Hawthorneite, Ba[Ti₃Cr₄Fe₄Mg]O₁₉; a new metasomatic magnetoplumbite-type mineral from the upper mantle

A new opaque mineral oxide is described from a veined, metasomatized, harzburgitic xenolith from a diamondiferous kimberlite pipe in Kimberley, South Africa. The new mineral, hawthomeite (~ 100 μm in maximum dimension), is in intimate association with lindsleyite (Ba,Sr)(Ti,Cr.. .)₂₁O₃₈, a member of the crichtonite group, accessory niobian chromian rutile, and magnesian chromian ilmenite in host-replacement-textured magne-sian chromite.

Hawthomeite is isostructural with magnetoplumbite Pb(Fe,Mn)₁₂O₁₉ and barium ferrite (BaFe,₁₂O₁₉). Typical electron-microbeam analyses of hawthorneite give TiO₂ = 22.2-23.6, Cr₂O₃ = 33-34.9, FeO = 23.3-24.9, BaO = 12-13, and MgO = 2.9-3.5, with K₂O, ZrO₂, Al₂O₃, MnO, and Nb₂O₅ in the range 0.1-0.5 (all wt%). A typical average composition is

or ideally Ba[Cr₄Ti₃Fe₄Mg]_Σ12O₁₉.

The space group is P6₃/mmc with Z = 2, and the strongest reflections are [d_calc(Å), I_calc, hkl] 2.616(100)(114), 2.765(85)(017), and 2.414 (49)(023). Hexagonal unit-cell parameters are a = 5.871(2) Å and c = 23.06(2) Å, and the structure comprises alternating spinel-type slabs and perovskite-type layers. The calculated density is 5.02 g/cm³.

Hawthomeite is pale gray in reflected light, is nonpleochroic with weak to moderate bireflectance, and has weak to moderate rotation tints in browns and grays. Quantitative spectral reflectance measurements for R_o and $Re′$ in air and oil immersion, respectively, are for 470 nm 18.3, 17.1 and 6.02, 5.25%; for 546 nm 17.5, 16.4 and 5.58, 4.88%; for 589 nm 17.3, 16.3 and 5.41, 4.75%; and for 650 nm 17.05, 16.0 and 5.34, 4.68%. VHN₁₀₀ = 801, with a range of five indentations = 772-835.

Hawthomeite is interpreted to have formed by reaction of a Ba-, K-, Ti-, Zr-, and Nb- enriched fluid or volatile-rich melt with magnesian chromite in previously depleted harz- burgite. An orientation relationship between spinel and hawthomeite is preserved such that [111]_Sp||[0001]_Haw and (224)_Sp||(3030)_Haw and in which (Ba + Ti) in fluid replaces (Mg + Cr) in spinel. Continued reaction yields lindsleyite. Conditions of formation are estimated at 900-1100 °C and 20-30 kbar.

Hawthomeite is the Ba analogue of K-characteristic yimengite (K(Cr,Ti.. .)₁₂O₁₉), which coexists with K-specific mathiasite of the crichtonite group. By contrast, the assemblage hawthorneite + lindsleyite is Ba specific. The small (M) cations in these minerals and in yimengite + mathiasite are Ti + Cr, with lesser Fe, Mg, Zr, and Nb. The similarities in these magnetoplumbite and crichtonite isotypes, and in the associated silicates and oxides, demonstrate a P-T crystallochemical underpinning that results from similarities in metasomatic fluids or melts, enrichment processes, and host rocks in widely separated subcontinental subcratonic Archean lithospheres.

The mineral is named for John Barry Hawthorne in honor of his contributions to upper- mantle studies.