The chemical formula of mineral Andalusite is indicated by Al2SiO5. Andalusite is known to crystallize in the orthorhombic crystal system. In optical mineralogy, the orthorhombic crystal system comprises crystals having three mutually perpendicular axes, of which all are of different lengths. When viewed under polarizing microscopes for mineralogists, Andalusite is commonly found having biaxial negative figure. The indices of mineral Andalusite range from 1.629 to 1.650. The birefringence of the mineral is usually within the range of about 0.009 to 0.013. The optic angle of Andalusite is at about 71 degree to 88 degree when viewed with the aid of the micrometer eyepiece of petrographic polarizing light microscope. When closely evaluated under polarized microscope used in optical mineralogy, Andalusite is commonly found exhibiting a moderately high positive relief.
Andalusite is most commonly found colorless in thin section and in grain mount. It is less commonly found pink, reddish pink, or slightly greenish which can be more splendidly exhibited when viewed under polarizing microscope for mineralogists. The hardness measure of mineral Andalusite when evaluated using the Mohs scale method is usually within the range of 6.5 to 7.5. The specific gravity measure of the mineral is also within the range of about 3.13 grams per cubic centimeters to 3.16 grams per cubic centimeters. Andalusite is commonly white, gray, pink, or rose in hand sample. It can be also found in shades of violet, yellow, or green. Colored varieties of Andalusite may show weak pleochroism when viewed under polarizing microscope and mostly in shades of pink and colorless. But then there are also some varieties that are pleochroic in shades of green and yellow. The color of the mineral becomes darker for the increasing amount of Mn(3+) and Fe(3+) in the composition. When Andalusite samples are rubbed on a white porcelain streak plate, it is commonly found leaving a white streak. When viewed in reflected light of the petrographic polarizing microscope, Andalusite is most commonly found exhibiting a vitreous luster.
Andalusite crystals are usually elongate prisms that are clearly exhibited when viewed under polarizing microscope used in optical mineralogy. These prisms have nearly square cross sections. One variety of Andalusite called chiastolite contains dark inclusions that form a cross along the diagonals of the prism. Andalusite also forms anhedral grains or highly irregular masses. There are also numerous inclusions of quartz, fine opaques, or other minerals that are commonly found with Andalusite when it is closely evaluated under petrographic polarizing microscopes.
Andalusite has two good prismatic cleavages that are clearly observed when viewed with the aid of polarized microscope used in optical mineralogy. These cleavages are nearly at right angles and are usually parallel to the prism faces in cross sections. The cleavage of Andalusite tends to control fragment orientation in grain mounts. There is also a poor cleavage that can be observed when Andalusite is closely evaluated with the aid of the petrographic polarizing microscopes for mineralogists. Twinning for Andalusite crystals are rare but can be possibly noticed when the mineral sample is meticulously evaluated using the polarized microscopes used in optical mineralogy.
The extinction of the mineral Andalusite is usually found parallel to the trace of the cleavage and the long dimension of the elongate crystals. These observations are made clearly visible under petrographic polarizing microscopes. The extinction of the basal sections is also found symmetrical. In optical mineralogy tests, it has been observed that for most Andalusite, the fast ray vibrates parallel to the trace of the cleavage and the long dimension of grains in elongate sections. However, there are some samples containing substantially more Mn(3+) and Fe(3+) content substituting for aluminum in the composition are found as biaxial positive mineral samples.
With the substantial substitution in the composition of the mineral, Andalusite also shows increasing range in its indices of refraction. The birefringence of Andalusite decreases to nearly zero and may possibly increase above that point depending on the composition. The most common Andalusite has birefringence within the range of about 0.009 to 0.013. This usually yields a first-order gray and white interference colors in standard thin section of the mineral.
Most Andalusite minerals are biaxial negative. Their optic angle is usually greater than 80 degree. Andalusite in basal sections may also exhibit acute bisectrix figures with metalopes outside the field of view of the polarized microscope. When closely evaluated under polarized microscope, the cleavage fragments of Andalusite usually produce a strongly off-center obtuse bisectrix figures. The optic axis dispersion of Andalusite is also found weak as observed under petrographic polarizing light microscope.
Mineral Andalusite could be possibly altered to sericite. Sericite is mostly fine-grained white mica. Andalusite may be also altered to chlorite or other phyllosilicates. It is also commonly found involved in metamorphic mineral reactions. Andalusite is most commonly found in contact and regional metamorphic rocks. It is less commonly found in granitic pegmatites and granite.
