Liddicoatite is a species within the tourmaline group, distinguished from the more familiar elbaite and dravite by its calcium-rich rather than sodium-rich X-site composition. The species was formally named in 1977 in honour of Richard T. Liddicoat, then chairman of the Gemological Institute of America, in recognition of his contribution to gemmological science and education. Most material in trade circulation comes from a single Madagascar locality, principally the Anjanabonoina pegmatites, and the species is best known to collectors for the spectacular triangular cross-section colour zoning that characterises slices cut perpendicular to the c-axis.

Composition and the tourmaline group

The tourmaline group is a complex set of borosilicate species sharing a common structural framework but differing in their X-site, Y-site and other occupancies. Liddicoatite occupies the calcium-dominant X-site position, with formula approximately Ca(Li,Al)3Al6(BO3)3Si6O18(OH)4, paralleling the lithium-aluminium chemistry of elbaite but with calcium where elbaite has sodium. The species is part of the lithium tourmaline subgroup and shares with elbaite the capacity for strong polychromatic colour zoning and an extensive range of body colours.

Visual character

The most distinctive visual feature of liddicoatite is concentric triangular colour zoning visible in basal slices. The cross-section reveals a series of nested triangular zones in pinks, reds, greens, browns, blues, yellows and colourless, sometimes with twenty or more discrete zones in a single specimen. These slices have become a benchmark image in introductory gemmological literature and a reliable seller in the mineral specimen market. Faceted material from liddicoatite is comparatively uncommon because the most striking visual property of the material is the slice rather than the cut stone, and faceted goods often miss the polychromatic effect that gives the species its appeal.

Sources

The Madagascar deposits at Anjanabonoina have been the dominant economic source since the species was described. Pegmatite exposures in the area have yielded crystals of decimetre-scale size with strong colour zoning. Smaller occurrences have been reported from Vietnam, Pakistan, Brazil and Russia, but Madagascar dominates the trade. Other reported occurrences should be evaluated against the species-defining calcium-dominant chemistry, since older trade descriptions and even some museum labels conflate liddicoatite with calcium-bearing elbaite or with intermediate compositions, and full chemical analysis is sometimes required to assign a sample correctly to the species.

Properties

Liddicoatite shares the general physical properties of the tourmaline group: hardness 7 to 7.5, specific gravity around 3.05, and refractive indices in the range 1.62 to 1.65 with birefringence around 0.02. The species is uniaxial negative and shows the strong dichroism characteristic of tourmaline, which can be used as a quick distinguishing test from many other coloured stones. Inclusions are typically growth-related liquid feathers, two-phase inclusions and tube-like cavities along the c-axis, the latter producing chatoyancy in cabochon material when sufficiently developed.

Trade significance

For the gem trade liddicoatite is principally a specimen and slice market rather than a faceting market. The triangular-zoned slices are marketed by mineral dealers at prices that reflect colour saturation, zone count and crystal size, with the most striking pieces fetching four-figure to low five-figure US dollar prices. Faceted material is collected by tourmaline specialists rather than by general coloured-stone buyers, and origin documentation matters: a Madagascar attribution adds credibility, although the trade is generally pragmatic about Madagascar-or-uncertain attributions where the material's appearance is consistent with that source.

Naming history and Richard T. Liddicoat

The species was approved as a distinct tourmaline species in 1977. Richard T. Liddicoat (1918 to 2002) was a long-serving director and later chairman of the Gemological Institute of America, the developer of the institute's diamond grading system and a central figure in twentieth-century gemmological education. The species name follows a long-established convention in mineralogy of recognising scientific contribution; comparable namings exist for other tourmaline-group species such as povondraite and rossmanite.