Serpentine H4Mg3Si2O9

Optical Properties:

Serpentine minerals may be either biaxial + or -, with 2V varying from 0 to 90 degrees. All members of the group have low birefringence (first order yellow maximum) and parallel extinction. The mineral habit is fibrous, and in plane polarized light grains are colorless to pale green. Grain size is typically too small to determine many optical properties.

Occurrence:

The serpentine group of minerals is formed by metamorphism and hydrothermal alteration of mafic minerals, such as olivine and pyroxene.

The photos above show serpentine in crossed polarizers (left) and plane polarized light (right). The rock shown is almost 100% serpentine; the equant crystal forms seen are serpentine pseudomorphs after olivine.

The picture below left (1.5 mm field of view, crossed polarizers) shows light gray stringers of serpentine altering clinopyroxene (at extinction). On the right is a clinopyroxene grain surrounded by gray serpentine (crossed polarizers).