Anorthite

specimen number:
1052569

location:
Antsohamamyquarry, Fotadrevo, Ampanihy, Atsimo-Andrefana, Madagascar

description:
A cut and polished slab of labradorite. This is a nice play of colors.

The Antsohamamy Quarry, located in the Toliara Province of southern Madagascar, is a premier source for high-quality iridescent feldspar. Geologically, the quarry is situated within a massive Precambrian anorthosite complex, which is a type of intrusive igneous rock composed almost entirely of plagioclase feldspar. These rocks formed over 500 million years ago during the Pan-African orogeny when magma cooled slowly deep within the earths crust, allowing for the development of large, coarse-grained mineral structures. Historically, the quarry has been an active site for artisanal and small-scale commercial mining for several decades, primarily targeting the decorative stone market. The operation is well-known for producing large blocks of chatoyant material that are exported globally for use in architectural surfaces, countertops, and lapidary arts. While industrial records for such remote locations are often sparse, the site remains a consistent and vital contributor to the gemstone and ornamental stone economy of Madagascar.

Anorthite is the calcium-rich endmember of the plagioclase feldspar mineral series, and its variety labradorite is specifically defined by having a calcium-to-sodium ratio between fifty and seventy percent. This mineral is famous for a spectacular optical phenomenon known as labradorescence, which is a vivid play of colors caused by the interference of light within microscopic, alternating layers of different feldspar compositions. Geologically, these layers form through a process called exsolution as the mineral cools, creating a natural diffraction grating that reflects brilliant flashes of blue, green, gold, and orange. In the specimens from Antsohamamy, the crystals often exhibit a deep, saturated iridescence that is highly prized by collectors and jewelers. Beyond its beauty, labradorite is a significant rock-forming mineral that provides important clues to the cooling history and chemical evolution of the mafic magmas from which it crystallized.

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