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1) In what type of rock are Chiastolites formed? In what areas do they form and under what conditions?
2) Please indicate the color of the chiastolites that you can see in the site (matrix and cross).Tell me their average size (diameter and length)
3) According to what you observe at the local, are the chiastoliths harder or softer than the matrix rock?
4) Add a photo to your local, another one that you can see an object, or one nick in a piece of paper
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History
Chiastolite, a variety of the mineral andalusite, began circulating in Europe as a souvenir brought back by pilgrims returning from Santiago de Compostela, probably in the second half of the 16th century. The peculiarity of its appearance, with a black cross, perfectly delineated, on a white or pink background, caught the attention of collectors of that time, so much so that it soon became part of the "cabinets of curiosities" and appeared under variations of the name “cross stone” in most of the catalogs published in the 17th and 18th centuries
Andalusite
This mineral is found in many metamorphic rocks, in which it does not always crystallize visibly, but when it does, it takes the form of prismatic crystals with a simple habit, a square base, or rounded crystals, columnar masses, radial or granular aggregates. The crystals can reach a considerable size, although those with gem character are unusual and small.
Andalusite is an aluminium silicate with the chemical formula Al2SiO5. Andalusite is a common mineral in contact metamorphic zones and forms at low pressures and over a wide range of temperatures. The minerals kyanite and sillimanite are polymorphs of andalusite, each occurring at different temperature-pressure regimes and therefore not occurring simultaneously in the same rock. It is transparent, of variable colour, prismatic, strongly pleochroic and used as a gemstone, although it rarely exceeds 2 carats. Hardness 7.0 to 7.5 and density 3.16 to 3.20.
Chiastolite
Chiastolite is a variety of andalusite whose crystals contain cross-shaped carbonaceous inclusions and is common in some metamorphic rocks.
Origen
The most classic and widely accepted theory about the metamorphic mineralogical process that allows these curious crosses to form is based on the thesis proposed by Frondel in 1934 about the selective incrustation of impurities in the crystals.10 According to this, andalusite crystals, which are fast growing, include carbonaceous impurities as they grow and these accumulate only at certain points (at the corners of the crystals). As the concentration of inclusions (mainly graphite) increases in those places, the growth of the crystal is inhibited (slowed down). This concentrates the inclusions in the characteristic Maltese cross because it produces reentrants, where the graphite is absorbed by the growth of the andalusite porphyroblast. This cycle (growth-delay-growth) is repeated and forms a pattern that resembles a graphite feather in the four radial distribution arms.
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