Andalusite
Andalusite is Al2SiO5: square prisms, hardness 6.5 to 7.5, density 3.13 to 3.16. Chiastolite is andalusite with a carbon cross; more manganese makes kanonaite.
Andalusite is aluminium silicate, Al2SiO5 on the September 2026 IMA list (G, 1798, Spain): orthorhombic prisms with nearly square cross sections, hardness 6.5 to 7.5, density 3.13 to 3.16, sharing its composition with kyanite and sillimanite. Chiastolite is andalusite with black carbonaceous inclusions in a cross (International Gem Society). Above 0.50 manganese per formula unit, it is kanonaite.
In short
- One of three Al2SiO5 rows. The September 2026 IMA-CNMNC Master List gives andalusite, Al2SiO5, G, 1798, Spain, and the Handbook of Mineralogy andalusite sheet calls it trimorphous with kyanite and sillimanite. The three-row count and the half-gram rule that separates kyanite by weight are on sillimanite; andalusite reads 13.61 to 13.67 g in water for 20.00 g there and here.
- The kanonaite line. The sheet says andalusite forms a series with kanonaite, Mn3+AlOSiO4 on the IMA list (A, 1976-047, Zambia). Under the IMA’s 50% rule for solid solutions (Nickel (1992), Solid solutions in mineral nomenclature), the name changes when Mn3+ passes 0.50 per formula unit: 22.42% Mn2O3 by weight, worked. The kanonaite sheet analyses sit at 0.76 and 0.79.
- Viridine and chiastolite are labels, not species. The International Gem Society calls viridine a grass to deep green andalusite variety that contains manganese, and chiastolite andalusite with black carbonaceous inclusions in cruciform patterns. A script finds no IMA row for either name. A viridine label gives no manganese figure, so it cannot place a stone against the kanonaite line.
- Weak or strong pleochroism? The Handbook sheet prints weak: X reddish pink, Y = Z greenish yellow. The gem society prints strongly pleochroic, olive green to flesh-red in Brazilian material, usually yellow, green and red. Both are printed here, attributed. The kanonaite sheet prints strong: yellow-green, bluish green, deep golden yellow.
- Documented failure: tourmaline by weight. The gem society warns that tourmaline shares andalusite’s hardness, colour range, specific gravity and pleochroism, and separates them by optic character, biaxial against uniaxial. Worked on the sheets, a 20.00 g dravite reads 13.40 to 13.71 g in water, a window that contains andalusite’s 13.61 to 13.67 g; elbaite tops out at 13.55 g, 0.06 g below.
| Name on the label | What it is (source) | IMA row? | Mn3+ pfu | Refractive indices | Pleochroism (source) | Optic character | Density; 20.00 g piece in water |
|---|---|---|---|---|---|---|---|
| Andalusite | Al2SiO5 (sheet; IMA) | Yes: G, 1798, Spain | Not in the Goat Mountain analysis | α 1.632, β 1.636, γ 1.643 | Weak (sheet); strong (IGS) | Biaxial (−) or (+) | 3.13–3.16; 13.61–13.67 g |
| Chiastolite | Grey andalusite with black carbonaceous inclusions in cruciform patterns (IGS) | No | Not given | Not given separately | Not given | Not given | Lower SG than other andalusite (IGS); no figure |
| Viridine | Grass to deep green andalusite that contains manganese (IGS) | No | Not given | 1.66–1.69 (IGS) | Not given | Not given | Not given |
| Kanonaite | (Mn3+,Al)AlSiO5 (sheet); Mn3+AlOSiO4 (IMA) | Yes: A, 1976-047, Zambia | 0.76; 0.79 | α 1.702, β 1.730, γ 1.823 | Strong (sheet) | Biaxial (+) | 3.395 calculated; 14.11 g |
| Sillimanite | Al2SiO5 (sheet; IMA) | Yes: G, 1824, USA | — | α 1.653–1.661, γ 1.672–1.683 | If coloured: X pale brown or pale yellow; Y brown or green; Z dark brown or blue (sheet) | Biaxial (+) | 3.23–3.24; 13.81–13.83 g |
| Kyanite | Al2OSiO4 (IMA); Al2SiO5 composition | Yes: A, 1967 s.p., Austria | — | α 1.710–1.718, γ 1.724–1.734 | Weak (sheet) | Biaxial (−) | 3.53–3.65; 14.33–14.52 g |
| Dravite (tourmaline) | Tourmaline group (sheet) | Yes: G, 1884, Slovenia | — | Not compared here | Very strong (sheet) | Uniaxial (−) | 3.03–3.18; 13.40–13.71 g |
| Elbaite (tourmaline) | Tourmaline group (sheet) | Yes: G, 1913, Italy | — | ω 1.633–1.651, ε 1.615–1.630 | O pink, pale green, pale to deep blue; E colourless, yellow, olive-green, purplish (sheet) | Uniaxial (−) | 2.90–3.10; 13.10–13.55 g |
What is andalusite? The row and the sheet
The row. The September 2026 IMA-CNMNC Master List gives Al2SiO5, status G for grandfathered, year 1798, country Spain. The country column records where a mineral was first described; it is not a type-locality field, and this page gives no type locality. The sheet derives the name from an occurrence in the Andalusia region, Spain.
On the sheet. Orthorhombic, 2/m 2/m 2/m. Euhedral crystals or columnar aggregates with nearly square cross sections, commonly elongated along [001], to 20 cm; also fibrous, compact or massive; twinning on {101}, rare. Cleavage good on {110}, poor on {100}; fracture uneven to subconchoidal; brittle. Hardness 6.5 to 7.5; density 3.13 to 3.16 measured. Pink, violet, yellow, green, white or grey; streak white; vitreous to subvitreous. Biaxial (−) or (+), α = 1.632, β = 1.636, γ = 1.643, 2V 71° to 86°, dispersion strong.
