Warkite, Ca₂Sc₆Al₆O₂₀, a new mineral in carbonaceous chondrites 

and a key-stone phase in ultra-refractory inclusions from the solar nebula

ABSTRACT

Warkite (IMA 2013-129) is a new Sc-rich ultra-refractory mineral in the rhönite group of the sapphirine supergroup. It has a P[bar]1  aenigmatite-type structure with a = 10.367 Å, b = 10.756 Å, c = 8.895 Å, α = 106°, β = 96°, γ = 125°, and Z = 2, and general formula of Ca₂(Sc,Ti,Al,Mg,Zr)₆Al₆O₂₀. Warkite occurs as micrometer-sized crystals in eleven ultra-refractory Ca,Al-rich inclusions (UR CAIs) from the CM, CV, CO, and CH chondrites. In the CM, CO, and CV CAIs, warkite in the cores coexists with a Ti-rich oxide, either perovskite or kangite; the cores are generally mantled and rimmed by davisite and/or Sc-diopside. In the CH CAIs and one CO CAI, warkite in the cores coexists with perovskite and grossite; the cores are mantled by grossite ± gehlenite, and rimmed by low-Sc, Al-diopside. Therefore, there are two basic families of warkite-bearing inclusions, those containing Sc-rich clinopyroxenes but no grossite and those containing grossite but no Sc-rich clinopyroxene. Scandian clinopyroxene in warkite-bearing CAIs generally formed by the reaction of warkite, which supplied most or all of the Sc, Ti, Al, and Ca, and a nebular gas that supplied Mg and SiO. The presence of Sc-rich clinopyroxenes may reflect exposure of some warkite-bearing CAIs to a dust-rich environment, which would enhance partial pressures of Si-, Mg-, and Ca-bearing species in the vapor and make it oxidizing relative to a dust-poor gas.  Warkite in grossite-bearing inclusions is generally Ti³⁺-enriched relative to those in davisite-bearing inclusions, consistent with their formation in relatively dust-poor environments. Warkite compositions are sensitive to the presence or absence of spinel, melilite, Sc-rich clinopyroxenes, and grossite. Warkite is an aluminate but often contains measurable Si content, which implies that this phase can potentially be used to constrain P_SiO in nebular environments, even if no silicates were present. Compositional variations of perovskite and warkite grains indicate a connection but, except for late-stage Fe exchange, they did not equilibrate with each other. The presence of at least two trends in Y-Sc among perovskites without corresponding trends in warkite suggests that at least some perovskite formed separately.

Warkite-bearing CAIs from CM2s and CO3.0s are uniformly ¹⁶O-rich (D¹⁷O ~ -23‰), whereas those from metamorphosed COs and CVs are isotopically heterogeneous: warkite, kangite, perovskite, melilite, and davisite are ¹⁶O-depleted to various degrees (D¹⁷O range from -22 to -2‰) relative to hibonite, spinel, and forsterite, all having ¹⁶O-rich compositions (D¹⁷O ~ -25 to -20‰). We infer that warkite-bearing CAIs originated in an ¹⁶O-rich nebular gas. Subsequently, CAIs from metamorphosed CVs and COs experienced O-isotope exchange with an ¹⁶O-depleted external reservoir, most likely aqueous fluids on the CV and CO chondrite parent asteroids; however, O-isotope exchange in the solar nebula cannot be excluded.  

Backscattered electron image of warkite and other phases in an inclusion in carbonaceous chondrite meteorite DOM 08006 from the Dominion Range, Antarctica/