Rubinite

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Formula: Ca₃Ti³⁺₂Si₃O₁₂
Nesosilicate (insular SiO₄ groups), garnet supergroup, titanium-bearing mineral
Crystal System: Isometric
Specific gravity: 3.63 calculated
Environments

Metamorphic environments
Meteorites

Rubinite occurs in calcium-aluminium rich inclusions in carbonaceous chondrite meteorites. It is among the first solid materials in the solar nebula formed either as a condensate or through crystallisation from an ¹⁶O-rich calcium-, aluminium- and titanium- rich melt under highly reduced conditions (AM 105.1923-1924).
It has also been found in a terrestrial environment in Israel. See below. Published in 2025.

Localities

At the Roadside Arad – Dead Sea, Wadi Zohar, Hatrurim Basin, Tamar Regional Council, Beersheba Subdistrict, Southern District, Israel, rubinite was found in a single sample of the phosphide-bearing breccia which was discovered in an exposure created during the construction of the Arad–Dead Sea road. In the breccia, xenoliths of altered sedimentary rocks ranging in size from a few centimetres to 0.5 m, cemented by flamite–gehlenite (± rankinite, pseudowollastonite) paralava, usually have a characteristic zonation.
The contact facies of black gehlenite–flamite or gehlenite–rankinite amygdaloidal paralava (zone 1) is represented by a light pseudowollastonite–gehlenite zone, 2 to 3 mm thick, intensely replaced by OH-bearing grossular (hydrogrossular) and hydrous silicates (zone 2). Contamination led to the formation of pseudowollastonite in the paralava. At the contact with the paralava, the xenolith itself often has a thin zone several millimetres thick, which usually differs in colour (zone 3) from the central parts of the xenolith (zone 4). This zone is interpreted as a zone of almost complete melting.
The xenolith consists of hydrogrossular, calcium hydrosilicates (predominantly tacharanite) and calcite; in some fragments the rock is enriched in gypsum, ettringite, whewellite and halite. Relict high-temperature minerals are represented by small grains of barringerite, murashkoite, perovskite, baghdadite, pseudowollastonite, cuspidine, osbornite, paqueite, fluorapatite, oldhamite and, rarely, gehlenite.
Numerous grains of rubinite occur in zone 4 of the xenolith, forming aggregates with relics of pseudowollastonite in the central part of the crystals. Reaction rims of rubinite up to 10 µm thick occur on the pseudowollastonite. There are rare inclusions of gehlenite and barringerite in the rubinite crystals and osbornite associated with the rubinite typically forms as a rim on barringerite. Garnet crystals occur in fragments of phosphide-rich rock and are associated with cuspidine, pseudowollastonite and paqueite, rarely with unusual blue Ti³⁺-bearing perovskite. Rubinite crystals overgrow a core of titanium-bearing grossular.
The formation of rubinite is induced by high-temperature processes during the interaction of hot paralava generated at temperatures above 1200°C with thermally altered fragments of carbonate-clay sedimentary rocks. Preliminary thermal alteration of the sedimentary rocks caused graphitisation of fishbone remains and replacement of numerous framboids (pellets, often of pyrite, forming spheroidal aggregates resembling a raspberry) by hematite, leading to the mass formation of phosphide and native iron aggregates as well as osbornite. Thermal alteration of the xenoliths resulted in the formation of a paralava contact facies enriched in pseudowollastonite and phosphides. The fine-grained matrix of the xenolith was mainly represented by gehlenite and hatrurite aggregates, probably with an impurity of oldhamite, lime and minerals of the mayenite supergroup. Rubinite is located in the central part of the xenolith. The majority of rubinite grains form a rim on pseudowollastonite.
Rubinite was formed by the reaction
3CaSiO₃ (pseudowollastonite) + Ti₂O₃ (tistarite) = Ca₃Ti³⁺₂Si₃O₁₂ (rubinite)
The reduction of Ti was due to the carbon released as a result of the decomposition of the graphitised fish bone remains
2Ti⁴⁺O₂ (anatase/brookite/rutile) + C/CO = Ti³⁺₂O₃ (tistarite) + CO/CO₂
Rubinite found in situ from the phosphide-bearing breccia of the Hatrurim Complex is at present (2025) the only authentic silicate with trivalent titanium formed on Earth under super-reduced conditions (MM 89.725–735).

At the type locality, the Vigarano meteorite, Vigarano Pieve, Vigarano Mainarda, Ferrara Province, Emilia-Romagna, Italy, rubinite was identified in calcium-aluminium rich inclusions in the carbonaceous chondrite meteorite as irregular subhedral crystals, ~0.5 to 1 μm in size. It occurs in an ultra-refractory fragment with zirconium-rich panguite, spinel and davisite-diopside, all enclosed within forsterite aggregate (AM 105.1923-1924).

At the Efremovka meteorite, Pavlodar, Pavlodar Region, Kazakhstan, rubinite was identified in calcium-aluminium rich inclusions in the carbonaceous chondrite as irregular subhedral crystals, ~1 to 20 μm in size. It occurs within gehlenitic melilite with perovskite, spinel and grossmanite in three inclusions (AM 105.1923-1924).

At the Allende meteorite, Pueblito de Allende, Chihuahua, Mexico, rubinite was identified in calcium-aluminium rich inclusions in the carbonaceous chondrite as irregular subhedral crystals, ~1 to 8 μm in size. It is found with primary gehlenitic melilite, perovskite, spinel, hibonite, corundum, davisite, grossmanite, diopside and eringaite, and with secondary anorthite, grossular and sodium-rich melilite (AM 105.1923-1924).

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