary

Introduction

Garnet is one of the most widely occurring minerals in the Earth. It is stable to temperatures (T) approaching. In almost all metamorphosed crustal rocks in which it occurs, the composition of garnet lies between the end members pyrope [Mg3Al2(SiO4)3], almandine [Fe2+3Al2(SiO4)3], spessartine [Mn3Al2(SiO4)3] and grossular [Ca3Al2(SiO4)3)]16 Such ‘common’ garnet, which is anhydrous, typically has a cubic structure (space group Ia-3d) and is optically isotropic[15]. Much rarer is so-called grandite garnet, a solid solution between grossular and andradite [Ca3Fe3+2(SiO4)3], and hydrogrossular garnet [Ca3Al2(SiO4)3−x(H4O4)x] These unusual compositions typically exhibit optical birefringence that is accompanied by oscillatory or sector zoning. Birefringence has rarely been noted in common garnet[20] In such cases, the anisotropy has been attributed to either externally-imposed strain or internal lattice strain, the latter due to the size difference between larger Ca cations and smaller Fe, Mg or Mn cations in the X site of the structure[20]. The differences in the refined unit-cell parameters were too small to unambiguously demonstrate a non-cubic symmetry

Methods

Results

Conclusion