Andalusite

Andalusite is a rock forming minerals and is an aluminium nesosilicate minerals with the chemical formula Al2SiO5.At higher temperatures and pressures, andalusite may convert to sillimanite.andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.

Physical Properties of Andalusite

Category                       Nesosilicates
Formula                         Al2SiO5
Crystal system          Orthorhombic
Color                               Pink, violet, yellow, green, white, gray; in thin section, colorless to pink or green
Crystal 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 scale                 6.5 – 7.5
Luster                            Vitreous
Streak                            White
Diaphaneity               Transparent to nearly opaque with inclusions
Specific gravity        3.17 (+/- .04)

Optical properties

double refractive, biaxial negative; chiastolite has anomalous aggregate reaction.
Refractive index nα = 1.629 – 1.640 nβ = 1.633 – 1.644 nγ = 1.638 – 1.650
Birefringence δ = 0.009 – 0.010
Pleochroism strongly trichroic
2V angle 71 – 86°
Dispersion r < v strong
Ultraviolet fluorescence

Occurrence of Andalusite

It is a common metamorphic mineral which forms under low pressure and low to high temperatures. The minerals kyaniteand sillimanite are polymorphs of andalusite, each occurring under different temperature-pressure regimes and are therefore rarely found together in the same rock. Because of this the three minerals are a useful tool to help identify the pressure-temperature paths of the host rock in which they are found. An example rock includes hornfels.

What is Chiastolite ?

Chiastolite is a variety of andalusite that contains black particles of graphite arranged in geometric patterns. The graphite is pushed aside by crystal growth within a rock that is being metamorphosed. As growth occurs, the particles become concentrated at crystal interfaces. The result can be a cross-shaped pattern within the mineral – similar to the “cross-stone” shown in the photo here. People have known about these cross stones for centuries and have valued them for their perceived religious or spiritual meaning. Attractive specimens are often cut and polished for use as amulets, charms, and novelty gems.

Uses of Andalusite

The rare transparent variety of Andalusite is used as a minor gemstone. Chiastolite cross-sections are often cut and polishedto bring out the distinctive cross-shaped pattern, and Chiastolite crystals may also be cut along their cross-section and used in jewelry as a Christian symbol.