The intermediate zone is one of the named layers in the standard zonation scheme used to describe granitic pegmatites. Pegmatites are coarse-grained igneous bodies that crystallise from the most fractionated, water-rich and incompatible-element-enriched fraction of a granitic melt, and many of the world's gem deposits, particularly those for tourmaline, beryl, spodumene and topaz, sit in zoned pegmatites. The intermediate zone, lying between the outer wall zone and the central core, often hosts the most economically interesting mineralogy.

The classic zonation scheme

A textbook zoned pegmatite displays, from the contact inwards, a fine-grained border zone a few centimetres thick, a coarser-grained wall zone, one or more intermediate zones, and a quartz-rich core. Each zone records a successive stage in the cooling and chemical evolution of the melt as it crystallises against the country rock. Border and wall zones tend to be quartz, feldspar and mica with minor accessory minerals. The core is usually massive quartz, sometimes with embedded crystals. Between them, the intermediate zones host the bulk of the rare-element minerals.

Why gems appear here

By the time crystallisation reaches the intermediate zone the melt has been concentrated in volatiles, fluxes and incompatible elements that the granitic system could not accommodate earlier. Boron, lithium, beryllium, fluorine, phosphorus and rare alkalis become enriched. These are precisely the elements that build tourmaline (boron), beryl (beryllium), spodumene and lepidolite (lithium), topaz (fluorine) and the suite of phosphates that accompany them. The textures are also right for gem material: cavities, miarolitic pockets and slow growth from a fluid-rich residual melt, which together produce the clear, well-formed, gem-quality crystals that miners pursue.

Pocket mineralisation

Within the intermediate zone, the most prized feature is the miarolitic pocket, a cavity left by exsolution of an aqueous fluid as the melt approached complete crystallisation. Pocket walls are lined with euhedral crystals that grew freely into open space, and it is in these pockets that the finest gem tourmaline, aquamarine, morganite, kunzite and topaz are recovered. The Himalaya mine in California, the Pala district, Minas Gerais in Brazil and the Paprok and Kunar areas of Afghanistan are all examples of pegmatite fields whose gem yield comes overwhelmingly from intermediate zone pockets.

Implications for sourcing

For the trade, knowing that a stone came from a pegmatite intermediate zone signals certain characteristics. The crystals are typically large enough for clean faceted material, the chemistry is rich enough that vivid colours are possible, and the pocket environment tends to leave distinctive inclusion suites such as etched surfaces, fluid trails and growth-tube remnants. Identifying a tourmaline as Brazilian, Mozambican or Afghan in origin often comes down to the inclusion fingerprint produced by its specific intermediate zone host.