Andrémeyerite, wilkinsonite, and other accessory minerals occurring in a residual melt derived from ijolite- and melilitolite-forming magma from the Cenozoic Podhorní vrch Hill volcano (Czechia)

Podhorn vrch Hill is a small, isolated complex product of alkaline volcanism located in the Eger (Ohe) Rift, a part of the European Cenozoic rift system.Within effusive units there occurr fine-to-coarse grained nepheline-clinopyroxene melilite pegmatitoid segregations or xenoliths, classified as ijolite and melilitolite (turjaite).Coarse-grained xenoliths have a cavernous texture.This investigation focuses on the minerals present as the latest filling of the interstices between rock-forming minerals.These infills are typified by a chaotic microstructure of mineral crystals and aggregates, the presence of partially devitrified glass, and the dendritic-to-skeletal crystals.Andrmeyerite occurrs as euhedral, skeletal crystals, up to 200 80 m in size, with a composition close to the ideal formula.Aenigmatite-wilkinsonite series minerals are present as short prismatic crystals.Ti 4+ in the formula unit varies from 0 to 1.46 apfu (on the basis of 40 oxygen anions), Fe 2+ from 9.30-5.39apfu, and Fe 3+ from 4.58-0.93apfu, indicating the presence of both aenigmatite and wilkinsonite.Minerals of the pyrochlore group are "oxynatropyrochlore" (Sr up to 0.35 apfu), and in some samples zero-valent-dominant pyrochlores rich in Na, Sr, and Ba.The accompanying materials include fayalite (Fa 0.92 ) and strongly peralkaline glass with high contents of Fe (15.7 wt.%), Ba (5.7 wt.%), and Sr (1.3 wt.%).Within the late-stage domains, primary fluorapatite was affected by metasomatic processes, whereas Ca 2+ is replaced by Sr 2+ in its outer parts, forming fluorcaphite, fluorsigaiite, and a yet IMA-unapproved mineral with Ca 3 Sr 2 (PO 4 ) 3 F composition.