Composition. Worked on standard atomic weights, Al2SiO5 is 37.08% SiO2 and 62.92% Al2O3, the ideal column the sheet prints. Its Goat Mountain, Idaho analysis reads SiO2 36.74 and Al2O3 62.70, with 0.36 Fe2O3 and traces of other oxides, and no manganese line.
When is andalusite kanonaite? The kanonaite line
The kanonaite line: andalusite in which manganese replaces more than half of one aluminium, above 0.50 Mn3+ per formula unit or 22.42% Mn2O3 by weight, is kanonaite. The andalusite sheet says the two form a series. The IMA list writes kanonaite Mn3+AlOSiO4, status A, 1976-047, Zambia, and the kanonaite sheet writes (Mn3+,Al)AlSiO5: one of andalusite’s two aluminium positions is the one manganese takes. Nickel (1992), Solid solutions in mineral nomenclature gives the IMA’s rule for solid-solution series: divided at 50 mole per cent, each name running from its end member to the midpoint, and different names for different elements dominant in specified structural sites.
Worked. At the line, Al1.5Mn0.5SiO5 has a formula mass of 176.02 and is 22.42% Mn2O3; end-member MnAlSiO5 is 41.55%. The kanonaite sheet’s Kanona, Zambia analysis has Mn2O3 32.2 and Mn3+ 0.76 on that site; Salmchâteau, Belgium has 33.48 and 0.79. Both are kanonaite with room to spare.
What the line changes in the hand. On the sheets, kanonaite is greenish black with a grey-green streak, hardness 6.5, calculated density 3.395 (14.11 g in water for 20.00 g), with strong pleochroism and indices of 1.702 to 1.823, against andalusite’s 1.632 to 1.643. Viridine, which the International Gem Society describes as manganese-bearing andalusite, has indices of 1.66 to 1.69 on the gem society’s panel, between the two.
Chiastolite, viridine and the other andalusite names
Chiastolite. The International Gem Society andalusite article (updated October 2021) describes chiastolite as grey crystals with black, carbonaceous inclusions in cruciform patterns, says the impurities give it a lower hardness and specific gravity than other andalusite, and derives the name from the Greek chiastos, arranged diagonally, after the letter chi. It also says chiastolite had been described in 1754, before the name andalusite. The Handbook sheet does not mention chiastolite.
Viridine and cat’s-eyes. The same article calls viridine the grass to deep green variety that contains manganese, gives it birefringence 0.029 against 0.007 to 0.011 for andalusite, and lists chatoyancy as very rare. A script over the September 2026 IMA list finds no line containing chiastolite or viridine: on a label each stands for andalusite, and a viridine label still needs an analysis to rule out kanonaite.
Under UV. Read ligature-aware, with hemimorphite and willemite as positive controls, the andalusite sheet records no fluorescence. The gem society records none in longwave, brown in shortwave for Lancaster, Massachusetts material, and dark green or yellow-green in shortwave for Brazilian brown-green gems. Neither source gives a cause, and this page supplies none; lamps are covered on fluorescent minerals explained.
Andalusite or tourmaline? Where weight fails
The optic-character rule: andalusite is biaxial and tourmaline uniaxial, and that, not weight, separates them. The International Gem Society says tourmalines share andalusite’s hardness, colour range, specific gravity range and moderate to strong pleochroism, so casual observation of pleochroic colours can confuse the two; its separating test is optic character. The sheets agree: andalusite biaxial (−) or (+); dravite and elbaite uniaxial (−).
Worked on the sheets. A 20.00 g piece reads 13.61 to 13.67 g in water as andalusite. Dravite, density 3.03 to 3.18, reads 13.40 to 13.71 g: its window contains andalusite’s entirely. Elbaite, 2.90 to 3.10, reads 13.10 to 13.55 g, 0.06 g below andalusite’s lowest figure. Hardness, 7 for both tourmalines against 6.5 to 7.5, does not help either. The tourmaline species are on tourmaline and how to tell if tourmaline is real; the gem society also records a synthetic quartz cut to imitate andalusite’s two colours.
Against the other Al2SiO5 minerals. Sillimanite reads 13.81 to 13.83 g and kyanite 14.33 to 14.52 g. The weights and the documented sillimanite overlap belong to sillimanite and are linked, not repeated. Cleavage helps on a broken edge: andalusite good on {110}, sillimanite perfect on {010}, kyanite perfect on {100}. The method is on cleavage or fracture: how to tell.
Where andalusite is found
The sheet’s Distribution field. Widespread; localities for good crystals: Hornachuelos, near Córdoba, Spain; the Lisens Alp, Selraintal, Tirol, Austria; Gefrees, Bodenmais and elsewhere in Bavaria, Germany; the Claggau quarry, Co. Galway, Ireland. In the USA, Lancaster, Worcester Co., Massachusetts; Leiperville, Delaware Co., Pennsylvania; around Custer, South Dakota; Fresno, near Ogilby in the Cargo Muchacho Mountains, and near Daltons Ranch, Madera Co., California. Mt. Howden, Bimbowrie, South Australia; Nawalapitiya, Sri Lanka; gem crystals from the Santa Teresa district, Espírito Santo, Brazil.
Setting. The Occurrence field: a result of contact metamorphism of argillaceous sediments, also in regionally metamorphosed schists; rare in granites and pegmatites, which nonetheless afford the largest crystals; detrital in some sandstones. Associates: kyanite, sillimanite, cordierite, corundum, garnet, tourmaline, mica. This page gives no pressure or temperature for the three polymorphs. The species pages are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.