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Geology is the Way

Andalusite

Orthorhombic

Al2SiO5

Andalusite is an aluminum nesosilicate and one of the three polymorphs of Al2SiO5 (the other two are kyanite and sillimanite). Andalusite occurs primarily in low-pressure/medium-temperature metapelitic rocks, but can be found also in some Al-rich granites. The name ‘andalusite’ was coined by Jean-Claude Delamétherie in 1798 in reference to the type locality, which was supposed to be in Andalusia, Spain. However, the actual type locality of the specimens studied by Delamétherie and other mineralogists of the time was likely El Cardoso de la Sierra (Guadalajara, Castilla–La Mancha), a village situated north of Madrid 300 km from the border of Andalusia. Nevertheless, the name remained the official designation of the mineral.

Structure and chemistry
The structure of andalusite contains two types of sites for Al: distorted octahedrons, where Al is surrounded by 6 oxygen atoms (light grey in the figure below), and trigonal bipiramids, where Al is in five-fold coordination (surrounded by 5 oxygens; dark grey color in figure). Octahedral sites form chains of edge-sharing octahedrons oriented parallel to the c-axis. The trigonal bipiramids, on the other hand, share their vertices only with Si-bearing tetrahedral sites and Al octahedral sites, surrounding the octahedral chains laterally on the {001} plane (i.e. the basal plane). As a result, tetrahedral sites and trigonal bipiramids alternate in the structure along the c-axis, so that tetrahedral sites and trigonal bipiramids are never connected to identical sites in the structure.
From a chemical point of view, andalusite deviates little from its pure chemical formula but it can contain up to 2 wt% of Fe3+, and even trivalent Mn (Mn3+), which can substitute Al. The Mn-rich variety of andalusite is known as viridine.

andalusite crystal structure
Crystal structure of andalusite seen on its basal plane (a-axis: horizontal; b-axis: vertical). In this 3D view, chains of light grey octahedrons oriented parallel to the c-axis (coming towards the viewer) are visible. Dark grey: Al trigonal bipiramids. Blue: Si tetrahedrons. Source: ruby.colorado.

CPL
CPL
CPL
PPL
PPL

⇔ slider. Basal section of andalusite in a migmatitic hornfels, with square-like outline and cleavage planes intersecting at orthogonal angle. Width: 3 mm. Calamita Schists. Punta Bianca, Island of Elba, Italy.

Properties
Habit: prismatic
Hardness: 6.5 – 7.5
Density: 3.14 – 3.16 g/cm3
Cleavage: {110} good (intersecting at 89°), {100} poor 
Twinning: {101} rare
Color: pink to red/rose-red, grey, violet, yellow, green. Dark-colored when clouded by inclusions.
Luster: vitreous
Streak: white
Alteration: sericite
In thin section…
α(//c): 1.633-1.642
β(//b): 1.639-1.644
γ(//a): 1.644-1.650
2Vα: 73-86°
Color: colorless or (more rarely) pale pink/green
Pleochroism: colored varieties show α
rose-pink, β/γ greenish yellow
Birefringence (δ): 0.009-0.012 (first-order grey)
Relief: moderate
Optic sign:
[Mindat]

Field features

Andalusite crystal habit
Sketch of an andalusite crystal showing the orientation of crystal axes, refractive indices, and cleavage planes. Modified after Optical Mineralogy: Principles and Practice.

Andalusite occurs predominantly in metamorphic rocks with sedimentary protoliths in contact aureoles and low-pressure/high-temperature metamorphic units. It can be recognized as prismatic crystals with square-like basal sections with two orthogonal sets of cleavage planes. The colors of andalusite are frequently obscured by the presence of inclusions or impurities and in rocks it often appears grey or dark-colored. Altered andalusite can appear white. A quite peculiar (but distinctive) type of andalusite is chiastolite, cross-shaped andalusite with inclusions of graphite concentrated in the nuclei of crystals and along two perpendicular planes oblique to the {110} cleavage planes.

Andalusite chiastolite
Andalusite (var. chiastolite) from unknown locality. Source: wikimedia.commons.
andalusite hornfels
Elongated prisms of andalusite surrounded mostly by biotite in a high-grade migmatitic hornfels. Calamita Schists. Punta Bianca, Island of Elba, Italy.
andalusite hornfels
Prismatic andalusite crystals in a metapelitic hornfels. Calamita Schists. Punta Bianca, Island of Elba, Italy.
andalusite cordierite lineation
Lineation defined by oriented grains of andalusite and cordierite in a micaschist. Calamita Schists. Remaiolo, Island of Elba, Italy.

