Andalusite

Andalusite is an aluminum silicate mineral (Al₂SiO₅) and one of three polymorphs of that composition, sharing its chemistry with kyanite and sillimanite. Andalusite belongs to the Al₂SiO₅ polymorph group alongside kyanite and sillimanite. All three share identical chemistry but form under different pressure-temperature conditions during regional metamorphism. Andalusite crystallizes at the low-pressure end of the spectrum, typically below 4 kilobars, in contact metamorphic zones where igneous intrusions bake surrounding aluminous shales and mudstones at temperatures between 400 and 700°C. This low-pressure, moderate-temperature window is why andalusite is found near the margins of granite intrusions rather than in the deeply buried high-pressure rocks where kyanite forms.

The chiastolite variety develops when growing crystals incorporate carbonaceous material from carbon-rich host sediments. As the crystal expands outward, dark inclusions are systematically displaced toward the crystal's center and the midpoints of each face, creating the cruciform pattern visible on cross-section. The cross is not a secondary alteration but a primary growth structure reflecting the orthorhombic symmetry of the expanding crystal and the distribution of organic material in the surrounding rock.

Transparent gem-quality andalusite typically grows in pegmatitic pockets or coarse-grained metamorphic rocks where slower crystallization allows larger, clearer crystals to develop. Brazil's Minas Gerais produces the finest examples, where aluminous schists underwent contact metamorphism at ideal pressure-temperature conditions for transparency.

The defining property is strong trichroic pleochroism: viewed along different crystallographic axes, andalusite shifts through red-orange, olive green, and yellow-brown all under the same light source. Rotate a faceted stone between your fingers and you can watch all three colors emerge. A dichroscope placed against the stone confirms the effect immediately. This is mechanistically distinct from alexandrite's color change, which requires different light sources to trigger chromium's absorption shift. Andalusite's multiple colors are always present simultaneously - it is the viewing angle, not the light source, that reveals them.

The chiastolite variety is unmistakable on cross-section: a charcoal-gray or black cross in a buff to clay-colored matrix, with arms aligning to the crystal's orthorhombic faces. No common mineral produces this exact pattern. In massive or opaque specimens, specific gravity of 3.16-3.20 and hardness around 7 distinguish andalusite from feldspar (lower SG, softer), topaz (similar hardness, higher SG at 3.5), and epidote (similar color range, higher SG).