Andalusite

Not from Andalusia

Andalusite is one of three aluminosilicate minerals with the identical chemical composition Al₂(SiO₄)O, often written Al₂(SiO₅), but it is really SiO₄ groups with an extra oxygen, not one ion of SiO₅. You might think andalusite came from the Andalusia region of Spain, but you’d be wrong. The original specimens came from El Cardoso, Guadalajara, Spain, but the French and German mineralogists (Jean-Claude Delamétherie and Abraham Gottlob Werner) describing the mineral in 1798 thought mistakenly that they were from Andalusia and named it for that region. Andalusite was subsequently found in Andalusia as well.  

A variety of andalusite called chiastolite (from Greek chiastos, meaning cross, referring to the Greek letter chi, Χ) had actually been described less formally 44 years earlier, but today chiastolite is just a variety of andalusite. But it’s an interesting variety, in which graphite (pure carbon) inclusions were deposited preferentially in certain crystallographic positions, the “corners” of the growing crystal. When the graphite coated a corner crystal face it inhibited growth there until the rest of the crystal “caught up,” and this cycle of growth, retardation, and growth is the likely explanation for the cross as well as the subtle darker, inclusion-rich spots at the corners. At least that’s what Clifford Frondel reportedly thought, and he was in my view a great mineralogist. Frondel’s concept was expressed in 1934 (Mineral Incrustations Upon the Edges and Corners of Crystals: American Museum Novitates No. 759; and in Selective incrustation of crystal forms: Am. Mineralogist, vol. 19, no. 7, pp. 316-329), but I cannot find an explicit reference to chiastolite in his work.   

This specimen at top is from Madera County, California, from metamorphic rocks of the Mother Lode gold belt. It is 33 mm in longest dimension on the polished face. Broadly speaking the Mother Lode belt is the result of a collision between a magmatic island arc (like the West Indies today) and the western margin of North America during Jurassic to Early Cretaceous time, around 170 to 130 million years ago.

The photo above shows a button using chiastolite, from excavations at the site of the fort at Jamestown, Virginia. The fort was established in 1607 and incorporated into James Towne in 1619. If this is as old as 1607, it probably came with a colonist from Europe, perhaps one of the Spanish localities. If it dates to a few decades later, it could have been imported from Massachusetts, where a well-known occurrence of chiastolite around what is now Lancaster in Worcester County was exploited by Native Americans as well as colonists as early as the 1640s (Hoffman and others, 1999, Symbols in stone: Chiastolites in New England archaeology: Bulletin of the Massachusetts Archaeological Society, v. 60 (1), p. 2). Photo by Jamie Ann Meyers.

The Massachusetts andalusite probably formed in the metamorphism resulting from the collision between North America and the Avalonia terrane (a sliver of the Gondwana supercontinent) in the Acadian Orogeny about 420 to 385 million years ago.

In thin section (a slice of rock about 30 µm or 0.03 mm thick) chiastolite makes beautiful images. The textbook-quality photo above is by Chris Gammons of a specimen collected by Joel Dietrich in the Flint Creek Range of southwest Montana USA. Andalusite there grew in a hornfels, a metamorphic rock that lacks the strong planar fabrics of gneiss and schist.

Hornfels forms under conditions of high temperature, but in the absence of significant directed pressure. The hornfels in the Flint Creek Range is the result of baking and metamorphism of aluminous sediments, mostly the Blackleaf Shale of middle Cretaceous age, deposited about 99 million years ago. The heat was provided by intrusive igneous sills that were injected into the shale about 85 to 76 million years ago.

The other two Al₂(SiO₄)O minerals, kyanite and sillimanite, have crystallography that differs from andalusite. Two of the defining aspects of a mineral are both a definite chemical composition AND a specific crystalline structure, so the three identical compositions are three different minerals.

The three aluminosilicates of identical chemical composition, kyanite, sillimanite, and andalusite, each form in discrete temperature-pressure realms, so they provide useful information about the metamorphic history of a rock. Temperature and pressure are often proportional to depth of burial, giving clues to the geologic history of collisions and subduction as well as igneous activity. The diagram above shows this relationship, with andalusite as the relatively low-pressure mineral over a relatively wide range of temperatures.

Minerals with identical composition but different crystallography are called polymorphs, meaning “many forms.”