Does staurolite have twinning?

Twinning: Staurolite is famous for its twinned crystals that form into the shape of a cross; staurolite forms two twin types; one that is nearly 90 degrees and one that is nearly 60 degrees. The 60 degree type is more common but the 90 degree type is the most sought after.

Is staurolite a cleavage or fracture?

Cleavage Distinct on {010}
Fracture Subconchoidal
Tenacity Brittle
Mohs scale hardness 7 – 7.5

Is staurolite metallic or nonmetallic?

Staurolite: Staurolite in schist from Little Falls, Minnesota. Specimen is approximately 4 inches (10 centimeters) across….

Physical Properties of Staurolite
Chemical Classification Silicate
Streak Colorless (harder than the streak plate)
Luster Vitreous, sometimes resinous

Where can you find staurolite?

Staurolite occurs in Canada; North Carolina, Virginia, and Georgia, U.S.; Brazil; Brittany, France; and Switzerland, especially along the Saint Gotthard Pass. For chemical formula and detailed physical properties, see silicate mineral (table).

What is a fairy cross?

Fairy Cross is another name for staurolite crystals, minerals found in old rocks of the Eastern U.S., and they are especially abundant in Fannin County. Their teardrops fell to the ground and formed tiny crosses of stone.

How do you identify staurolite in a thin section?

Distinguishing Features It is vitreous and has a grey streak. Staurolite’s hand sample has characteristic penetration twinning and unique crystal habit. The crystals are brown, red or yellow in color. May resemble tourmaline in thin section, but tourmaline is uniaxial.

Why is it called Fairy Cross?

Staurolite gets its name from two Greek words, stauros meaning cross, and lithos, meaning stone. The name was coined because of the stone’s curious tendency to form twins that cross at various angles.

How do you get a fairy cross?

Dig with your hands along the soft dirt at the base of the trees. The cross stones are the same color as the dirt, so use your hands to sift the stones from the soil until you find a cross shaped stone about the size of a small marble, usually less than an inch in length.

What is staurolite used for?

For centuries it’s been highly valued by Christians as a talisman for good luck and protection. As a resemblance to a crucifix, Staurolite used to sit atop Church entryways in the Middle Ages. An old folklore tale about this stone ties it back to its ancient name, Fairy Stones.

When does transformation twinning occur in a crystal?

Transformation Twins – Transformation twinning occurs when a preexisting crystal undergoes a transformation due to a change in pressure or temperature. This commonly occurs in minerals that have different crystal structures and different symmetry at different temperatures or pressures.

When to use twinning and Pericline twinning in alkali feldspar?

Similarly the combination of albite twinning and pericline twinning in alkali feldspar results when high temperature sanidine (monoclinic) transforms to low temperature microcline (triclinic).

When do albite and Pericline twinning occur in quartz?

Dauphiné and Brazil twinning in quartz commonly forms this way during a decrease in temperature. Similarly the combination of albite twinning and pericline twinning in alkali feldspar results when high temperature sanidine (monoclinic) transforms to low temperature microcline (triclinic).

Why is twinning an important feature of mineral identification?

Twinning is important to recognize, because when it occurs, it is often one of the most diagnostic features enabling identification of the mineral. What happens is that lattice points in one crystal are shared as lattice points in another crystal adding apparent symmetry to the crystal pairs.