Oxo-magnesio-hastingsite, NaCa2(Mg2Fe3+

A. N. Zaitsev, E. Yu Avdontseva, S. N. Britvin, A. Deméný, Z. Homonnay, T. E. Jeffries, J. Keller, V. G. Krivovichev, G. Markl, N. V. Platonova, O. I. Siidra, J. Spratt, T. Vennemann

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Oxo-magnesio-hastingsite, ideally NaCa2(Mg2Fe 3+ 3)(Al2Si6)O22O 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/cm3. Ferri-kaersutite is biaxial (-), a = 1.706 (2), b = 1.715(2), g = 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 (Na0.67K0.33)Σ1.00(Ca 1.87Na0.14Mn0.01)Σ2.02(Mg 3.27Fe3+ 1.25Ti0.44Al 0.08)Σ5.04(Al1.80Si6.20O 22)(O1.40OH0.60)Σ2.00. 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, b = 105.278(1)o, V = 914.77(5) Å3, 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) (202), 2.349 (29) (351) 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.

Original languageEnglish
Pages (from-to)2773-2792
Number of pages20
JournalMineralogical Magazine
Issue number6
Publication statusPublished - Aug 1 2013


  • Deeti cone
  • Gregory rift
  • H and O isotopic composition
  • Mössbauer and IR spectroscopy
  • Oxo-magnesio-hastingsite
  • Tanzania
  • X-ray diffraction
  • chemical composition
  • crystal structure
  • new mineral
  • oxo amphibole group

ASJC Scopus subject areas

  • Geochemistry and Petrology

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    Zaitsev, A. N., Avdontseva, E. Y., Britvin, S. N., Deméný, A., Homonnay, Z., Jeffries, T. E., Keller, J., Krivovichev, V. G., Markl, G., Platonova, N. V., Siidra, O. I., Spratt, J., & Vennemann, T. (2013). Oxo-magnesio-hastingsite, NaCa2(Mg2Fe3+. Mineralogical Magazine, 77(6), 2773-2792. https://doi.org/10.1180/minmag.2013.077.6.06