Original paper

Ferronigerite with dominant substitution TiSn1 in muscovite+chlorite aggregate from massive quartz nodule associated with a petalite-rich aplite-pegmatite of the Barroso-Alvao pegmatite field, Northern Portugal

Novák, Milan; Vieira, Romeu; Lima, Alexandre Škoda; Martins, Taniados; Anjos Ribeiro, Maria


Zoned to homogeneous, euhedral to subhedral tabular crystals (up to ∼ 100 μm in size) of ferronigerite-6N6S ≫ ferronigerite-2N1S with Ti-enriched core occur in fine-grained muscovite+chlorite aggregate from a quartz nodule enclosed in a mica schist host. The quartz nodule is located close to the petalite-rich aplite-pegmatite dike Lousas from the Barroso-Alvão pegmatite field. Ferronigerite-6N6S exhibits dominant substitution TiSn−1, Sn/(Sn+Ti)=0.99-0.61 and Fe > Zn > Mg > Mn. Normalization to 66 cations yielded ΣSn+Ti from 5.94 apfu in Ti-rich to 6.64 apfu in Sn-rich compositions and high Σnegative charges 190.43 and 191.95; normalization to 186 negative charges gave ΣSn+Ti 5.80-6.43 and low Σcations = 63.96-64.46, respectively. Calculation of Li by normalization to 66 cations and 192 negative charges (OH = 6) yielded ΣSn+Ti = 5.73-6.21 apfu and Σoxides = 99.20-100.86. These data are closest to the ideal formula of ferronigerite-6N6S although calculated Li contents of 0.88 to 1.12 wt.% Li2O (2.35-3.12 apfu) seem to be too high. The quartz nodule consists of an aggregate of zoned tourmaline, medium-grained aggregate of muscovite and fine-grained aggregate of muscovite+chlorite (likely a pseudomorph after cordierite-sekaninaite). This fine-grained aggregate contains accessory ferronigerite, fluorapatite, paragonite, beryl, staurolite, and rutile with rare small grains (≤1 μm) of Fe, Mn, Y, Nb, Ta, and Ti-oxides. The studied assemblage represents a new paragenetic type of nigerite showing obvious peraluminous signature, high amount of Fe and Mg along with close relationship to a highly evolved granitic rock. The geochemical signature of this assemblage indicates influx of highly evolved fluids enriched in F, P, B, Sn, Zn, Be, Nb, Ta and potentially Li into the quartz nodule.