Bytownite

Bytownite
A somewhat rounded, alluvial crystal of bytownite from the Dorado Mine, Casas Grandes, Chihuahua, Mexico (size 3.7 x 2 x 1.3 cm)
General
Category	Plagioclase feldspar
Formula (repeating unit)	(Ca,Na)[Al(Al,Si)Si2O8]
Strunz classification	09.FA.35
Dana classification	76.01.03.05
Crystal system	Triclinic
Space group	Triclinic pinacoidal 1
Unit cell	a = 8.178 Å, b = 12.870 Å, c = 14.187 Å; α = 93:5° β = 115:9° γ = 90:63°; Z = 8
Identification
Color	Colorless, white, gray
Crystal habit	Rarely as crystals flattened on [010], commonly as cleavable masses or anhedral grains in massive aggregates
Twinning	Common Albite, Carlsbad, and Pericline twinning
Cleavage	Perfect on [001], good on [010], imperfect on [110]
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scale hardness	6 – 6.5
Luster	Vitreous, pearly on cleavages
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.72 – 2.74
Optical properties	Biaxial (+/-)
Refractive index	nα = 1.563 - 1.572 nβ = 1.568 - 1.578 nγ = 1.573 - 1.583
Birefringence	δ = 0.010 - 0.011
2V angle	Measured: 86°, Calculated: 80° to 88°
Dispersion	r > v strong
References

Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals. It is usually defined as having "%An" between 70 and 90. Like others of the series, bytownite forms grey to white triclinic crystals commonly exhibiting the typical plagioclase twinning and associated fine striations.

The specific gravity of bytownite varies between 2.74 and 2.75. The refractive indices ranges are nα=1.563 – 1.572, nβ=1.568 – 1.578, and nγ=1.573 – 1.583. Precise determination of these two properties with chemical, X-ray diffraction, or petrographic analysis are required for identification.

Occurrence

Bytownite from Crystal Bay, Minnesota

Bytownite is a rock forming mineral occurring in mafic igneous rocks such as gabbros and anorthosites. It also occurs as phenocrysts in mafic volcanic rocks. It is rare in metamorphic rocks. It is typically associated with pyroxenes and olivine.

The mineral was first described in 1835 and named for an occurrence at Bytown (now Ottawa), Canada. Other noted occurrences in Canada include the Shawmere anorthosite in Foleyet Township, Ontario, and on Yamaska Mountain, near Abbotsford, Quebec. It occirs on Rùm island, Scotland and Eycott Hill, near Keswick, Cumberland, England. It is reported from Naaraodal, Norway and in the Bushveld complex of South Africa. It is also found in Isa Valley, Western Australia.

In the USA it is found in the Stillwater igneous complex of Montana; from near Lakeview, Lake County, Oregon. It occurs in the Lucky Cuss mine, Tombstone, Arizona; and from the Grants district, McKinley County, New Mexico. In the eastern US it occurs at Cornwall, Lebanon County, Pennsylvania and Phoenixville, Chester County, Pennsylvania.

Uses

Bytownite is occasionally used in jewelry. As a gemstone, bytownite is usually faceted with the transparent gems varying in color from a pale, straw yellow to a light brown. A variety from Mexico has been marketed under the trademarked name "Golden Sunstone" (but is distinct from the various sunstone gemstone varieties).

References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
• Mineral galleries

External links

Chisholm, Hugh, ed. (1911). "Bytownite" . Encyclopædia Britannica (11th ed.). Cambridge University Press.