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



• Clarke, D. B., McKenzie, C. B., Muecke, G. K. & Richardson, S. W. (1976): Magmatic andalusite from the South Mountain Batholith, Nova Scotia. Contributions to Mineralogy and Petrology 56, 279–287.
• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• B. W D. Yardley, W S. Mackenzie, C. Guilford: Atlante delle rocce metamorfiche e delle loro microstrutture. Zanichelli editore

Photo
calamita(1).jpg

Andalusite crystal with pink core. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)
calamita(2).jpg

Andalusite crystal rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
calamita(17).jpg

Andalusite crystals. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
calamita(18).jpg

Andalusite crystals. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
calamita(19).jpg

Andalusite crystal rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
calamita(20).jpg

Andalusite crystal with pink core. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). PPL image, 10x (Field of view = 2mm)
calamita(22).jpg

Andalusite crystals rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
calamita(23).jpg

Andalusite crystals rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
calamita(24).jpg

Andalusite crystals rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
calamita(48).jpg

Andalusite crystals. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
calamita(49).jpg

Andalusite crystals. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
calamita(50).jpg

Andalusite crystals rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
calamita(53).jpg

Andalusite crystals rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
calamita(54).jpg

Andalusite crystals rimmed by sericite. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). XPL image, 10x (Field of view = 2mm)
calamita(55).jpg

Andalusite and Garnet crystals. Leucogranitic Dyke, Capo Calamita, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
andalusitemagmatica(2).jpg

Andalusite crystal with pink core. PPL image, 2x (Field of view = 7mm)
andalusitemagmatica(3).jpg

Andalusite crystal. XPL image, 2x (Field of view = 7mm)
andalusitemagmatica(4).jpg

Andalusite crystal with pink core. PPL image, 10x (Field of view = 2mm)
andalusitemagmatica(5).jpg

Andalusite crystal. XPL image, 10x (Field of view = 2mm)
andalusitemagmatica(6).jpg

Andalusite crystal. PPL image, 10x (Field of view = 2mm)
andalusitemagmatica(7).jpg

Andalusite crystal with sericite alteration. XPL image, 10x (Field of view = 2mm)
andalusitemagmatica(8).jpg

Andalusite crystal. PPL image, 10x (Field of view = 2mm)
andalusitemagmatica(9).jpg

Andalusite crystal with sericite alteration. XPL image, 10x (Field of view = 2mm)
andalusitemagmatica(10).jpg

Andalusite crystal with pink core. PPL image, 10x (Field of view = 2mm)
andalusitemagmatica(11).jpg

Andalusite crystal with sericite alteration. XPL image, 10x (Field of view = 2mm)
andalusitemagmatica(13).jpg

Andalusite crystal. PPL image, 10x (Field of view = 2mm)
andalusitemagmatica(12).jpg

Andalusite crystal with sericite alteration. XPL image, 10x (Field of view = 2mm)