Astrophyllite

Abstract: We describe titanite with unusually high contents of Na, Nb, and Zr from a hydrothermal natrolite-rich vein cutting kalsilite‐ nepheline syenite at Mount Rasvumchorr, Khibina peralkaline complex, in the Kola Alkaline Province, Russia. This titanite is associated with astrophyllite, ceriobetafite, yttrobetafite, henrymeyerite, and banalsite. We recognize four generations of titanite: nearly stoichiometric titanite-I, titanite-II, containing up to 16.4 wt.% Nb2O5 (0.25 apfu Nb) and 3.2% Na2O (0.21 apfu Na), titanite-III, with up to 9% ZrO2 (0.15 apfu Zr) and zoned from 12.4 to 2.2% Nb2O5, and (Nb‐Zr)-poor titanite-IV, with up to 3.6% Al2O3 and 2.2% Fe2O3. In titanite-II, substitution of Nb at the octahedral site is accompanied by Na at the seven-fold site and, probably, by (F,OH) ‐ and vacancies. A compositional analogue of titanite-III, and synthetic titanite containing 0.25 apfu Zr, similar to the most Zr-rich titanite known, have been prepared by standard ceramic techniques, and their crystal structure determined by Rietveld refinement of powder X-ray-diffraction patterns. The synthetic variants doped with Zr, or with Zr, Na, and Nb, adopt space group A2/a, and consist of distorted YO7 polyhedra, XO6 octahedra, and SiO4 tetrahedra. The (Ca0.85Na0.15) (Ti0.70Zr0.15Nb0.15)OSiO4 analogue of natural titanite from the Rasvumchorr zeolite vein contains the least-distorted coordination polyhedra. The polyhedra in Ca(Ti0.75Zr0.25)OSiO4 are moderately distorted, and the most strongly distorted polyhedra are those in the CaTiOSiO4 end-member.

Abstract: Aenigmatite is common in many trachytes, phonolites and agpaitic nepheline syenites. Petrographic evidence suggests that the aenigmatite in these rocks arises by the reaction of Ti-magnetite with a peralkaline silica-undersaturated liquid, and it is postulated that a no-oxide field, where aenigmatite is stable, exists in alkaline undersaturated magmas. This field is similar to that found in silicic liquids but lies below the FMQ buffer curve in \(T - f_{{\text{O}}_{\text{2}} } \) space and is probably confined within narrow limits of temperature and oxygen fugacity. The hydrated equivalent of aenigmatite is possibly astrophyllite and the latter mineral is frequently associated with Na-amphiboles in natural rocks. This suggests that the stability field of astrophyllite is similar to that of Na-amphiboles with respect to temperature and \(f_{{\text{O}}_{\text{2}} } \).

Abstract: The Astrophyllite Bay Complex in East Greenland (part of the Palaeogene North Atlantic Igneous Province) consists of an alkaline diorite plug, with detached trachyandesitic pillows, surrounded by co-magmatic syenite that was emplaced into Archaean basement. The diorite intrusion has yielded a 47.11 ± 0.68 Ma Rb-Sr isochron age. Saw-cut profiles through pillow-syenite-gneiss sections have been taken to resolve close spatial elemental and isotopic (Sr-Nd-Hf-Pb-O) variations. The diorite and syenite formed from alkaline basaltic, mantle-derived, melts with complex histories of prolonged assimilation and fractional crystallisation. Each evolved to different extents in separate magma chambers during the establishment of new plumbing systems in the Kangerlussuaq area. The diorite is dominated by lower crustal, granulite facies contamination, whereas the syenite shows evidence for greater degrees of upper crustal amphibolite facies contamination, indicating stalling and fractionation of magmas at different levels within the crust. The syenite and diorite magmas were subsequently emplaced as separate pulses into the basement gneisses at Astrophyllite Bay giving rise to superimposed local contamination trends between pillow/syenite and syenite/gneiss, respectively.