Original paper

The aenigmatite-rhönite mineral group

Kunzmann, Thomas

European Journal of Mineralogy Volume 11 Number 4 (1999), p. 743 - 756

33 references

published: Jul 16, 1999
manuscript accepted: Mar 29, 1999
manuscript received: Dec 8, 1997

DOI: 10.1127/ejm/11/4/0743

BibTeX file

ArtNo. ESP147051104003, Price: 29.00 €

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Abstract The aenigmatite-rhönite mineral group consists of eight minerals: Aenigmatite, rhönite, serendibite, kri- novite, welshite, dorrite, wilkinsonite and h0gtuavite. The general chemical formula of the minerals in this group may be written as {X2} [Y6] (Z6) O20, with {X} eightfold coordinated Na+, Ca2+ and [Y] sixfold coordinated Mg2+, Fe2+, Fe3+, Ti4+, AP+, Mn2+, Cr3+, Ti3+, Ca2+, Sb5+, Nb5+ and As5+, and (Z) fourfold coordinated Si4+, Al3+, Fe3+, Be2+ and B3+. There are two subgroups: a sodic group, including the minerals aenigmatite, krinovite and wilkinsonite, and a calcic group with rhönite, serendibite, dorrite, welshite and hpgtuavite. 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, skams, limestone-basalt contacts and meteorites, but mostly as accessories. Experimental data on stability are available only for aenigmatite and rhönite. Aenigmatite was synthesized at 700°C/ 1000 bars and 750°C/500 bars by Thompson & Chisholm (1969) and Lindsley (1969). The oxygen fugacity is con- strained be lower than the fayalite-quartz-magnetite = FQM buffer. Rhönite is stable from 850°-1000°C/l bar to at least 5 kbar, 900 to IIOO0C (Kunzmann, 1989). There is no limit on oxygen fugacity. In alkali-basaltic rocks, the sta- bility is restricted to pressures lower than 600 bars and temperatures from 840 to 1200°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: I: Siiv + NaVIII ^ AlIV + СаVIII; 2: Siiv + Mgvl ^ Aliv+Aivi; 3: TiVI + MgvI ⇄ 2A1VI; 4: MgVI Fe2+VI; 5: AIV ⇄ BIV; 6: SiIV + BeIV ⇄ 2 AlIV; 7: Sb5+VI + 2 MgVI ⇄ 3 Fe3+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 SiIV + 2 NaVIII 2 AlIV + 2 CaVIII; II: 2 SiIV + 2 (M2+)VI ^ 2 AlIV + 2 (M3+)VI; III: 2 Ti4+VI + 2 (M2+)VI ⇄ 4 (M3+)VI. This results in a rectangular poly- hedron for the aenigmatite-rhönite group, in which ten sub-volumes can be assigned to ten end-members.