Deerite has been found in layers of Fe-rich metasediments, metamorphosed under conditions of blueschist to eclogite facies (T = ca. 480°C). Geochemical comparisons with other deerites described in the literature show that the Ile de Groix deerites most closely resemble the Mn-poor, alpine type, and Sifnos deerites. The P-T conditions of the Ile de Groix metamorphism do not conform with the stability field of deerite. In the assemblages examined here deerite seems to be a relic from earlier stages of prograde metamorphism.

Deerite in two meta-ironstones from Papua New Guinea coexists with quartz-albite-riebeckite-dannemorite-magnetite-spessartine-stilpnomelane-apatite-Ca/Mn carbonates and quartz-albite-crossite-ilmenite-spessartine-stilpnomelane-apatite respectively. Geochemical comparisons in the system FeO-MnO-MgO suggest that the P.N.G. deerites most closely resemble Franciscan types. However, the metamorphic assemblages more closely resemble Alpine types indicating metamorphism close to the lower P-T limit of deerite stability. Consistent differences in whole-rock and internal oxidation ratios between the two specimens suggest that fo2 may be important in determining deerite chemistry. A redox equation is suggested linking changes in deerite internal oxidation ratio with variable OH content.

Calderitic garnet and manganoan deerite have been found in Al-poor, Fe-Mn-rich quartzites associated with metabasic schists on the island of Andros, Cycladic blueschist belt, Greece. Calderitic garnet (30–45 mole % calderite, 55–70 mole % andradite, 0–2 mole % spessartine) occurs in paragenesis with quartz, rhodonite, rhodochrosite, Ni-Mn-bearing magnetite (2–20 mole % trevorite, 6–12 mole % jacobsite, 73–91 mole % Fe3O4), baryte, and orthite. Deerite is associated with quartz, garnet (rich in spessartine, almandine, and andradite), phengite, aegirine, magnetite (containing up to 12 mole % jacobsite), and crossite/riebeckite. Deerite contains 33.0–34.4 wt. % SiO2, 0.53–5.5 wt. % TiO2, 0.09–0.31 wt. % Al2O3, 11.9–33.9 wt. % MnO, 17.5–44.2 wt. % FeOtot, and 1.5–2.8 wt. % MgO. MnO contents correspond to 29–77 cation % Mn2+ per total R2+ cations. Deerites from Andros are thus closer to the hypothetical end-member mangano-deerite, Mn122+Fe3+6Si12O40(OH)10, than any other analysis of deerite so far published.

Deerite, a typical mineral of Fe-rich metacherts metamorphosed under blueschist conditions, is not rare, but known occurrences have up to now been restricted mainly to the Tethyan collisional zone and the Western Cordillera of North America. We describe a first occurrence in the high-pressure nappes of the Escambray Massif, Cuba, in the assemblage deerite + Mg-Al-poor riebeckite + magnetite + quartz ± garnet ± phengite ± aegirine. This assemblage typically forms during exhumation and accompanies late, stress-free annealing of the quartz matrix. Mg-Al-poor riebeckite overgrows older, large, oriented crystals of glaucophane, ferroglaucophane and Mg-Al-rich riebeckite (‘crossite’) during deerite formation. Early-formed hematite was largely replaced by magnetite. Deerite is very close to ideal composition, attaining >99% Si12O40(OH)5, allowing direct application of the experimentally determined P-T-fO2 stability field (Lattard and Le Breton, 1994). In combination with oxygen-isotope thermometry on magnetite-quartz, the crystallization conditions of the deerite-bearing assemblage can be constrained to ∼470°C, >15 kbar, and an oxygen fugacity restricted closely to the quartz-fayalite-magnetite buffer (fO2 ≈ 10−23 bar). Thus, the late-stage P-T path ofthe metacherts mirrors a steep P-T gradient of 10°C/km or less, requiring subduction ofthis part ofthe Antillean Island Arc to be still active during exhumation of the Escambray nappes.