Overview
Bustamite is a calcium-manganese chain silicate valued by collectors for its soft pink to brownish-red colour and, at several localities, its vivid red fluorescence under ultraviolet light. It belongs to the pyroxenoid family, the same structural group as rhodonite, and the two minerals are often confused. Bustamite was named after the Mexican mineralogist and statesman Jose Maria Bustamante. Its naming history is unusual: the original Mexican material from Puebla proved to be a mixture of other minerals, so the type locality was reassigned to the Franklin zinc deposit in New Jersey, where genuine, well-characterised bustamite is moderately common.
Composition & structure
Bustamite is a single-chain silicate (a pyroxenoid) with calcium and divalent manganese as its essential cations, commonly written CaMn(SiO3)2 or, on a three-tetrahedra repeat, CaMn2(Si3O9). Iron and zinc may substitute for some of the manganese; iron-rich material grades toward the related species ferrobustamite. The structure is built from kinked chains of silica tetrahedra repeating every three units, which gives the mineral its triclinic symmetry and its set of good cleavages. Bustamite forms part of a structural group that also includes wollastonite, with which it shares broad similarities.
| Formula | CaMn(SiO3)2, i.e. CaMn2+(Si3O9) repeat |
| Crystal system | Triclinic |
| Mohs hardness | 5.5 to 6.5 |
| Lustre | Vitreous to subvitreous, sometimes waxy |
| Colour | Pale to deep pink, orange-pink, brownish-red (pink fades in sunlight) |
| Type locality | Franklin Mine, Franklin, Sussex County, New Jersey, USA |
Formation & occurrence
Bustamite forms chiefly by metamorphism of manganese-bearing sediments and within manganese-rich ore bodies. At Franklin and the neighbouring Sterling Hill deposit, it crystallised during the high-grade metamorphism of an ancient zinc-manganese orebody and occurs intergrown with rhodonite, tephroite, willemite, franklinite and calcite. At Broken Hill in Australia it appears in the metamorphosed lead-zinc-silver ores, sometimes associated with galena. In both settings bustamite is a product of regional metamorphism rather than igneous crystallisation, and its presence signals manganese-enriched protoliths.
Identification & similar species
The most common point of confusion is rhodonite, which shares a pink colour and a pyroxenoid structure. Bustamite is generally paler, often more orange or brownish in tone, and tends toward bladed or fibrous aggregates rather than the blocky crystals of rhodonite; optically it is biaxial negative whereas rhodonite is biaxial positive. A useful field clue at Franklin is fluorescence: bustamite there glows red under longwave ultraviolet light, while associated willemite glows green. Its pink colour can fade with prolonged sun exposure, so specimens are best stored away from strong light.
Notable localities & collecting
The Franklin and Sterling Hill mines of New Jersey are the definitive source of well-studied bustamite and remain favourites for fluorescent-mineral collectors. Broken Hill in New South Wales, Australia, has produced fine iron-bearing crystals studied for their structure. Bustamite is also reported from manganese and skarn deposits in Japan, South Africa, Sweden and elsewhere. Gem-quality pink material is occasionally cut as a collector curiosity, but because the mineral is relatively soft and the colour light-sensitive, it is more often appreciated as cabinet specimens, particularly the fluorescent Franklin associations.