Ferri-kaersutite, NaCa2(Mg3TiFe3+)(Si6Al2)O22O2, a new anhydrous amphibole from Harrows Peak, Northern Victoria Land, Antarctica

• Published in: American Mineralogist
• Main Authors: GENTILI S., BIAGIONI, CRISTIAN, COMODI P., PASERO, MARCO, McCAMMON C., BONADIMAN C.
• Other Authors: Gentili, S., Biagioni, Cristian, Comodi, P., Pasero, Marco, Mccammon, C., Bonadiman, C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2016
• Subjects: Ferri-kaersutite; new mineral; crystal chemistry; oxo-amphibole; Victoria Land; Antarctica; Antarc*
• Online Access: https://hdl.handle.net/11568/744091; https://doi.org/10.2138/am-2016-5204

Ferri-kaersutite, ideally NaCa2(Mg3TiFe3+)(Si6Al2)O22O2, is a new oxo-amphibole from Harrow Peaks, Northern Victoria Land, Antarctica. It occurs as brown prismatic crystals, up to 200 μm in length, with a vitreous luster, and a perfect {110} cleavage. Ferri-kaersutite is associated with forsterite, diopside, and Cr-bearing spinel. Chemical analyses, by a combination of electron microprobe, SIMS, and 57Fe Mössbauer spectroscopy, gave the following results (in wt%): SiO2 41.69, TiO2 5.30, Al2O3 13.65, Cr2O3 0.09, Fe2O3 4.52, MgO 15.54, CaO 11.03, MnO 0.11, FeO 2.83, Na2O 2.88, K2O 0.96, H2O 0.70, F 0.24, Cl 0.08, O=(F,Cl) –0.12, sum 99.50. On the basis of 24 anions per formula unit, the formula is (Na0.816K0.179)Σ0.995(Ca1.726Fe2+0.258Mn0.014)Σ1.998(Mg3.383Fe2+0.088Ti0.582Fe3+0.497Al0.439Cr0.011)Σ5.00 (Si6.089Al1.911)Σ8.00O22[O1.187(OH)0.682F0.111Cl0.020]Σ2.00. Ferri-kaersutite is monoclinic with space group C2/m. Its unit-cell parameters are a = 9.8378(8), b = 18.0562(9), c = 5.3027(4) Å, β = 105.199(9)°, V = 908.99(13) Å3, Z = 2. The five strongest reflections in its X‐ray powder diffraction pattern [d in Å (relative visual intensity, hkl)] are: 8.4 (s, 110), 3.379 (ms, 131), 3.115 (ms, 310), 2.707 (s, 151), 2.598 (ms, 061). The crystal structure of ferri-kaersutite has been refined on the basis of 1783 observed reflections [Fo > 4σ(Fo)] with a final R1 = 0.038. The relatively large equivalent isotropic displacement parameter at M(1), with respect to those at M(2) and M(3) sites, together with the short M(1)–O(3) distance, suggest the occurrence of Ti4+ at the M(1) site, whereas the small octahedral distortion at this site suggests a low Fe3+ occupancy. This element is mainly hosted at the M(2) and M(3) sites. The occurrence of amphiboles in the magma source region is notably relevant. The melting of Ti-rich amphibole in the lithosphere and subsequent degrees of melt/host peridotite reactions are able to produce melts that account for the compositional spectrum ranging from extreme alkaline lavas to the ...