Goldschmidtite, (K,REE,Sr)(Nb,Cr)O3: A new perovskite supergroup mineral found in diamond from Koffiefontein, South Africa

Abstract

Goldschmidtite is a new perovskite-group mineral (IMA No. 2018-034) with the ideal formula (K,REE,Sr)(Nb,Cr)O3. A single grain of goldschmidtite with a maximum dimension of ∼100 μm was found as an inclusion in a diamond from the Koffiefontein pipe in South Africa. In addition to the dark green and opaque goldschmidtite, the diamond contained a Cr-rich augite (websteritic paragenesis) and an intergrowth of chromite, Mg-silicate, and unidentified K-Sr-REE-Nb-oxide. Geothermobarometry of the augite indicates that the depth of formation was ∼170 km. The chemical composition of gold-schmidtite determined by electron microprobe analysis (n = 11, WDS, wt%) is: Nb2O5 44.82, TiO2 0.44, ThO2 0.10, Al2O3 0.35, Cr2O3 7.07, La2O3 11.85, Ce2O3 6.18, Fe2O3 1.96, MgO 0.70, CaO 0.04, SrO 6.67, BaO 6.82, K2O 11.53, total 98.53. The empirical formula (expressed to two decimal places) is (K0.50La0.15Sr0.13Ba0.09Ce0.08)Σ0.95(Nb0.70Cr0.19Fe0.05Al0.01Mg0.04Ti0.01)Σ1.00O3. Goldschmidtite is cubic, space group Pm3m, with unit-cell parameters: a = 3.9876(1) Å, V = 63.404(6) Å3, Z = 1, resulting in a calculated density of 5.32(3) g/cm3. Goldschmidtite is the K-analog of isolueshite, (Na,La)NbO3. Raman spectra of goldschmidtite exhibit many second-order broad bands at 100 to 700 cm–1 as well as a pronounced peak at 815 cm–1, which is possibly a result of local ordering of Nb and Cr at the B site. The name goldschmidtite is in honor of the eminent geochemist Victor Moritz Goldschmidt (1888–1947), who formalized perovskite crystal chemistry and identified KNbO3 as a perovskite-structured compound.

Natural cubic perovskite, Ca(Ti,Si,Cr)O3–δ, a versatile potential host for rock-forming and less-common elements up to Earth’s mantle pressure

“…In view of a possible formation of cubic perovskite at high pressure, it is noteworthy that the comparable Si and Cr contents were previously encountered only in those perovskite-group minerals that come from the inclusions in mantle diamonds (Kopylova et al 1997;Walter et al 2008;Nestola et al 2018;Meyer et al 2019). This similarity might be occasional, because there are no geological evidences for surficially exposed mantle lithologies or remnants of a large-scale impact structure across the area of the Dead Sea Transform (Garfunkel and Ben-Avraham 1996;French and Koeberl 2010).…”

“…The Raman spectrum of cubic perovskite is depicted in Figure 7 (Guennou et al 2010) and goldschmidtite, (K,REE,Sr)(Nb,Cr)O 3 , from an inclusion in diamond from Koffiefontein, South Africa (RRUFF R190009, Meyer et al 2019). The absence of bands in the OH-stretching region of the cubic perovskite spectrum (Fig.…”

“…The two reported Cr-bearing perovskite- perovskite. Like Si-bearing perovskites, both these minerals were described as inclusions within mantle diamonds (Kopylova et al 1997;Meyer et al 2019). Goldschmidtite has a structure of ideal cubic ABO 3 perovskite.…”

Heamanite-(Ce), (K0.5Ce0.5)TiO3, a new perovskite supergroup mineral found in diamond from Gahcho Kué, Canada

“…Even if other mantle materials, such as mantle xenoliths and exposed mantle rocks, are directly observable, the deeper into the earth that we want to sample, the more we have to rely upon diamond to provide material for study. For this reason, the investigation of diamonds and their inclusions has allowed the discovery of the natural occurrences of high-pressure phases previously known only from experimental studies (Harte et al 1999;Stachel et al 2000;McCammon 2001;Brenker et al 2002;Harte and Hudson 2013;Pearson et al 2014;Anzolini et al 2016;Nestola et al 2018), and also has led to the new mineral species: jeffbenite (Mg 3 Al 2 Si 3 O 12 , previously known as TAPP, Nestola et al 2016), ice-VII (H 2 O, Tschauner et al 2018), goldschmidtite [(K,REE,Sr)(Nb,Cr)O 3 , Meyer et al 2019] and breyite (CaSiO 3 with the walstromite structure-type, Brenker et al 2021). This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

“…Heamanite-(Ce) represents the K-analog of loparite-(Ce), whose ideal composition is (NaCe)Ti 2 O . It is the sixth perovskite-structured mineral to occur in Earth's mantle, along with perovskite sensu stricto (CaTiO 3 ), K-REE-Cr-rich tausonite (Kopylova et al 1997a,b), bridgmanite [(Mg,Fe)SiO 3 , Harte et al 1999;Tschauner et al 2014], CaSiO 3 -perovskite (Nestola et al 2018), and goldschmidtite [(K,REE,Sr)(Nb,Cr)O 3 , Meyer et al 2019]. We describe the physical, chemical, optical and structural properties of the new mineral heamanite-(Ce) and suggest a possible mechanism of formation.…”