Site distribution of Fe2+and Fe3+in the axinite mineral group: New crystal-chemical formula
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Giovanni B. Andreozzi
,
Sergio Lucchesi
Abstract
A set of nine samples of axinite, selected from 60 specimens from worldwide localities, were investigated by single-crystal X-ray diffraction, electron and ion microprobe, and 57Fe Mössbauer spectroscopy. The selected samples cover the compositional join from almost pure ferroaxinite (80%) to pure manganaxinite (95%). A new crystal-chemical formula for the axinite mineral group is proposed: VI[X1 X2 Y Z1 Z2]2IV[T1 T2 T3 T4 T5]2O30(OwOH1-w)2, where VI and IV are coordination numbers; X1 = Ca and very minor Na; X2 = Ca (in axinites) or Mn (in tinzenite); Y = Mn (in manganaxinite and tinzenite), Fe2+ (in ferroaxinite) or Mg (in magnesioaxinite), with minor Al and Fe3+; Z1 = Al and Fe3+; Z2 = Al; T1, T2, and T3 = Si; T4 = Si (and presumably very minor B); T5 = B and minor Si. Charge unbalance (w), due to heterovalent substitutions, is compensated for by O2- → OH- substitution.
From ferroaxinite to manganaxinite, cell volume increases linearly from 568.70 to 573.60 Å3. This is mainly due to an increase in the <Y-O> mean distance from 2.220 to 2.255 Å, which is directly related to the Mn population (up to 1.89 apfu). Fe3+ concentrations, as determined by 57Fe Mössbauer spectra at 80 K, sub-regularly increase up to 0.27 apfu, and three cases are evidenced: (1) Fe3+ << Fe2+ (or no Fe3+), in ferroaxinite; (2) Fe3+ < Fe2+, in intermediate compositions, and (3) Fe3+ > Fe2+ (or only Fe3+), in manganaxinite.
Chemical and structural data were co-processed via a computer minimization program to obtain the cation distribution scheme. Adopting the Hard-Sphere Model, empirical cation-oxygen distances were refined for every cation in the axinite structure. The results revealed that Fe2+ is ordered at the octahedral Y site (up to 1.61 apfu), whereas Fe3+ is ordered at the octahedral Z1 site (up to 0.26 apfu) and is almost absent in the smallest Z2 site, which is fully populated by Al. The observed Fe3+ partitioning is in agreement with the structural results, which show that the Z1 octahedron is always larger than Z2. Moreover, no Fe3+ is found at the tetrahedral sites, but Si → B substitution occurs at T5. The continuous Y dimensional increase from ferroaxinite to manganaxinite involves progressive enlargement of the edge-sharing Z1 octahedron. As a consequence, the Z1Fe3+ → Z1Al3+ substitution is structurally favored toward manganaxinite and points to a new end-member with the suggested name “ferri-manganaxinite.”
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- First-principle study of polytype structures of 1:1 dioctahedral phyllosilicates
- Calorimetry of liquids in the system Na2O-Fe2O3-SiO2
- A Mössbauer study of the oxidation state of Fe in silicate melts
- Nanometer-scale measurements of Fe3+/ΣFe by electron energy-loss spectroscopy: A cautionary note
- The most reduced rock from the moon, Apollo 14 basalt 14053: Its unique features and their origin
- Quantitative analysis of ammonium in biotite using infrared spectroscopy
- Lizardite-chlorite structural relationships and an inferred high-pressure lizardite polytype
- V oxidation state and coordination number in silicate glasses by XAS
- Structure of the fluorapatite (100)-water interface by high-resolution X-ray reflectivity
- Origin of diffuse superstructure reflections in labuntsovite-group minerals
- Isosymmetric structural phase transition of orthoenstatite: Molecular dynamics simulation
- Crystal nucleation in hydrous rhyolite: Experimental data applied to classical theory
- Investigation of polytypes in lepidolite using electron back-scattered diffraction
- Iron partitioning between basaltic melts and clinopyroxene as a function of oxygen fugacity
- The synthesis of zeolite-P, Linde Type A, and hydroxysodalite zeolites from paper sludge ash at low temperature (80 °C): Optimal ash-leaching condition for zeolite synthesis
- High-pressure viscometry of polymerized silicate melts and limitations of the Eyring equation
- Biological control on calcite crystallization: AFM investigation of coccolith polysaccharide function
- Order parameter variation through the C2/m-P21/m phase transition in cummingtonite
- Arsenate sorption on schwertmannite
- Evidence on the structure of synthetic schwertmannite
- Crystal-chemical characterization of tourmalines from the English Lake District: Electron-microprobe analyses and Mössbauer spectroscopy
- Crystal chemistry and paragenesis of compositionally unique (Al-, Fe-, Nb-, and Zr-rich) titanite from Afrikanda, Russia
- Site distribution of Fe2+and Fe3+in the axinite mineral group: New crystal-chemical formula
- Epidote-rich talc-kyanite-phengite eclogites, Sulu terrane, eastern China: P-T-fO₂estimates and the significance of the epidote-talc assemblage in eclogite
- Quantitative mineralogy of the Yukon River system: Changes with reach and season, and determining sediment provenance
- Exsolution of thortveitite, yttrialite, and xenotime during low-temperature recrystallization of zircon from New Caledonia, and their significance for trace element incorporation in zircon
- Mineralogical characterization and genesis of hydrothermal Mn oxides from the flank of the Juan the Fuca Ridge
- Synchrotron X-ray diffraction study of the structure of shafranovskite, K2Na3(Mn,Fe,Na)4[Si9(O,OH)27](OH)2⋅nH2O, a rare manganese phyllosilicate from the Kola peninsula, Russia
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