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Allanite-(Ce) alterations in the Mukinbudin Feldspar Quarries pegmatite, Mukinbudin, Western Australia

The Mukinbudin area, Western Australia, comprises a pegmatite field with pegmatite bodies classified as NYF-type [1]. Allanite-(Ce) has been found in Mukinbudin Feldspar Quarries in form of metamict masses. The allanite masses are round-shaped being up to tens of centimeters in diameter and show no distinct crystal faces. The mineral is black, brittle and glassy in appearance. It has distinctive ochre to brown coating with red tint which is up to few millimeters thick. Along with allanite, the mineral assemblage comprises albite, beryl, biotite, chalcopyrite, fergusonite-(Y), magnetite, microcline, monazite, pyrolusite, quartz, and zircon. XRD pattern of the allanite sample showed that the mineral is heavily metamictized with just a dozen diffraction poor-resolved maxima yielding a unit cell a=8.99 (2), b = 5.76 (2), c = 10.28(3) Å, β = 115.1(2)°, V=482(2) Å3. The volume of the unit cell exceeds the one of recrystallized allanite (V ~ 478 Å3) due to metamictization. The coating around allanite masses is composed mainly of bastnäsite based on XRD pattern. SEM analysis showed veinlets throughout the investigated allanite grain, whereby some of them could be observed macroscopically. Preliminary chemical analyses (EDS) showed a typical allanite-(Ce) content. The veinlets inside allanite are mostly composed of titanite. Throughout the grain there are lot of ThSiO4 inclusions which are frequently aligned in almost straight-line clusters. In some cases, ThSiO4 inclusions are surrounded by tiny grains of titanite and an Y-Ti-Fe oxide. The rim zone of the allanite masses is distinct from the rest of the grain showing a more complex composition. Generally, the rim is composed of altered allanite-(Ce) that is intersected by veins of bastnäsite-(Ce) and mica (Figure 1). ED spectra yielded fluorine in the composition of bastnäsite, but IR spectra showed also the presence of OH group, which in part could also originate from the altered allanite-(Ce) and mica.

allanite-(Ce); alteration; bastnäsite-(Ce)

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