Almandine-1O and Spessartine-1O in the Franciscan blueschists from Laytonville Quarry, northern California: crystallographic features

Occurrences of anhydrous garnets enriched in almandine-spessartine-grossular component showing sector-zoned birefringence are increasingly reported in low-grade metamorphic rocks in localities around the world [1, 2]. Despite several proposed hypotheses [1, 3, 4], the origin of optical anisotropy in garnet have remained unclear for a long time. Recently, reduction from cubic to orthorhombic symmetry due to Al-Fe³⁺ ordering in octahedral sites has been demonstrated to cause the birefringence of garnets from the Cazadero blueschists (Franciscan Complex, USA), strengthening the idea that garnets directly grew non-cubic in low-T metamorphic environments [1, 2].

Stilpnomelane-garnet metasediments from Laytonville Quarry (Franciscan Complex, USA), equilibrated at T < 400°C, contain a new, especially illustrative example of direct growth of non-cubic, low-T, almandine-spessartine-grossular solid solution garnets. Garnets in these rocks show optically well-defined sector zoning under polychromatic polarizing light, not corresponding to any chemical zonation. Chemically, these crystals have a spessartine-rich core, typical Mn bell-shaped distributions, and almandine-rich rims.

Crystal structure refinements, performed on grains separated from single birefringent sectors (n = 2) analyzed by single-crystal X-ray diffraction (SCXRD), determined these garnets as I2/a12/d orthorhombic (Fddd unconventional setting) with pseudo-tetragonal (which is in turn pseudo-cubic) unit-cell parameters (c > a, b; c -a = 0.002 to 0.005 Å). Slight cation ordering between Al and Fe³⁺ within octahedral sites leads to reduction from cubic to orthorhombic symmetry, where Y1 and Y2 sites are occupied by 5% and 1% of Fe³⁺, respectively. This result is supported by cation-anion bond distances, which are longer in cation sites with greater Fe³⁺, and by EPMA data processed by flank method, which indicates Fe³⁺/ΣFe of ~5 to 8% in these almandine-rich garnets. These observations further support symmetry lowering as the cause of the optical anisotropy.

Overall, the present crystallographic investigation on Laytonville Quarry samples confirms the results obtained in garnets from Cazadero blueschists [2], reinforcing the idea that common almandine-spessartine garnets grow non-cubic at low-T conditions due to a small but non-negligible andradite component coupled with Al-Fe³⁺ octahedral site ordering. Our results recommend reassessment of garnet thermodynamics properties and urge a revision to the nomenclature and classification of this key mineral in the lithosphere, in agreement with current IMA – CNMNC rules [5]. Therefore, we propose to name the orthorhombic Fe- and Mn-rich garnet end-members as almandine-1O and spessartine-1O, and to distinguish them from their cubic analogues (almandine-1C and spessartine-1C).

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

[2] Lorenzon S et al. (2025) EGU2025

[3] Griffen DT et al. (1992) Am Min 77: 399-406.

[4] Hofmeister AM et al. (1998) Am Min 83: 1293-1301

[5] Nickel EH and Grice JD (1998) Miner Petrol 64(1): 237-263