Anjanabonoina (also rendered Anjanabonoïna in French-language literature) is a pegmatite locality in the central highlands of Madagascar, situated in the Betafo district of the Vakinankaratra region. It is internationally recognised as the type locality — and pre-eminent source — for liddicoatite, the calcium-dominant member of the tourmaline supergroup. Specimens from Anjanabonoina are distinguished by their extraordinary polychrome zoning, which follows the trigonal symmetry of the crystal and produces cross-sections of almost architectural complexity. The locality is documented in Gems & Gemology and GIA literature as one of Madagascar's most mineralogically significant gem deposits, and its crystals occupy prominent positions in major natural-history museum collections worldwide.

Geological Setting

The Anjanabonoina deposit occurs within a suite of late Neoproterozoic to Cambrian granitic pegmatites that intruded the high-grade metamorphic basement of central Madagascar during the Pan-African orogenic event, roughly 500–550 million years ago. These pegmatites are notably enriched in lithium, calcium, fluorine, and rare-earth elements — a geochemical signature that distinguishes them from the more common elbaite-bearing pegmatites found elsewhere on the island and in classic localities such as Pala (California) or Minas Gerais (Brazil).

Liddicoatite, the species that defines the deposit's fame, was formally described as a distinct tourmaline species in 1977 and named in honour of Richard T. Liddicoat, long-time president of the Gemological Institute of America. Its defining characteristic is the substitution of calcium for sodium in the X crystallographic site, combined with elevated fluorine content. Anjanabonoina crystals represent the reference standard against which liddicoatite from other localities is compared.

Crystal Morphology and Colour Zoning

The crystals from Anjanabonoina are typically prismatic with well-developed trigonal symmetry, and it is in cross-section — perpendicular to the c-axis — that their most celebrated feature is revealed. Polished slabs and wafers cut across the crystal display concentric triangular zones of colour that may include combinations of pink, red, green, blue, yellow, brown, and colourless sectors, all arranged in patterns that mirror the three-fold symmetry of the tourmaline structure. No two crystals are identical; the zoning records the changing chemistry of the pegmatitic fluid from which the crystal grew, layer by layer, over geological time.

Several distinct zoning patterns have been described in the literature:

• Concentric triangular zoning — the most common and most prized pattern, in which successive colour bands follow the outline of the triangular prism faces.
• Sector zoning — colour differences between the three alternating prism sectors, producing a pinwheel or trilobate appearance.
• Core-to-rim zoning — a single colour transition from centre to margin, analogous to the watermelon pattern seen in elbaite but expressed with triangular rather than circular geometry.
• Patchy or irregular zoning — less systematic colour distribution, often associated with late-stage fluid infiltration or partial resorption of the crystal.

Colour in liddicoatite is primarily attributed to trace manganese (producing pinks and reds), iron (greens and blues), and combinations thereof. The calcium-rich composition and elevated fluorine content are thought to influence the saturation and stability of these colours relative to sodium-dominant elbaite from other localities.

Collector Specimens versus Faceted Gems

The overwhelming majority of Anjanabonoina material reaches the market as mineral specimens rather than faceted gemstones. Intact prismatic crystals with vivid polychrome zoning command substantial premiums among mineral collectors, and polished cross-section slabs — sometimes called tourmaline wafers — are among the most recognisable collector items in the gem-mineral trade. Museum-quality slabs displaying sharp, saturated triangular zoning in three or more colours are considered among the most visually complex natural objects produced by any pegmatite system.

Faceted liddicoatite from Anjanabonoina is comparatively rare. The complex zoning that makes crystals so desirable as specimens also creates significant challenges for the lapidary: transparent, inclusion-free zones of sufficient size to yield a clean faceted stone are uncommon, and cutting across the zoning to capture multiple colours in a single gem requires both skill and a willingness to sacrifice much of the rough. When faceted stones are produced, they may display bicolour or tricolour effects depending on the orientation chosen. Such gems are sold primarily to specialist collectors rather than into mainstream jewellery channels.

Mining and Trade

Mining at Anjanabonoina has been conducted on an artisanal and small-scale basis for decades. The remote location — accessible only by difficult highland tracks — limits the scale of operations and contributes to the relative scarcity of top-quality material in the international market. Production is irregular, and the supply of exceptional polychrome crystals has always been limited relative to collector demand.

Material from the locality enters the international market principally through Antananarivo, Madagascar's capital, and through mineral show networks in Tucson, Munich (Mineralientage), and Denver. Provenance documentation for individual crystals is inconsistent, as is typical of artisanal production, though the distinctive triangular zoning of Anjanabonoina liddicoatite is sufficiently characteristic that experienced dealers and gemmologists can identify the locality on visual grounds alone.

Madagascar as a whole is one of the world's most mineralogically diverse gem-producing nations, and Anjanabonoina occupies a singular position within that broader context: it is the locality that effectively defined a tourmaline species and established a benchmark for polychrome complexity that no other deposit has surpassed.

Gemmological Identification

Liddicoatite from Anjanabonoina can be distinguished from elbaite and other tourmaline species by a combination of properties:

• Refractive indices: typically n_o 1.635–1.651, n_e 1.615–1.632, with birefringence of approximately 0.018–0.020 — slightly higher than typical elbaite.
• Specific gravity: approximately 3.02–3.10, somewhat elevated relative to elbaite (c. 3.06 on average), reflecting the substitution of the heavier calcium ion for sodium.
• Chemical analysis: definitive identification requires electron microprobe or LA-ICP-MS analysis confirming calcium dominance at the X site and elevated fluorine content. Routine gemmological instruments cannot distinguish liddicoatite from elbaite with certainty.
• Characteristic zoning: the triangular cross-sectional zoning visible in slabs and, under magnification, in faceted stones is strongly indicative of Anjanabonoina origin, though not strictly diagnostic of the species.

Major gemmological laboratories do not routinely issue species-level tourmaline reports distinguishing liddicoatite from elbaite unless the client specifically requests chemical analysis. Collector-oriented laboratories and research institutions are more likely to provide species-level identification for significant specimens.

Scientific Significance

Beyond their aesthetic appeal, Anjanabonoina crystals have contributed substantially to the scientific understanding of tourmaline crystal chemistry and growth dynamics. Studies of the zoning patterns in liddicoatite from this locality have been used to model the evolution of pegmatitic fluids over time, treating each colour band as a chemical record of changing temperature, pressure, and melt composition. The crystals thus function as natural archives of pegmatite petrogenesis, and they continue to be subjects of mineralogical and geochemical research.

Further Reading

• Gems & Gemology — Tourmaline from Madagascar (GIA)
• GIA Gem Encyclopedia: Tourmaline