Kimzeyite is a rare end-member of the garnet group with the idealised composition Ca3(Zr,Ti)2(Si,Al,Fe)3O12, in which the trivalent Y site (octahedral) of the garnet structure is occupied dominantly by zirconium rather than aluminium, iron or chromium. It was first described from Magnet Cove, Hot Spring County, Arkansas, in 1961, in a publication by Charles Milton, Blanche Ingram and Lewis Blade of the U.S. Geological Survey. The name commemorates the Kimzey family of Magnet Cove, owners of the property on which the type material was collected. Kimzeyite is recognised as a distinct mineral species by the International Mineralogical Association and is the zirconium analogue of schorlomite (titanium-rich) and andradite (iron-rich) within the garnet supergroup.

Composition and structure

The defining substitution in kimzeyite is the entry of zirconium into the octahedral Y site of the garnet structure. The substitution is charge-balanced by tetrahedral substitutions, typically aluminium and iron(III) replacing silicon at the Z site. Pure end-member kimzeyite has not been described in nature; natural specimens are intermediate compositions in which Zr is the dominant Y-site cation but is accompanied by Ti and Fe. Crystals are typically dodecahedral or trapezohedral, opaque to translucent, and dark brown to nearly black in colour, with a sub-metallic to vitreous lustre. The hardness is approximately 7 to 7.5 on the Mohs scale, and the specific gravity is in the range of 4.0 to 4.1, distinctly higher than the more familiar pyrope-almandine series.

Occurrence

Kimzeyite is associated with alkaline igneous environments. The type locality at Magnet Cove is part of an ijolite-carbonatite-nepheline syenite intrusion of Cretaceous age, and the mineral occurs in carbonatite and ijolite assemblages there. Other reported occurrences include the Kovdor and Afrikanda alkaline-ultramafic complexes in the Kola Peninsula of Russia, the Oka carbonatite of Quebec, and a small number of African carbonatite localities. In all of these settings the mineral is rare, accessory in character, and not formed in volumes that would make it of any economic significance.

Why it is not a gem

Despite belonging to the garnet group, kimzeyite is not a gemstone in any practical sense. The natural material is opaque to at most translucent, dark in colour, and rarely formed in crystals of more than a few millimetres. Optical clarity, attractive colour and adequate size, the three minimum requirements for a faceted gemstone, are not met by any kimzeyite material yet described. Collectors of garnet-group rarities prize good kimzeyite crystals as a mineral specimen, but the species does not enter the cut-stone or jewellery trade. Its interest is therefore mineralogical and academic, expanding the chemical range of the garnet supergroup, rather than commercial.

Position within the garnet group

Within the modern classification of the garnet supergroup (Grew et al., 2013), kimzeyite is placed in the garnet group sensu stricto, in a sub-group of zirconium-bearing species that also includes kerimasite (Ca3Zr2(SiO4)2(Fe2O7) end-member with iron-rich tetrahedral sites). The recognition of these species reflects the increasing chemical and structural complexity that has been documented in garnet over the past several decades, well beyond the classical six 'gem garnets' (pyrope, almandine, spessartine, grossular, andradite, uvarovite) that dominate the gemmological literature. For the working trade, kimzeyite is best filed as a reminder that the garnet structure can accept a far wider range of cations than the gemstone market alone would suggest.