Plagioclase is series of framework silicate minerals in feldspar group. Plagioclase is a continuous series of solid solutions known as the plagioclase feldspar series, rather than a specific mineral with a particular chemical composition. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral’s crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or ‘record-groove’ effect.
Name: From the Greek plagios – “oblique” and klao – “I cleave” in allusion to the obtuse cleavage angles of the good cleavages.
Polymorphism & Series: Low- and high-temperature structural modi¯cations are recognized.
Mineral Group: Feldspar group, plagioclase series.
Environment: Igneous and metamorphic rocks. Group name for Na, Ca feldspars.
Contents
Plagioclase series members
The composition of plagioclase feldspar is typically indicated by the general anorthite (% An) or albite (% Ab) fraction and is easily determined by measuring the refractive index or peel angle within the crushed particles by measuring the refraction angle under a thin section. The deflection angle is an optical characteristic and varies according to the albite fraction (Ab). There are several plagioclase feldspars in the series between albite and anorthite.
Feldspar classification
This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite
| Plagioclase Group Minerals | ||
| Mineral | % Albite | % Anorthite |
| Albite | 100-90% Ab | 0-10% An |
| Oligoclase | 90-70% Ab | 10-30% An |
| Andesine | 70-50% Ab | 30-50% An |
| Labradorite | 50-30% Ab | 50-70% An |
| Bytownite | 30-10% Ab | 70-90% An |
| Anorthite | 10-0% Ab | 90-100% An |
Plagioclase Feldspar Chemical Properties
| Chemical Classification | Silicate |
| Chemical Composition | NaAlSi3O8 – CaAl2Si2O8 |
Plagioclase Feldspar Physical Properties
| Color | Usually white or gray. Also colorless, yellow, orange, pink, red, brown, black, blue, green. |
| Streak | White |
| Luster | Vitreous. Pearly on some cleavage faces. |
| Cleavage | Perfect in two directions that intersect at approximately 90 degrees. |
| Diaphaneity | Translucent to transparent |
| Mohs Hardness | 6 to 6.5 |
| Crystal System | Triclinic |
| Tenacity | Brittle |
| Specific Gravity | 2.6 to 2.8 |
Albite Optical Properties
| Color / Pleochroism | White to gray, bluish, greenish, Reddish; may be chatoyant. |
| 2V: | Measured: 45° , Calculated: 76° to 82° |
| RI values: | nα = 1.528 – 1.533 nβ = 1.532 – 1.537 nγ = 1.538 – 1.542 |
| Optic Sign | Biaxial (+) |
| Birefringence | δ = 0.010 |
| Relief | Low |
| Dispersion: | Low |
Anorthite Optical Properties
| Color / Pleochroism | White, grayish, reddish; colorless in thin section |
| 2V: | Measured: 78° to 83°, Calculated: 78° |
| RI values: | nα = 1.573 – 1.577 nβ = 1.580 – 1.585 nγ = 1.585 – 1.590 |
| Optic Sign | Biaxial (-) |
| Birefringence | δ = 0.012 – 0.013 |
| Relief | Moderate |
| Dispersion: | r < v weak |
Plagioclase Occurrence
Plagioclase feldspar group minerals are the most common rock-forming minerals. They are importantly dominant minerals in most igneous rock. They are major constituents in a wide range of intrusive and extrusive igneous rocks including granite, diorite, gabbro, rhyolite, andesite, and basalt. Plagioclase minerals are important constituents of many metamorphic rocks, such as gneiss, where they can be inherited from an igneous protolith or formed during the regional metamorphism of sedimentary rocks.
Plagioclase is a common clast produced during the weathering of igneous and metamorphic rocks. It can be the most abundant clast in sediments located close to their source area and decreases in abundance downstream. This decrease is partly because quartz is more physically and chemically durable than feldspar and persists in greater relative quantities downstream in eroded sediments.
Plagioclase Uses Area
- Plagioclase minerals are important constituents of some building stone and crushed stone such as granite and trap rock.
- Some rare specimens of plagioclase exhibit optical phenomena that make them highly desirable gem materials.
- Moonstone is a name given to a gem material that consists of very thin, alternating layers of orthoclase (an alkali feldspar) and albite (a plagioclase feldspar).
- Some specimens of labradorite exhibit a schiller effect, which is a strong play of iridescent blue, green, red, orange, and yellow colors when moved under a source of incident light.
Distribution
Anorthite
A widely distributed rock-forming mineral. Classic occurrences include:
- from Monte Somma and Vesuvius, Campania; on Mt. Monzoni, Val di Fassa, Trentino-Alto Adige; and from the Cyclopean Islands, Italy.
- At Tunaberg, SÄodermanland, Sweden. From near Lojo, Finland.
- At Bogoslovsk and Barsowka, Ural Mountains, Russia.
- On Miyakejima Island, Tokyo Prefecture; at Toshinyama, Tochigi Prefecture; the Zao volcano, Yamagata Prefecture; Otaru, Hokkaido; and other places in Japan.
- In the USA, on Great Sitkin Island, Aleutian Islands, Alaska; from Grass Valley, Nevada Co., California.
- On Amitok Island, Labrador, Newfoundland, Canada.
Albite
Widespread; a few localities for good crystals are:
- In Switzerland, from St. Gotthard, Ticino and Tavetsch, GraubuÄnden. From Roc Tourne, near Modane, Savoie, France.
- On Mt. Greiner, Zillertal, Tirol, Austria.
- At Baveno, Piedmont, and in the P¯tschtal, Trentino-Alto Adige, Italy.
- From Mursinka, Ural Mountains, and Miass, Ilmen Mountains, Southern Ural Mountains, Russia.
- In the USA, at Haddam and Middletown, Middlesex Co., Connecticut; Amelia, Amelia Co., Virginia; from Diana, Lewis Co., and Dekalb, Macomb, and Pierrepont, St. Lawrence Co., New York. On Prince of Wales Island, Alaska; in the Pala and Mesa Grande districts, San Diego Co., California.
- At Bathurst, and Wicklow Township, Hastings Co., Ontario, Canada.
- From Virgem da Lapa and Morro Velho, Minas Gerais, Brazil.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
- Mindat.org. (2019). Orpiment: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
- Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].
