Pegmatites: giant-crystal rocks, tourmaline, beryl and kunzite

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Crystals bigger than life

A pegmatite is an igneous rock with a very coarse texture: its crystals usually exceed one centimeter and can reach several meters. Most are granitic in composition, made of quartz, feldspar and mica, but with enormous grains. The word therefore describes a texture before it describes a composition. It comes from the Greek pêgma (“that which is fixed or assembled”), and was used by René Just Haüy in 1822 for “graphic pegmatite”, in which quartz and feldspar intergrow in patterns that resemble written characters.

Pegmatites stand apart among igneous rocks (see igneous rocks): they supply most of the spectacular crystals, including many gemstones.

How do they form?

When a large body of granitic magma cools, it crystallises gradually. The liquid left at the end is poor in the major rock-forming components and rich in water, volatile elements (boron, fluorine, phosphorus) and rare elements (lithium, beryllium, cesium, tantalum, niobium). This residual liquid, which has low viscosity, is injected into the cracks of the surrounding rock and crystallises from the outside inward. Atoms move easily through it and feed a few large crystals rather than many small ones.

Specialists do not fully agree on the details. The classic model (1960s) invokes the separation of an aqueous phase from a magma saturated in water. More recent work, such as that of David London, proposes a liquid strongly cooled below its crystallisation temperature, whose viscosity, lowered by fluxing elements (boron, lithium, fluorine, phosphorus), slows the formation of new crystals and favors the growth of a few. The question remains open, notably regarding the rate of cooling.

Zones and pockets

Large pegmatites are often zoned like an onion: a fine-grained border zone, a wall zone, intermediate zones with very large crystals (feldspars, micas), and sometimes a massive quartz core. Some contain cavities, the pockets, where crystals grew freely and ended in sharp faces: this is where gemstones and collector crystals are found. The pockets are often filled with clay, which crystal hunters must remove with care.

Modern classifications mainly distinguish two families: LCT pegmatites (rich in lithium, cesium, tantalum) and NYF pegmatites (niobium, yttrium, fluorine).

Minerals and gems found in pegmatites

Mineral or stone	Interest	Mohs hardness
Tourmaline (elbaite and others)	Multicolored crystals, sometimes bicolored or “watermelon”; copper-bearing Paraíba tourmaline (Brazil) has been famous since the late 1980s	7 to 7.5
Beryl: aquamarine, morganite, heliodor	Elongated crystals, sometimes of large size	7.5 to 8
Topaz	Transparent crystals, sometimes very large	8
Spodumene: kunzite, hiddenite	Pink kunzite described at Pala (California) in 1902, named after the gemologist George F. Kunz; perfect cleavage, hence fragile	6.5 to 7
Lepidolite	Pink to lilac lithium-bearing mica, a source of lithium	2.5 to 3.5
Amazonite, moonstone, smoky quartz, rose quartz	Famous amazonite from Pikes Peak (Colorado); smoky quartz in large crystals	6 to 7
Spessartine garnet, apatite, columbite-tantalite, cassiterite	Collector minerals and ores	variable

See our pages pink tourmaline, morganite, kunzite, lepidolite and aquamarine.

Records

Pegmatites hold the largest crystals known, according to compilations that must still be checked case by case: a spodumene more than 12 meters long in the Black Hills of South Dakota (Etta mine), a beryl of about 380 tonnes in Madagascar (Malakialina), and large masses of feldspar in Karelia that are said to weigh thousands of tonnes. The Naica cave in Mexico, with its gypsum crystals over 11 meters long, is not a pegmatite: it results from a different process (see the monoclinic system).

In France and worldwide, and what they are used for

Pegmatites occur in Limousin, the Massif Central, Brittany and the Alps. Montebrasite, a lithium phosphate mineral, is named after Montebras in the Creuse, where it was described in the 19th century. Worldwide, they are mined for their feldspar (ceramics, glass), mica, high-purity quartz and, increasingly, lithium (batteries): some major spodumene deposits, for example in Western Australia, are pegmatites.

Pegmatites also have a sensitive side. Coltan (columbite-tantalite) and cassiterite, the ores of tantalum and tin, come in part from artisanal mines in countries affected by conflict. See responsible stone extraction.

Safety, collecting and observation

• Beryl dust. Beryl dust contains beryllium, which is toxic if inhaled. Never grind or sand raw beryl; a cut and polished stone poses no problem.
• Kunzite and topaz: perfect cleavage, so avoid knocks and prolonged sunlight (kunzite can fade in light).
• Collecting in the field. It is regulated in France and elsewhere: nature reserves, quarries and private land. Always ask for permission.
• Safe observation. On a block of coarse-grained granite or in the mica flakes of river sand, look for mica “books” that flake apart under a fingernail. See cleavage.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Mineralogy for beginners.