Publication Date:
2013-08-22
Description:
Oxo-magnesio-hastingsite, ideally NaCa 2 (Mg 2 $${\mathrm{Fe}}_{3}^{3+}$$ )(Al 2 Si 6 )O 22 O 2 , is a new anhydrous amphibole from the Deeti volcanic cone in the Gregory rift (northern Tanzania). The mineral occurs as megacrysts up to 12 cm in size in crystal-rich tuff. Oxo-magnesio-hastingsite is brown with a vitreous lustre and has a perfect {110} cleavage. The measured density is 3.19(1) g/cm 3 . Ferri-kaersutite is biaxial (–), α = 1.706 (2), β = 1.715(2), = 1.720(2) (Na light, 589 nm). 2V (calc.) = 73°. Dispersion: r 〉 v, weak; orientation: Y = b ; Z ^ c = 8°; pleochroism: strong, Z: dark brown, Y: brown, X: light brown. The average chemical formula of the mineral derived from electron microprobe analyses, Mössbauer spectroscopy and direct water determination is (Na 0.67 K 0.33 ) 1.00 (Ca 1.87 Na 0.14 Mn 0.01 ) 2.02 (Mg 3.27 $${\mathrm{Fe}}_{1.25}^{3+}$$ Ti 0.44 Al 0.08 ) 5.04 (Al 1.80 Si 6.20 O 22 )(O 1.40 OH 0.60 ) 2.00 . It has monoclinic symmetry, space group C2/m and unit-cell parameters a = 9.8837(3), b = 18.0662(6), c = 5.3107(2) Å, β = 105.278(1)°, V = 914.77(5) Å 3 , Z = 2. The five strongest powder-diffraction lines [ d in Å, ( I/Io ), hkl ] are: 3.383 (62) (131), 2.708 (97) (151), 2.555 (100) (2I02), 2.349 (29) (3I51) and 2.162 (36) (261). The isotopic composition of H and O, as well as the concentration of trace elements in oxo-magnesio-hastingsite suggest its formation from a melt originated from a mantle source metasomatized by slab-derived fluids.
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
Permalink