Almandine-1O and spessartine-1O in the Franciscan blueschists from Laytonville Quarry, northern California: petrographic, optical and compositional features

The best example of optically anisotropic, sector-zoned garnet has just been discovered in stilpnomelane-garnet ironstones from Laytonville Quarry. The analyzed sample was metamorphosed under low-T (<400 °C) blueschist facies conditions, and consists of garnet and stilpnomelane in similar amounts, with minor quartz and accessory titanite, apatite, sulfides and ilmenite.

Garnet is subhedral to euhedral and < 300 µm in diameter. It locally shows rim dissolution and replacement by stilpnomelane. Optically, all garnet crystals show a weak birefringence even under conventional crossed polars. Analysis by polychromatic polarization confirms the already known sector-zoned pattern of birefringence, with six pairs of opposed pyramidal sectors displaying equal optical orientation. The twelve pyramids define the overall rhombic dodecahedral shape of the crystals. The optical sector zoning is accompanied by a subtle oscillatory concentric zoning, more developed at crystal rims where chemical zoning in Fe and Mn is strongest.

Optical measurements reveal that the birefringence in this garnet is 0.00053, and indicate that the optic axes in each sectors are oriented tangentially and form angles of 90° and 60° to each other.

Chemically, garnet displays regular concentric growth zoning with a well-developed bell-shaped Mn profile, but with a reversal at the rim. Considering all iron as FeO, typical compositions are: core = Sps₄₃Alm₄₁Grs₁₅Pyr₀₁ and rim = Alm₆₁Sps₂₀Grs₁₇Pyr₀₂. Notably, the pyrope content is extremely low, and X_Mg is < 0.03. The chemical zoning has no relationship with the optical sector zoning. Rather, some steps in the chemical zoning profile overlap with the optical concentric oscillations.

Ferric iron in the garnet was measured by electron microprobe using the flank method: the Fe³⁺/Fe_tot is in the range 5-8 %. It follows that the andradite component is not negligible, and decreases the grossular content, so that the compositional zoning becomes: core = Alm₃₉Sps₄₃Grs₁₃Pyr₀₁Adr₀₃ and rim = Alm₆₀Sps₂₁Grs₁₁Pyr₀₂Adr₀₇. Therefore, the garnet can be classified as spessartine in the inner core, and almandine in the rest of each crystal. The measured Fe³⁺ content of garnet has been used in the subsequent refinement of the crystal structure.

Transmission FTIR spectra recorded from garnets show weak absorption in the OH-stretching region, suggesting garnets contain trace amounts of OH and no molecular H₂O. However, as garnets contain numerous inclusions, it is not clear whether the observed OH-signal is due to garnet or OH-bearing mineral inclusions.

Analysis of the blueschist-facies rocks from the Laytonville Quarry deepens our knowledge of non-cubic Fe-Mg-Mn-Ca garnets increasingly observed in low-T metamorphic rocks, and allows discussion of the relationships of (non) parallelism among their optic and crystallographic axes.