Labradorite is an unusual mineral. It can display a beautiful iridescent play of colors, caused by internal fractures in the mineral that reflect light back and forth, dispersing it into different colors. This effect, known as labradorescence, gives Labradorite its appeal and fame. Specimens sold to collectors are usually polished or sliced by dealers to fully bring out this effect. Sliced slabs are sometimes sold by dealers in water, which enhances the effect.
Labradorite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. Labradorite is an intermediary member of this series. Labradorite is considered by some authorities as a variety of Anorthite rather then a separate mineral. The acclaimed Dana’s System of Mineralogy lists Labradorite as an individual mineral, whereas the IMA does not recognize it as individual mineral species, but rather a sodium-rich variety of Anorthite.
Sodium calcium aluminum silicate. The ratio of sodium to calcium is about 7:3. The amount of aluminum atoms are between 1 and 2, and the amount of silicon atoms are between 3 and 2.
In the Plagioclase Feldspar series, Labradorite contains between 30 and 50 percent Albite (Ab), and between 50 and 70 percent Anorthite (An).
White, gray, light blue, light green, pale orange-red, black, usually with a strong multicolored display of purple, blue, and green schillers. A variety from Finland known as Spectrolite, shows the Schiller effect with dark reds, orange, yellow, blues and green color flashes.
6 – 6.5
Crystal Forms and Aggregates
Labradorite rarely forms in crystals. When it does, they are generally tabular and often twinned. Most commonly occurs massive , grainy , as elongated fragments, as chunky masses, and rounded.
Transparent to translucent
2.69 – 2.72
Vitreous to pearly
2,1 – basal ; 2,1 – prismatic ; 3,1 – pinacoidal. The cleavage angle is about 90º.
Conchoidal to uneven
Silicates; Tectosilicates; Feldspar Group
Play of color, hardness, cleavage, and crystal forms
In igneous environments in diabase, and in contact metamorphic rocks
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One of my favorite rocks.