Labradorite

Lore & Background

Labradorite is a calcium-rich member of the plagioclase feldspar series, with an anorthite content between 50 and 70 percent. It typically appears as clear, white, or grey grains that are blocky or lath-shaped. The mineral has a triclinic crystal system, a white streak, a specific gravity ranging from 2.68 to 2.72, and a refractive index between 1.559 and 1.573. Twinning is common, and it exhibits three cleavage directions: two are good to perfect and nearly at right angles, while the third is poor. Its defining characteristic is labradorescence, an iridescent optical effect caused by a lamellar structure formed through phase exsolution within the Bøggild miscibility gap. This effect is visible only when the lamellae are separated by 128 to 252 nanometers and requires a very slow cooling rate to allow ion diffusion. Not all labradorite displays this effect, as composition and cooling history must be suitable. The mineral is most common in mafic igneous rocks such as basalt and gabbro, and it forms the bulk of anorthosite bodies. It also occurs in metamorphic amphibolites and as detrital grains in some sediments. Common associated minerals include olivine, pyroxenes, amphiboles, and magnetite. The type locality is Paul’s Island near Nain, Labrador, Canada, and it has been reported from Poland, Norway, Finland, Madagascar, China, Australia, Slovakia, and the United States. A rare variety known as spectrolite, mined in Finland, exhibits a richer and more varied range of iridescent colors than typical labradorite.

Reader's Guide

Labradorite is significant as a common feldspar in mafic igneous rocks and as the primary constituent of anorthosite bodies. Its optical phenomenon, labradorescence, has been studied since the early 20th century, with key contributions from Bøggild and Lord Rayleigh. The effect is restricted to plagioclases of specific composition (calcic labradorite and bytownite) and requires slow cooling, meaning not all labradorite displays it. The variety spectrolite, from Finland, became a local industry after World War II, with gem cutting and an annual show established in Ylämaa. Labradorite occurs worldwide, including in Madagascar, China, Australia, Slovakia, and the United States. Its legacy lies in both its geological abundance and its aesthetic value as a gem material.

Did You Know?

• Labradorite was first identified on Paul's Island near Nain, Labrador, Canada.
• The term labradorescence was coined by Ove Balthasar Bøggild.

Mineralogical Identity & Crystallographic Character

Labradorite occupies the intermediate-to-calcic segment of the plagioclase feldspar family, defined by an anorthite content between 50 and 70 percent. Its formula (Ca, Na)(Al, Si)4O8 reflects the solid-solution nature of the plagioclase series, where calcium and sodium substitute for one another across the compositional spectrum. Like every member of this family, it crystallizes in the triclinic system and exhibits three cleavage directions—two nearly perpendicular and of good to perfect quality, while the third is notably weaker. The mineral typically presents as blocky or lath-shaped grains in shades of clear, white, or grey. Its specific gravity falls between 2.68 and 2.72, the streak is white as expected of silicate minerals, and the refractive index spans 1.559 to 1.573. Twinning is a common feature, adding visual complexity to the crystal. Together these properties distinguish labradorite from its more sodic or more calcic neighbors within the plagioclase continuum.

The Science of Labradorescence

The shimmering, iridescent play of color known as labradorescence (or schiller) arises from a very specific internal architecture. The effect is produced by a phase-exsolution lamellar structure that forms within what is termed the Bøggild miscibility gap. For the optical phenomenon to be visible, the separation between these lamellae must fall between roughly 128 and 252 nanometers; the lamellae need not be perfectly parallel, and the structure lacks long-range order. Two key conditions must align: the plagioclase must possess the right composition (calcic labradorite at 50–70 percent anorthite, or bytownite at roughly 70–90 percent), and the host rock must have cooled extremely slowly so that calcium, sodium, silicon, and aluminum ions could diffuse and separate into the lamellar pattern. Because both requirements are restrictive, many labradorites show no labradorescence at all, and some labradorescent specimens are actually bytownite rather than labradorite. The term was coined by Ove Balthasar Bøggild, and Robert Strutt, 4th Baron Rayleigh, also contributed to understanding the effect in the early 1920s.

Global Occurrence & Host Rocks

The geological type locality for labradorite is Paul's Island, near the town of Nain in Labrador, Canada—the region that gave the mineral its name. Beyond that classic setting, the mineral has been documented in Poland, Norway, Finland, Madagascar, China, Australia, Slovakia, and the United States, among other locations. In terms of host rocks, labradorite is the dominant feldspar variety in common mafic igneous rocks, particularly basalt and gabbro. It also forms the near-entire composition of the rarer anorthosite bodies. Metamorphic amphibolites and certain detrital sediments contain labradorite as well. In igneous settings it is typically accompanied by olivine, pyroxenes, amphiboles, and magnetite. The mineral's prevalence in these widespread rock types means that labradorite grains are encountered routinely in geological samples worldwide, even when the iridescent variety is far less common.

Spectrolite: A Finnish Legacy

Spectrolite is a rare, highly iridescent variety of labradorite distinguished by a far richer palette of colors than the predominantly blue-grey-green tones seen in Canadian or Madagascan material. The name has become so closely tied to Finnish material that it now functions as a brand name for labradorite mined exclusively in Finland, though the term is sometimes misapplied to any labradorite displaying a vivid play of color regardless of origin. The variety was described by Finnish geologist Aarne Laitakari (1890–1975), who spent years investigating its origin. The pivotal discovery came in 1940, when his son Pekka, working on the Salpa Line fortifications at Ylämaa in south-eastern Finland, uncovered a spectrolite deposit. Quarrying resumed after World War II and grew into a meaningful local industry. In 1973 the first cutting and polishing workshop opened in Ylämaa, followed by a dedicated gem centre offering training. Mayor Esko Hämäläinen also launched an annual Gem and Mineral Show, cementing the town's identity around this remarkable stone.