Andalusite- Al₂SiO₅

Andalusite, named by Jean-Claude Delamétherie in 1798 for the supposed type locality, "Andalusia region" (Spain), is an aluminium mineral with the chemical formula Al₂SiO₅. Andalusite occurs as an accessory mineral in a wide range of felsic peraluminous A/CNK = molar [(Al₂O₃)/(CaO + Na₂O + K₂O)] > 1 extrusive and intrusive igneous rocks (including rhyolites, aplites, granites, pegmatites, and anatectic migmatites).

Controls on the formation of magmatic Andalusite

Metaluminous felsic igneous rocks do not contain primary magmatic andalusite, but peraluminous felsic igneous rocks can. The effect of excess Al₂O₃ in the magma is two-fold:

(1) it creates a more favourable T–P range for andalusite crystallization by depressing the granite solidus, enlarging the region of overlap with the andalusite stability field;
(2) it creates a more favourable compositional condition because, the more peraluminous a magma is, the greater is the probability that it will become saturated in Al₂SiO₅ and crystallize andalusite.

Of the many possible controls on the formation of andalusite (excess Al₂O₃, water concentration and fluid evolution, high Be–B–Li–P, high F, high Fe–Mn–Ti, and kinetic considerations), the two most important factors appear to be excess Al₂O₃ and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt).

Optical properties:
• Color: Colourless or pale pink
• Form: Usually occurs as elongate prisms with square cross section
• Cleavage: (001) good appearing as traces parallel to the prism edge in prismatic sections but intersecting at right angle in basal section
• Interference colors: Low, first order (like quartz)
• Alteration: Commonly sercite
• Relief: Moderate

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Andalusite crystal with pink core. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)	Andalusite crystal rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystals. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
Andalusite crystals. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)	Andalusite crystal rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystal with pink core. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)
Andalusite crystals rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystals rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystals rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
Andalusite crystals. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)	Andalusite crystals. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)	Andalusite crystals rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
Andalusite crystals rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystals rimmed by sericite. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite and Garnet crystals. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
Andalusite crystal with pink core. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)	Andalusite crystal. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)	Andalusite crystal with pink core. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)
Andalusite crystal. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystal. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)	Andalusite crystal with sericite alteration. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
Andalusite crystal. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)	Andalusite crystal with sericite alteration. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystal with pink core. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)
Andalusite crystal with sericite alteration. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)	Andalusite crystal. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)	Andalusite crystal with sericite alteration. Leucogranitic dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)