Zirconolite from Larvik Plutonic Complex, Norway, its relationship to stefanweissite and nöggerathite, and contribution to the improvement of zirconolite end-member systematics

Abstract

Abstract The very first description of zirconolite, originally called polymignyte, discovered in alkaline pegmatites within the Larvik Plutonic Complex (LPC), Norway, was published almost 200 yr ago. We studied zirconolite from three occurrences located in this region using modern techniques—X-ray powder diffraction (XRPD) upon thermal annealing of initially radiation-damaged mineral, electron probe microanalysis, and Mössbauer spectroscopy. The initial XRPD pattern lacked any sharp diffraction maxima, as the accumulated radiation dose exceeded a critical value of ca. 1016 α-decays/mg. Annealing at 400–800 °C induced recrystallization to a transitional, cubic phase interpreted to have a disordered, defect fluorite structure [space group Fm3m; unit-cell parameters: a = 5.1047(4) Å, V = 133.02(2) Å3], with its XRPD pattern being very similar to that of cubic ZrO2. Rietveld analysis of the XRPD pattern obtained after a phase transition at 900 °C shows a mixture of -3O [wt. fraction of ca. 60%, space group Cmca, unit-cell parameters: a = 7.2664(8) Å, b = 14.1877(15) Å, c = 10.1472(12) Å, V = 1046.1(2) Å3], and -3T [wt. fraction of ca. 40%, space group P3121, unit-cell parameters: a = 7.2766(6) Å, c = 17.0627(15) Å, V = 752.42(11) Å3] zirconolite polytypes. However, the crystal habits of zirconolite from the LPC show a distinct orthorhombic symmetry. Although their chemical compositions are far from the ideal zirconolite composition and a large number of elements are involved in high concentrations [up to ca. 17 wt% REE2O3, ≤7 wt% ACTO2, ≤18 wt% Me25+O5, ≤9 wt% Me2+O + Me23+O3, Fe3+/(Fe3++Fe2+) ≈ 0.2, where ACT = Th + U, Me5+ = Nb ± Ta, Me2+ = Fe2+ ± Mg, Me3+ ≈ Fe3+], the compositional variability is relatively limited. To quantitatively describe the two distinct compositional trends observed, we introduced a concept called “edgemembers,” so that mixing is approximated to occur between two terminal compositions situated at two edges of the zirconolite composition space. These marginal compositions were determined from observed compositional trends, i.e., heterovalent substitution of Me5+ for Ti in octahedral sites and ACT enrichment associated with increasing Ti/Me5+ ratio. This approach provides general substitution vectors for both, Håkestad-type mode (ACT + 3 Ti + Me3+ = Ca + 3 Me5+ + Me2+), and Stålaker-type mode (0.7 ACT + 0.5 REE + 0.9 Ti + 0.7 Me3+ + 0.05 Zr = 1.2 Ca + 1.3 Me5+ + 0.3 Me2+ + 0.05 Mn). In terms of chemical composition, the studied zirconolite corresponds to recently approved zirconolite-related minerals stefanweissite (for Ca > REE) and nöggerathite (for REE > Ca). Based on a careful analysis of zirconolite composition space, we show that our observed Håkestad-type compositional trend, as well as a high number of published zirconolite compositions worldwide (with Me2+ + Me3+ sum of ca. 1 atom per 14 O), can be well approximated by a modified end-member set comprising Ca2Zr2Me25+TiMe2+O14, REE2Zr2Ti3Me2+O14, CaACTZr2Ti3Me2+O14, CaREEZr2Ti3Me3+O14, and Ca2Zr2Ti2Me5+Me3+O14 without a need to involve the ideal zirconolite formula Ca2Zr2Ti4O14. The redefined composition space constrained by end-members from this set, together with ideal zirconolite, may be representative of the vast majority of more than 450 published zirconolite compositions worldwide with Me2+ + Me3+ totaling ≤1 atom per 14 O. The equation XMe3+ = 2 – 2XREE* – 3XMe5+* provides an independent calculation of iron oxidation state or XMe3+ = Me3+/(Me2+ + Me3+) for this remarkable group of zirconolites, where XREE* is derived from XREE = REE/(REE + Ca) and XMe5+* from XMe5+ = Me5+/(Me5+ + Ti). Understanding the oxidation state of iron in zirconolite may be helpful to characterize redox conditions during its crystallization.