Oxo-magnesio-hastingsite, NaCa₂(Mg₂Fe³⁺₃ )(Al₂Si₆)O₂₂O₂, a new anhydrous amphibole from the Deeti volcanic cone, Gregory rift, northern Tanzania

Abstract

Oxo-magnesio-hastingsite, ideally NaCa2(Mg2Fe3+3)(Al2Si6)O22O2, 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 (–), α = 1.706 (2), β = 1.715(2), γ = 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(Ca1.87Na0.14Mn0.01)Σ2.02(Mg3.27Fe3+1.25Ti0.44Al0.08)Σ5.04(Al1.80Si6.20O22)(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) (), 2.349 (29) () 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.

Lumping and splitting: Toward a classification of mineral natural kinds

“…Prior to that major effort, we lump 26 calcic amphibole species, including those with (Mg,Fe 2+ ,Fe 3+ ,Al,Si) as the essential tetrahedral and octahedral cations, as well as Na-and K-bearing species and OH, F, and Cl varieties, into "hornblende." Calcic amphiboles excluded from the hornblende group include 4 species of the compositionally distinct Si-poor sadanagaite group (Shimazaki et al 1984;Nikondrov et al 2001), which come from sub-silicic skarns, and oxo-magnesiohastingsite (Zaitsev et al 2013), which is a product of prior amphibole dehydration and oxidation. We acknowledge that the lumping of most calcic amphiboles into one root mineral kind, "hornblende," is a preliminary decision, as a number of compositional attributes point to specific paragenetic modes.…”

Potassic-Hastingsite from the Kedrovy District (East Siberia, Russia): Petrographic Description, Crystal Chemistry, Spectroscopy, and Thermal Behavior

“…Deeti volcanic cone, Gregory rif, northern Tanzania [21] O- Mg-Hs This work is devoted to the description of a new occurrence of potassic-hastingsite found in the Kedrovy district (East Siberia, Russia). Despite the fact that the mineral and the name were approved by IMA-CNMMN in 2018, and the description of the holotype was published in 2020 [10], this species of hastingsite was widely known and reported in early works.…”

“…The observed variations in the end-member formula recognized by the IMA for hastingsite are reported in Table 1. Detailed crystal-chemical properties of the minerals have been investigated, sometimes using a multi-technique approach consisting of electron probe microanalysis, Mössbauer spectroscopy, and single-crystal X-ray refinements [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. However, relatively few studies on their structural behavior under non-ambient conditions have been made [22,23].…”

Oxo-mangani-leakeite from the Hoskins mine, New South Wales, Australia: occurrence and mineral description

“…Moreover, accurate characterization of the amount and the mechanism of O 2incorporation in amphiboles provides useful constraints both for the recalculation of their unit formula and for the correct determination of their petrogenetic conditions. The first oxo-end-member identified (in 1884; Dana, 1892) compositions referring to root-names kaersutite (ferri-kaersutite, Gentili et al, 2014 ) and hastingsite (oxomagnesio-hastingsite, Zaitsev et al, 2013 ) are typical of igneous rocks and of crystallization of calcium amphiboles under HT and low f H 2 O or highly oxidizing conditions. However, amphibole compositions with O 2dominant among W anions have more recently also been found in sodium amphiboles.…”