The aenigmatite-rhonite mineral group

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.