Garnet, not always the archetypal cubic mineral: new crystallographic evidences from Cazadero (USA) and the SW Tauern Window (Italy)

Garnet, being one of the most widespread minerals in the Earth, plays a crucial role for the investigation of many geological processes. Garnet occurs in several ultramafic to felsic rocks (e.g. mantle peridotites, granulites and metamorphic rocks) and as detrital mineral in sediments [1], and crystallize from the crust to the Transition Zone in the mantle thanks to its broad P-T stability field (P ⁓25 GPa and T ⁓2000 °C [2, 3]). These features, along with the chemical variability and the peculiar resistance, make garnet an essential tool to determine the P-T-t history and the evolution of rocks.

Garnet is commonly defined as the archetypal cubic mineral, since it typically presents Ia-3d space group and isotropic optical properties under cross polarized light [4]. Despite that, occurrences of birefringent garnets have been reported [5, 6, 7]. In most cases, these “uncommon” samples have specific chemical characteristics, such as hydrogrossular (Ca₃Al₂(SiO₄)_3-x(H₄O₄)_x) and “grandite” (solid solution between grossular and andradite (Ca₃(Al,Fe³⁺)₂(SiO₄)₃)) compositions. In these specimens, the optical birefringence is explained as a consequence of symmetry reduction, from cubic to tetragonal or orthorhombic, possibly due to cation ordering in octahedral sites or to the presence of a significant hydrogarnet component [5, 6, 7].

Recent findings of sector-zoned birefringent and anhydrous garnets with almandine-grossular, (Fe²⁺,Ca)₃Al₂(SiO₄)₃, composition in blueschist- and greenschist-facies metamorphic rocks from several worldwide localities (e.g. Farinole, Cazadero, Jenner and eastern Italian Alps) suggested that optically anisotropic, probably not-cubic garnets could be more common than generally assumed [8]. Cesare et al. [8] proposed that these garnets could initially grow tetragonal in low-grade (T <450 °C) rocks, with possible implications about the use of this mineral as a marker for the processes occurring in such geological contexts. However, the causes of symmetry reduction (from cubic to tetragonal) and of the associated birefringence are not clear and a more detailed investigation is required.

Here, we report new crystallographic studies on sector-zoned and anhydrous garnets with almandine-grossular composition in low-grade metamorphic metabasites from Cazadero (USA) and metapelites from the SW Tauern Window (Italy). The investigation was carried out combining both single-crystal X-ray and electron diffraction techniques. In addition, electron energy-loss spectroscopy (EELS) has been performed to detect the presence and amounts of Fe³⁺ or Mn³⁺ in the studied garnets. Our results suggest that birefringence is caused by a symmetry reduction from cubic to orthorhombic system, as proposed by [7], connected with twinning and/or subsequent exsolution processes. The reduction of symmetry is due to the ordering of cations and is supported by the statistical analyses of diffracted intensities.

 

References:

[1] Baxter EF et al. (2013) Elements 9: 415-419

[2] Ringwood AE (1991) Geochim Cosmochim Acta 55: 2083-2110

[3] Wood BJ et al. (2013) Elements 9: 421-426

[4] Grew ES et al. (2013) Am Min 98: 785-810

[5] Allen FM and Buseck PR (1988) Am Min 73: 568-584

[6] Antao SM (2013) Powder Diffr 28(4): 281-288

[7] Xu H et al. (2023) Am Min 108: 572-583

[8] Cesare B et al. (2019) Sci Rep 9: 14672