Two new opaque minerals are described from metasomatized peridotites and heavy media concentrates from four kimberlites in the Republic of South Africa. Lindsleyite and mathiasite are the Ba and K members of the AM21O38 series, characterized by predominant A-specific, large cations (Sr = crichtonite; Na = landauite; Pb = senaite; Ca = loveringite, and U + REE = davidite), and by M-formula small cations which are dominantly Ti, Cr, Fe, Mg, Zr, and Nb. Approximately 70% of the A-site is filled by either Ba or K in the new minerals, and selective solid solution is present with Ba-Sr and K-Ca exchange. Typical electron microbeam analyses based on 38 oxygens, give the following formulae:
REE in both minerals share the unusual characteristic of davidite and loveringite in possessing LREE- and HREE-enrichment, with depletion of the middle lanthanides.

The space group for both minerals is R3 or R3 and the strongest reflections are 2.13, 1.80, 1.59, and 1.44Å (all 100) for lindsleyite, and 2.25(90), 2.14 (100), and 1.44 Å (100) for mathiasite. Hexagonal unit cell parameters for lindsleyite are a = 1.037 nm, c = 2.052 nm, V = 1.911 nm3, and for mathiasite are a = 1.035 nm, c = 2.058 nm, V = 1.909 nm3. Calculated densities for the Ba and K members are 4.63 and 4.60 gm/cm3, respectively.

Both minerals are black and have a metallic luster and conchoidal fracture. The minerals are pale tan in reflected light, exhibit weak reflection pleochroism and weak-moderate reflection anisotropy. Spectral reflectances for lindsleyite and mathiasite are very similar and are comparable to ilmenite and rutile. R1 and R2 in air and oil, respectively, for lindsleyite are: 470 nm = 17.9-18.3%, 5.9-6.1%; 546 nm = 17.0-17.3%; 5.3-5.5%; 589 nm = 16.7-17.0%, 5.1-5.3%; 650 nm = 16.5-16.9%, 5.0-5.2%. Similar data for mathiasite are: 470 nm = 18.3-19.0%, 5.9-6.4%; 546 nm = 17.1-17.9%, 5.3-5.8%; 589 nm = 16.7-17.5%, 5.1-5.6%; 650 nm = 16.5-17.3%, 5.0-5.5%. The mean Vickers microhardness value for mathiasite is 1505 Kg/mm2; lindsleyite is judged to be comparable.

Associated characteristic minerals are K-richterite, phlogopite, Nb-Cr-rutile and Nb-Cr-Mg-ilmenite, together with olivine, orthopyroxene, Cr-diopside, and Mg-Cr-spinel. Garnet is typically absent. The new minerals formed by metasomatizing fluids enriched in K + Ba + REE + Ti that infiltrated source rocks depleted in Al and contaminated perhaps by Cr prior to the eruption of kimberlite, providing important potential insights to the dynamics of fluid movements, and the compositions of fluids in the upper mantle.

Lindsleyite is named in honor of Professor Donald H. Lindsley, and mathiasite in honor of Professor Morna Mathias, for their respective contributions to experimental mineralogy and the petrology of alkalic suites. The minerals and the names have been approved by the Commission on New Minerals and Mineral Names of the I.M.A.

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