These pictures are of a polished specimen of Labradorite from Finland. The left picture shows the bluish gray color of Labradorite and with a change in the angle of lighting, the right picture shows the iridescence typically found in these species of Plagioclase.
Labradorite, as well as
other Plagioclase feldspars, often produce noticeable striations on cleavage
faces. These striations are often an important distinguishing factor in
the identification of the group of feldspars.
LABRADORITE is a member of the Plagioclase Feldspar Series. The basic properties of the family are as follows:
| SPECIES | % SODIUM | % CALCIUM | SPECIFIC GRAVITY | COLOR | OCCURRENCE |
| ALBITE | 100-90 | 0-10 | 2.60-2.65 | White to colorless | Granite pegmatites |
| OLIGOCLASE | 90-70 | 10-30 | 2.63-2.66 | White to slightly greenish | Monzonites, Granodiorites, also in granitic pegmatites |
| ANDESINE | 70-50 | 30-50 | 2.66-2.68 | Colorless to white to greenish yellow | Andesites and Diorites |
| LABRADORITE | 50-30 | 50-70 | 2.68-2.72 | Grayish, bluish, greenish with iridescence along cleavage planes | Both occur as small to large grains in gabbros and anorthosites. |
| BYTOWNITE | 30-10 | 70-90 | 2.72-2.74 | White to grayish | |
| ANORTHITE | 10-0 | 90-100 | 2.74-2.76 | White to grayish and reddish | Limestones that have undergone contact metamorphism |
THE FOLLOWING INFORMATION APPLIES TO ALL THE SPECIES IN THE PLAGIOCLASE SERIES.
CRYSTALLOGRAPHY Triclinic
CRYSTAL GROWTH AND HABIT The plagioclase feldspars typically form thin, tabular crystals. They are found also in large cleavable masses or they can be massive. Twinning is common producing striations that can be seen along cleavage faces.
HARDNESS 6 - 6.5
STREAK White
BREAKABILITY Cleavage is very good in 1 direction and good in another producing blocky fragments with angles of about 90o. This series of feldspars has and uneven fracture and is brittle.