Andalusite in thin section
Andalusite is easily identified for its moderate relief, transparency at PPL, first-order grey (maximum pale yellow) colors at CPL, prismatic habit, and characteristic square-shaped basal section with two sets of {110} cleavage planes intersecting almost perpendicularly. The presence of Fe in the structure may give rise to pink and red andalusite varieties, which are pleochroic. In general, even little quantities of Fe produce a very pale (but noticeable) pink color in andalusite, which stands out in comparison with feldspar and quartz. On the other hand, Mn in the structure produces pale green colors and pleochroism. Andalusite is an optically negative mineral with a negative sign of elongation (length fast) and straight extinction.

CPL
CPL
CPL
PPL
PPL

CPL
CPL
CPL
PPL
PPL

⇔ sliders. Basal (left) and prismatic (right) section of andalusite in a schist. The basal section shows a nearly square-shaped outline and two perpendicular sets of cleavage planes. In the elongated prismatic section, cleavage planes appear parallel. Note the pale pink color at PPL. Width: 1.2 mm. Calamita Schists. Praticciolo, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

⇔ slider. Prismatic andalusite crystal, surrounded by recrystallized quartz in a deformed pegmatite. Andalusite shows an unusually intense pink color that vary in hue highlighting zoning patterns within the crystal. Width: 3 mm. Calamita Schists. Pontimento, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

⇔ slider. Aggregate of andalusite and biotite crystals surrounded by sericite and quartz in a hornfels. Width: 3 mm. Calamita Schists. Punta Bianca, Island of Elba, Italy.


Video. Elongated andalusite grain, with cleavage planes outlining the orientation of the long axis. The insertion of the lambda plate shows that andalusite is length fast (negative elongation). Width: 3 mm. Calamita Schists. Punta Bianca, Island of Elba, Italy.

Examples of andalusite-bearing rocks

Andalusite porphyroblasts
Schist with andalusite grains (rotated porphyroblasts) surrounded by biotite, white mica, and quartz. Many andalusite grains contain abundant ilmenite inclusions that outline an internal foliation.
Sample: andalusite schist
Assemblage: andalusite, biotite, muscovite, quartz, ilmenite, tourmaline
Locality: Calamita Schists. Praticciolo, Island of Elba, Italy

CPL
CPL
CPL
PPL
PPL

⇔ slider. Width: 3 mm. Calamita Schists. Praticciolo, Island of Elba, Italy.

Occurrence
As the low-pressure/medium-temperature polymorph of Al2SiO5, andalusite occurs primarily in high-Al metamorphic rocks, especially metapelites and metasandstones, metamorphosed at hornfels or low-pressure amphibolite-facies conditions. Within these rocks, andalusite frequently occurs associated with cordierite, biotite, and white mica. Andalusite can also occur in peraluminous igneous rocks such as rhyolites and granitoids but it is unclear whether its presence is related to contamination, e.g. by xenolithic metapelitic material, or it is produced by igneous crystallization. In any case, andalusite is present also in some pegmatites, leucogranites, and veins associated with igneous intrusions, where it appears to have a hydrothermal origin.

Burnham, C. W., & Buerger, M. J. (1961). Refinement of the crystal structure of andalusite. Zeitschrift Für Kristallographie-Crystalline Materials115(1-6), 269-290.
Clarke, D. B., Dorais, M., Barbarin, B., Barker, D., Cesare, B., Clarke, G., … & Woodard, H. H. (2005). Occurrence and origin of andalusite in peraluminous felsic igneous rocks. Journal of Petrology46(3), 441-472.
Papeschi, S., Musumeci, G., Massonne, H. J., Bartoli, O., & Cesare, B. (2019). Partial melting and strain localization in metapelites at very low-pressure conditions: The northern Apennines magmatic arc on the Island of Elba, Italy. Lithos350, 105230.
Pattison, D. R. (1992). Stability of andalusite and sillimanite and the Al2SiO5 triple point: constraints from the Ballachulish aureole, Scotland. The Journal of Geology100(4), 423-446.

Resources

        

Mineral Properties
Minerals

 

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