Chiastolite ( ) is a variety of the

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

andalusite Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétherie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the sp ...

with the

chemical composition A chemical composition specifies the identity, arrangement, and ratio of the chemical elements making up a compound by way of chemical and atomic bonds. Chemical formulas can be used to describe the relative amounts of elements present in a com ...

Al₂SiO₅. It is noted for its distinctive cross-shaped black inclusions of

graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

. The presence of these crosses caused the mineral to be used as a gem. Chiastolite specimens were distributed throughout Europe from the

16th century The 16th century began with the Julian calendar, Julian year 1501 (represented by the Roman numerals MDI) and ended with either the Julian or the Gregorian calendar, Gregorian year 1600 (MDC), depending on the reckoning used (the Gregorian calend ...

, as an amulet or souvenir provided by the pilgrims returning from

Santiago de Compostela Santiago de Compostela, simply Santiago, or Compostela, in the province of Province of A Coruña, A Coruña, is the capital of the autonomous communities of Spain, autonomous community of Galicia (Spain), Galicia, in northwestern Spain. The city ...

(Saint James of Compostella), in Spain. In the old books of mineralogy, chiastolite appears it is cited with the name of l''apis crucifer'' or ''lapis cruciatur'', cross stone. The first figure of a chiastolite appears in Laet's book ''De Gemmis et Lapidibus'', published in 1648. The chiastolite specimens sold to the pilgrims came from

Asturias Asturias (; ; ) officially the Principality of Asturias, is an autonomous communities of Spain, autonomous community in northwest Spain. It is coextensive with the provinces of Spain, province of Asturias and contains some of the territory t ...

, where it is very abundant, in large specimens, in the area of Boal. In areas around

Georgetown, California Georgetown (formerly Growlersburg) is a census-designated place (CDP) in El Dorado County, California, United States. It is the northeasternmost town in the California Mother Lode. The population was 2,546 in the 2020 census, up from 2,367 in 20 ...

, US, metamorphosed sediments contained andalusite and chiastolite in a graphite-rich metasediment. The chiastolite crystals have been pseudomorphically altered by a mixture of

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

, paragonite and margarite. The calcium-rich margarite tends to form along the graphite-rich crosses or bands within the chiastolite. Mineralogically the occurrence is important because all three ''white mica'' phases are present in an equilibrium assemblage. There are several theories regarding the formation of the chiastolite cross, however the most widely accepted theory, proposed by Frondel in 1934, suggests that there is a selective attachment of impurities at the rapidly growing corners of andalusite crystals. As the concentration of these impurities (which consist primarily of graphite) increases, the growth of the crystal is slowed. This concentrated impurity deposit forms a re-entrant as it is absorbed by the growth of the andalusite porphyroblast. The cycle of growth-retardation-growth then repeats itself, creating a featherlike pattern of graphite along four radiating "arms".

References

External links

* Aluminium minerals Nesosilicates Orthorhombic minerals {{silicate-mineral-stub Gemstones