Andalusite
Aluminosilicate index mineral used as a refractory and gemstone.
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. It was originally called andalousite by the mineralogist Delamétherie, who mistakenly believed it came from Andalusia, Spain; the actual source was later identified as El Cardoso de la Sierra, in the Spanish province of Guadalajara. Andalusite is trimorphic with kyanite and sillimanite, serving as a lower-pressure, mid-temperature polymorph and an important index mineral for metamorphic geology. At higher temperatures and pressures, andalusite can transform into sillimanite, making these three minerals useful for determining the pressure-temperature history of the host rock. Andalusite is a common metamorphic mineral that forms under low pressure and low to high temperatures, particularly in pelitic rocks like mica schist. The world’s highest concentration of andalusite is found in the Glomel mine in Côtes-d’Armor, France, which accounts for a quarter of global production, while South Africa holds the largest known deposits. Notable varieties include chiastolite, known since at least the sixteenth century, which contains dark carbon or clay inclusions that form a cross-shaped pattern when cut in cross-section; pilgrims returning from Santiago de Compostela carried it as a souvenir to many European countries. Viridine is a green variety where manganese substitutes for aluminium, giving it its color, while kanonaite is a greenish-black related mineral. A clear variety from Brazil and Sri Lanka can be faceted into gemstones that display a muted play of red, green, and yellow colors due to strong pleochroism, not iridescence. Industrially, andalusite is used as a refractory material in furnaces, kilns, and other high-temperature processes.
- chemical_formula
- Al2SiO5
- crystal_system
- Nesosilicate
- polymorphs
- Kyanite, sillimanite
- varieties
- Chiastolite, viridine, kanonaite
- key_property
- Index mineral for pressure-temperature paths
- major_deposits
- Glomel mine, France (25% of global production); South Africa (largest known deposits)
Lore & Background
Andalusite was named by Delamétherie based on a locality error, as the specimens studied came from El Cardoso de la Sierra, not Andalusia. The mineral is trimorphic with kyanite and sillimanite, and at higher temperatures and pressures it may convert to sillimanite. Its presence in metamorphic rocks helps geologists identify the depth and pressure conditions under which the host rock formed. Notable varieties include chiastolite, which contains dark carbon or clay inclusions forming a cruciform pattern in cross-section; this stone was carried as a souvenir by pilgrims returning from Santiago de Compostela as early as the sixteenth century. Viridine is a green variety where manganese 3+ substitutes for aluminium, and kanonaite is a greenish-black mineral with approximate composition (Mn0.76Al0.23Fe0.02)AlSiO5. A clear variety from Brazil and Sri Lanka can be cut into gemstones, exhibiting strong pleochroism that produces red, green, and yellow colors. Andalusite is a common metamorphic mineral found under low pressure and low to high temperatures, particularly in pelitic rocks such as mica schist. The world's highest concentration is in the Glomel mine in France, which accounts for 25% of global production, while South Africa holds the largest known deposits.
Reader's Guide
Andalusite holds significance primarily as a metamorphic index mineral, alongside its polymorphs kyanite and sillimanite. Because each polymorph forms under distinct temperature-pressure regimes, their presence in a rock allows geologists to reconstruct the pressure-temperature history of the host rock. This makes andalusite a key tool in petrology and structural geology. Its industrial use as a refractory material in furnaces and kilns adds economic importance. The mineral's varieties also have historical and gemological interest: chiastolite was collected by medieval pilgrims, and faceted andalusite displays unusual pleochroism. The initial misidentification of its locality—attributed to Andalusia but actually from Guadalajara—is a notable episode in mineralogical history. The Glomel mine in France and South African deposits dominate global supply, underscoring the mineral's role in both science and industry.
Did You Know?
- Andalusite was originally misidentified as coming from Andalusia, Spain, but the actual source was El Cardoso de la Sierra in Guadalajara.
- The variety chiastolite contains dark inclusions that form a cruciform pattern in cross-section and was carried by pilgrims from Santiago de Compostela as early as the sixteenth century.
- Andalusite is trimorphic with kyanite and sillimanite, and at higher temperatures and pressures it may convert to sillimanite.
- The Glomel mine in France accounts for 25% of global andalusite production.
Frequently Asked Questions
What is Andalusite?
Andalusite is an aluminium nesosilicate with the formula Al2SiO5, forming as the lower-pressure, mid-temperature stable phase of that system. It sits in the same trimorphic trio as kyanite and sillimanite, all three sharing identical chemistry but different crystal structures.
What are Andalusite's polymorphs?
Kyanite and sillimanite are the two other polymorphs of Al2SiO5 alongside Andalusite. Andalusite specifically crystallizes under lower-pressure, moderate-temperature metamorphic conditions, distinguishing it from its higher-pressure or higher-temperature siblings.
Where are the major Andalusite deposits located?
The Glomel mine in France supplies roughly a quarter of global Andalusite output, while South Africa hosts the largest known deposits. The mineral's name is a historical mix-up: Delamétherie assumed the type specimen came from Andalusia, Spain, but it actually originated at El Cardoso de la Sierra in Guadalajara.
What is Chiastolite and how does it relate to Andalusite?
Chiastolite is a visually striking variety of Andalusite in which fine inclusions of hematite or ilmenite form a characteristic cross pattern inside the crystal. It is grouped with other recognized varieties such as viridine and kanonaite, all of which share the same Al2SiO5 chemistry.
Why do geologists care about Andalusite?
Andalusite serves as an index mineral, meaning its presence in a rock assemblage pinpoints the specific pressure-temperature window the rock passed through during metamorphism. This makes it a critical diagnostic tool for reconstructing the thermal and pressure history of metamorphic terranes.
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