Garnet is a key mineral of several metamorphic and igneous rock types in the lithosphere. Despite being traditionally considered the archetypal cubic mineral, crystallizing in Ia<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\:\overline{3}\)</EquationSource> </InlineEquation>d space group, there is increasing evidence that almandine and spessartine garnets from low-temperature metapelites and metabasites are optically anisotropic, and therefore possibly not cubic. Here, we study the best-known occurrence of optically anisotropic garnet - the metabasites of the Franciscan Complex (Cazadero, USA) - integrating polychromatic polarization microscopy (PPM), electron microprobe analysis (EMPA) with and without flank method for Fe3+/ΣFe estimation, field emission scanning electron microscopy coupled with focus ion beam (FIB−FESEM), electron energy-loss spectroscopy (EELS), single-crystal X-ray diffraction (SCXRD) and electron backscatter diffraction (EBSD). The result of this multi-analytical approach is a comprehensive optical, chemical and crystallographic characterization of these birefringent garnets. The Cazadero garnet has optical sector zoning according to twelve pyramidal sectors forming a rhombic dodecahedron. Within sectors, a concentric oscillatory zoning is also observed. From a chemical point of view, no sector zoning is detected, whereas the concentric oscillatory zoning is maintained. Chemical maps and profiles show a typical growth distribution characterized by a bell-shaped distribution of Mn. Considering the Fe3+ contents measured by both the EMPA flank method and EELS, a distinctive chemical zoning ranges from Alm53Sps20Grs16Pyr01Adr10 in the core to Alm66Sps04Grs20Pyr03Adr07 in the rim. Therefore, the analyzed garnets are almandine with a non-negligible andradite component, that reaches up to 10 mol%. Accurate X-ray diffraction refinements reveal that garnets are orthorhombic with I2/a12/d (Fddd) space group, with pseudo-tetragonal unit cell (c – a from − 0.009 Å to 0.014 Å). Symmetry lowering is due to Al-Fe3+ ordering within the octahedral sites, which is also supported by Y site distortion (difference in average < Y – O> distance from 0.0021 to 0.0041 Å). Our study provides new constraints on the origin of anhydrous almandine-rich garnets from low-grade metamorphic rocks occurring worldwide: we propose that their non-cubic structure (orthorhombic at Cazadero) is a growth feature, and is given by their Fe3+ content, coupled with low Mg and high Ca.

Low-temperature, anhydrous (Fe-Ca)-rich garnets do not grow cubic: new evidence from blueschists of the Franciscan Complex (Cazadero, USA)

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2026年6月27日

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Sofia Lorenzon,
Enrico Mugnaioli,
Cristian Biagioni,
Dominik C. Hezel,
Kenneth J. T. Livi,
Tommaso Tacchetto,
Fabrizio Nestola,
Bernardo Cesare

摘要

Garnet is a key mineral of several metamorphic and igneous rock types in the lithosphere. Despite being traditionally considered the archetypal cubic mineral, crystallizing in Ia \(\:\overline{3}\) d space group, there is increasing evidence that almandine and spessartine garnets from low-temperature metapelites and metabasites are optically anisotropic, and therefore possibly not cubic. Here, we study the best-known occurrence of optically anisotropic garnet - the metabasites of the Franciscan Complex (Cazadero, USA) - integrating polychromatic polarization microscopy (PPM), electron microprobe analysis (EMPA) with and without flank method for Fe³⁺/ΣFe estimation, field emission scanning electron microscopy coupled with focus ion beam (FIB−FESEM), electron energy-loss spectroscopy (EELS), single-crystal X-ray diffraction (SCXRD) and electron backscatter diffraction (EBSD). The result of this multi-analytical approach is a comprehensive optical, chemical and crystallographic characterization of these birefringent garnets. The Cazadero garnet has optical sector zoning according to twelve pyramidal sectors forming a rhombic dodecahedron. Within sectors, a concentric oscillatory zoning is also observed. From a chemical point of view, no sector zoning is detected, whereas the concentric oscillatory zoning is maintained. Chemical maps and profiles show a typical growth distribution characterized by a bell-shaped distribution of Mn. Considering the Fe³⁺ contents measured by both the EMPA flank method and EELS, a distinctive chemical zoning ranges from Alm₅₃Sps₂₀Grs₁₆Pyr₀₁Adr₁₀ in the core to Alm₆₆Sps₀₄Grs₂₀Pyr₀₃Adr₀₇ in the rim. Therefore, the analyzed garnets are almandine with a non-negligible andradite component, that reaches up to 10 mol%. Accurate X-ray diffraction refinements reveal that garnets are orthorhombic with I2/a12/d (Fddd) space group, with pseudo-tetragonal unit cell (c – a from − 0.009 Å to 0.014 Å). Symmetry lowering is due to Al-Fe³⁺ ordering within the octahedral sites, which is also supported by Y site distortion (difference in average < Y – O> distance from 0.0021 to 0.0041 Å). Our study provides new constraints on the origin of anhydrous almandine-rich garnets from low-grade metamorphic rocks occurring worldwide: we propose that their non-cubic structure (orthorhombic at Cazadero) is a growth feature, and is given by their Fe³⁺ content, coupled with low Mg and high Ca.