Abstract

The aenigmatite-rhonite mineral group consists of eight minerals: Aenigmatite, rhonite, serendibite, krinovite, welshite, dorrite, wilkinsonite and hogtuavite. The general chemical formula of the minerals in this group may be written as {X2} [Y6] (Z6) O 20 , with {X} eightfold coordinated Na (super +) , Ca (super 2+) and [Y] sixfold coordinated Mg (super 2+) , Fe (super 2+) , Fe (super 3+) , Ti (super 4+) , Al (super 3+) , Mn (super 2+) , Cr (super 3+) , Ti (super 3+) , Ca (super 2+) , Sb (super 5+) , Nb (super 5+) and As (super 5+) , and (Z) fourfold coordinated Si (super 4+) , Al (super 3+) , Fe (super 3+) , Be (super 2+) and B (super 3+) . There are two subgroups: a sodic group, including the minerals aenigmatite, krinovite and wilkinsonite, and a calcic group with rhonite, serendibite, dorrite, welshite and hogtuavite. The general features of the crystal structure are common to all the minerals of this group. These minerals occur in a wide range of rock types, e.g. alkaline lavas, sodium-rich intrusives, granitic gneisses, skarns, limestone-basalt contacts and meteorites, but mostly as accessories. Experimental data on stability are available only for aenigmatite and rhonite. Aenigmatite was synthesized at 700 degrees C/1000 bars and 750 degrees C/500 bars by Thompson & Chisholm (1969) and Lindsley (1969). The oxygen fugacity is constrained be lower than the faylite-quartz-magnetite = FQM buffer. Rhonite is stable from 850 degrees -1000 degrees C/1 bar to at least 5 kbar, 900 to 1100 degrees C (Kunzmann, 1989). There is no limit on oxygen fugacity. In alkali-basaltic rocks, the stability is restricted to pressures lower than 600 bars and temperatures from 840 to 1200 degrees C (Kunzmann, 1989). The chemistry of this group is complex, due to the flexibility of the structure. The structural formulae of 192 available analyses can be described in terms of seven substitutions: 1: Si IV +Na VIII <--> Al IV +Ca VIII ; 2: Si IV +Mg VI <--> Al IV +Al VI ; 3: Ti VI +Mg VI <--> 2Al VI ; 4: Mg VI <--> Fe (super 2+VI) ; 5: Al IV <--> B IV ; 6: Si IV +Be IV <--> 2Al IV ; 7: Sb (super 5+VI) +2Mg VI <--> 3Fe (super 3+VI) . The theoretical number of end-members (and names) resulting from these seven substitutions is immense. A simplified nomenclature is proposed here based on three substitutions. I: 2 Si IV +2Na VIII <--> 2Al IV +2Ca VIII ; II: 2Si IV +2(M (super 2+) ) VI <--> 2Al IV +2(M (super 3+) ) VI ; III: 2Ti (super 4+VI) +2(M (super 2+) ) VI <--> 4(M (super 3+) ) VI . This results in a rectangular polyhedron for the aenigmatite-rhonite group, in which ten sub-volumes can be assigned to ten end-members.

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