Manganoan deerite and calderitic garnet from high-pressure metamorphic Fe-Mn-rich quartzites on Andros Island, Greece

Abstract

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 ~ Fe304) , baryte, and orthite. Deerite is associated with quartz, garnet (rich in spessartine, almandine, and andradite), phengite, aegirine, magnetite (containing up to 12 mole ~o jacobsite), and crossite/riebeckite. Deerite contains 33.0-34.4 wt. ~ SiO2, 0.53-5.5 wt. ~ TiO 2, 0.09-0.31 wt. ~ A1203, 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 ~ Mn 1+ per total R 2+ cations. Deerites from Andros are thus closer to the hypotheticat end-member mangano-deerite, 2 + 3 + 9 Mn12 Fe6 $112040(OH)10, than any other analysis of deerite so far published.

Crystal chemistry of natural Μn3+-bearing calderite-andradite garnets from Otjosondu, SWA/Namibia

“…amphibolite and granulite facies (Lattard and Schreyer, 1983;Dasgupta et al, 1987;Reinecke, 1987). Garnets with considerable amounts of the calciente molecule Mn 3 + Fe 3 + (Si0 4 ) 3 are much rarer.…”

Deerite from Papua New Guinea

“…samples most closely resemble the Franciscan types, being characterised by higher MnO values, but they differ in that they contain significantly higher levels of MgO. In this respect they resemble manganoan deerites from the Cycladic blueschists of Greece which contain from 1.5 to 2.8% MgO (Reinecke, 1987). Evidence of whole-rock chemical control is suggested by the differing compositions of the P.N.G.…”

Deerite from Ile de Groix, Brittany, France

“…Fig. 7 shows a plot of some published deerite analyses (Worthing, 1987;Reinecke, 1987; former data recalculated by Verni6 et al, 1986), together with those from Ile de Groix. The ideal formula of deerite requires 12Si per unit, but microprobe analyses recalculated to 30 cations often reveal this value to be higher.…”

“…According to Muir Wood (1979) it can be caused by variations in the hydroxyl content and octahedral cation vacancies. In the deerites examined here, the Si content varies (Agrell and Gay, 1979;Bocquet and Forette, 1973;Dal Piaz and Molin, 1978;Muir Wood, 1979), open asterisks--Shuksan Unit (Brown, 1974), closed asterisks--Sifnos (Schliestedt, 1978), open squares--Ile de Groix (Kienast, 1983; this study), closed squares--Corsica (Verni6 el al., 1986), open circles--Papua New Guinea (Worthing, 1987), closed circles--Andros (Reinecke, 1987 …”

“…Each particular deerite seems to be stable only in a relatively narrow range of fo2 conditions (Lattard and Schreyer, 1981;cf. Verni6 etal., 1986, andReinecke, 1987). This paper reports on the occurrence of deerite in Fe-rich metacherts from Ile de Groix, Brittany, France.…”