Rubinite, Ca₃Ti³⁺₂Si₃O₁₂, a new mineral in CV3 carbonaceous chondrites and a refractory garnet from the solar nebula

Rubinite (IMA 2016-110) is a recently discovered Ti³⁺-dominant refractory mineral in the garnet group from the solar nebula. It has the Ia₃d garnet-type structure with a = 12.19(1) Å, and Z = 8, and end-member formula of Ca₃Ti³⁺₂Si₃O₁₂. Rubinite was identified as micrometer-sized crystals in five refractory Ca,Al-rich inclusions (CAIs) from the CV3 carbonaceous chondrites Allende, Efremovka, and Vigarano. In the Vigarano CAI V3, it occurs in the central portion of an ultrarefractory fragment with Zr,Y,Sc-oxide, spinel and davisite-diopside, all enclosed within an amoeboid olivine aggregate. In the Allende Compact Type A (CTA) CAI AE01-01, it occurs with gehlenitic melilite, perovskite, spinel, hibonite, davisite, grossmanite, and diopside. In Efremovka, rubinite occurs within gehlenitic melilite with perovskite, spinel, and grossmanite in three CTA CAIs E101, E105, and 40E-1 (in a compound CAI). Rubinite is present in spinel-poor regions in all four of the Efremovka and Allende CAIs but it is in contact with spinel in the Vigarano inclusion.

The mean chemical composition of type rubinite in Allende is (in wt%) CaO 32.68, Ti₂O₃ 14.79, TiO₂ 13.06, SiO₂ 28.37 Al₂O₃ 3.82, Sc₂O₃ 1.80, Na₂O 1.01, ZrO₂, 0.80, MgO 0.79, V₂O₃ 0.61, FeO 0.53, Y₂O₃ 0.07, Cr₂O₃ 0.05, total 98.38, giving rise to an empirical formula of (Ca_2.94Na_0.08)(Ti³⁺_1.04Ti⁴⁺_0.59Sc_0.13Mg_0.10V_0.04Fe_0.04Zr_0.03)(Si_2.38Al_0.38Ti⁴⁺_0.24)O₁₂, where Ti³⁺ and Ti⁴⁺ are partitioned based on stoichiometry. Efremovka rubinite has a similar composition with a mean empirical formula of (Ca_2.97Na_0.06)(Ti³⁺_1.05Ti⁴⁺_0.66Mg_0.12Sc_0.09Zr_0.03V_0.03Y_0.01Fe_0.01)(Si_2.36Al_0.48Ti⁴⁺_0.16)O₁₂. Vigarano rubinite is much more Y-, Sc-, and Zr-rich, having an empirical formula of (Ca_1.89Y_0.83Mg_0.28)(Ti³⁺_0.59Sc_0.50Zr_0.72Mg_0.2V_0.02Cr_0.01)(Si_1.64Al_1.18Ti⁴⁺_0.07Fe_0.06)O₁₂. All rubinites are Ti³⁺-rich but a significant amount (11–46%) of the Ti is 4⁺.

In the Efremovka CTAs, spinel is ¹⁶O-rich (Δ¹⁷O ~ –24‰); rubinite and perovskite show limited ranges of Δ¹⁷O (from –24 to –16‰; most analyses range from –24 to –20‰); melilite and grossmanite are the most ¹⁶O-depleted minerals (Δ¹⁷O range from ~ –10 to –4‰ and from –8 to –5‰, respectively). In the Allende CTA AE01-01, spinel and hibonite are ¹⁶O-rich (Δ¹⁷O ~ –24‰); melilite, rubinite and perovskite show large ranges in Δ¹⁷O (from –23 to –3‰, from –21 to –6‰, and from –14 to – 2‰, respectively); grossmanite is uniformly ¹⁶O-depleted (Δ¹⁷O ~ –3‰).

Rubinite formed under highly reducing conditions in the solar nebula by gas-solid condensation and by crystallization from a Ca, Al, and Ti-rich melt. Subsequently, most rubinite grains in the Allende CAI and some in the Efremovka CAIs may have experienced O-isotope exchange to a various degree with an ¹⁶O-depleted (Δ¹⁷O ~ – 2‰) aqueous fluid on the CV chondrite parent asteroid. However, crystallization from a Ca,Al,Ti-rich melt that recorded O-isotope exchange: with nebular gas with variable Δ¹⁷O or post-crystallization O-isotope with such gas cannot be excluded. The mineral name is in honor of Alan E. Rubin (b. 1953), a cosmochemist at University of California, Los Angeles (UCLA), USA, for his many contributions to research in cosmochemistry and mineralogy of meteorites.