Andalusite#Varieties

{{Short description|Aluminium nesosilicate mineral}}

{{Infobox mineral

| name = Andalusite

| category = Nesosilicates

| image = Andalusite - Malacheta, Minas Gerais, Brazil.jpg

| imagesize = 260px

| alt =

| caption = Andalusite

| formula = Al₂SiO₅

| molweight =

|IMAsymbol=And| strunz = 9.AF.10

| system = Orthorhombic

| class = Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)

| symmetry = Pnnm

| unit cell = a = 7.7980 Å, b = 7.9031 Å
c = 5.5566 Å; Z = 4

| color = Pink, violet, yellow, green, white, gray; in thin section, colorless to pink or green

| habit = As euhedral crystals or columnar aggregates having nearly square cross sections; fibrous compact to massive

| twinning = On {101}, rare

| cleavage = Good on {110}, poor on {100}

| fracture = uneven to subconchoidal

| mohs = 6.5–7.5

| luster = Vitreous

| polish =

| refractive = n_α = 1.629 – 1.640 n_β = 1.633 – 1.644 n_γ = 1.638 – 1.650

| opticalprop = double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.

| birefringence = δ = 0.009 – 0.010

| dispersion = r < v strong

| pleochroism = strongly trichroic

| 2V = 71–86°

| fluorescence= non-fluorescent

| absorption =

| streak = White

| gravity = 3.17 (± .04)

| density =

| melt =

| fusibility =

| diagnostic =

| solubility =

| diaphaneity = Transparent to nearly opaque with inclusions

| references = Gemological Institute of America, GIA Gem Reference Guide 1995, {{ISBN|0-87311-019-6}}http://rruff.geo.arizona.edu/doclib/hom/andalusite.pdf Handbook of Mineralogy

}}

Andalusite is an aluminium nesosilicate mineral with the chemical formula Al₂SiO₅. This mineral was called andalousite by Delamétherie,{{Cite journal|last=Delamétherie|first=Jean Claude|date=1798|title=Sur une pierre de l'Andalousie|journal=Journal de Physique, de Chimie d'Histoire Naturelle et des Arts|volume=46|pages=386–387}} who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the specimens studied were actually from El Cardoso de la Sierra, in the Spanish province of Guadalajara, not Andalusia.{{Cite book|title=Minerales y Minas de España. Vol. IX. Silicatos|last=Calvo|first=Miguel|publisher=Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo|year=2018|isbn=978-84-8321-883-9|location=Madrid, Spain|pages=91–94}}

Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.{{Cite journal|last=Whitney, D.L.|date=2002|title=Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey|journal=American Mineralogist|volume=87|issue=4|pages=405–416|doi=10.2138/am-2002-0404|bibcode=2002AmMin..87..405W|s2cid=131616262}}

{{al2sio5 phase diagram}}

File:Round faceted andalusite 1.jpg

==Varieties==

The variety chiastolite commonly contains dark inclusions of carbon or clay which form a cruciform pattern when shown in cross-section. This stone was known at least from the sixteenth century, being taken to many European countries, as a souvenir, by pilgrims returning from Santiago de Compostela.{{Cite journal|last=Calvo|first=Miguel|date=2016|title=El "lapis crucifer", "piedra de cruz de Compostela": un elemento importante de los patrimonios geológico y cultural del NW de España|journal=De Re Metallica|volume=6|pages=67–79}}

Viridine is a green variety of andalusite in which manganese 3+ substitutes for aluminium, the same change is also responsible for the colour.{{cite journal |last1=Abs-Wurmbach |first1=I. |last2=Langer |first2=K. |last3=Seifert |first3=F. |last4=Tillmanns |first4=Ε. |title=The crystal chemistry of (Mn 3+ , Fe 3+ )-substituted andalusites (viridines and kanonaite), (Al 1-x-y Mn x 3+ Fe 3+ y ) 2 (O{{!}}SiO 4 ): crystal structure refinements, Mössbauer, and polarized optical absorption spectra |journal=Zeitschrift für Kristallographie - Crystalline Materials |date=1 October 1981 |volume=155 |issue=1–4 |pages=81–114 |doi=10.1524/zkri.1981.155.14.81|s2cid=201671728 }}{{Cite web |title=Minerals Colored by Metal Ions |url=http://minerals.gps.caltech.edu/color_causes/Metal_Ion/index.html |access-date=2023-03-01 |website=minerals.gps.caltech.edu}} Kanonaite is a greenish-black mineral related to andalusite and having the approximate composition {{chem2|(Mn0.76Al0.23Fe0.02)AlSiO5}}.{{cite journal |last1=Vrána |first1=S. |last2=Rieder |first2=M. |last3=Podlaha |first3=J. |title=Kanonaite, (Mn 0.76 3+ Al0.23Fe 0.02 3+ )[6]Al[5][O{{!}}SiO4], a new mineral isotypic with andalusite |journal=Contributions to Mineralogy and Petrology |date=1978 |volume=66 |issue=3 |pages=325–332 |doi=10.1007/BF00373417|s2cid=95925117 }}

A clear variety found in Brazil and Sri-Lanka can be cut into a gemstone.{{Cite web |url=http://www.gemstone.org/gem-by-gem/english/andalusite.html |title=International Colored Gem Association: Anadalusite |access-date=2006-07-13 |archive-url=https://web.archive.org/web/20060717224731/http://www.gemstone.org/gem-by-gem/english/andalusite.html |archive-date=2006-07-17 |url-status = dead}} Faceted andalusite stones give a play of red, green, and yellow colors that resembles a muted form of iridescence, although the colors are actually the result of unusually strong pleochroism.{{cite journal |last1=Fernandes |first1=Shyamala |last2=Choudhary |first2=Gagan |title=Gem-quality andalusite from Brazil |journal=Gems & Gemology |date=Summer 2009 |volume=45 |issue=2 |pages=120–129 |doi=10.5741/GEMS.45.2.120 |url=https://www.gia.edu/doc/Summer-2009-Gems-Gemology-Gem-Quality-Andalusite-from-Brazil.pdf |access-date=30 October 2021}}