Bytownite is a calcium rich member of the plagioclase

Plagioclase

Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

 solid solution

Solid solution

A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...

 series of feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s. It is usually defined as having "%An

Anorthite

Anorthite is the calcium endmember of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl2Si2O8.-Mineralogy :...

" between 70 and 90. Like others of the series, bytownite forms grey to white triclinic crystals commonly exhibiting the typical plagioclase twinning

Crystal twinning

Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....

 and associated fine striations.

The specific gravity

Specific gravity

Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

 of bytownite varies between 2.74 and 2.75. The refractive indices

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 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

Petrography

Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. Petrographic descriptions start with the field notes at the...

 analysis are required for identification.

Occurrence

Bytownite is a rock forming mineral occurring in mafic

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

 igneous rock

Igneous rock

Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...

s such as gabbro

Gabbro

Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

s and anorthosite

Anorthosite

Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...

s. It also occurs as phenocryst

Phenocryst

thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...

s in mafic volcanic rock

Volcanic rock

Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...

s. It is rare in metamorphic rock

Metamorphic rock

Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...

s. It is typically associated with pyroxene

Pyroxene

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...

s and olivine

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

.

The mineral was first described in 1835 and named for an occurrence at Bytown

Bytown

Bytown is the former name of Ottawa, Canada's capital city. It was founded on on September 26, 1826, incorporated as a town on January 1, 1850, and superseded by the incorporation of the City of Ottawa on January 1, 1855. The founding was marked by a sod turning, and a letter from Governor General...

 (now Ottawa

Ottawa

Ottawa is the capital of Canada, the second largest city in the Province of Ontario, and the fourth largest city in the country. The city is located on the south bank of the Ottawa River in the eastern portion of Southern Ontario...

), Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

. Other noted occurrences in Canada include the Shawmere anorthosite in Foleyet Township, Ontario

Ontario

Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

, and on Yamaska Mountain,
near Abbotsford

Abbotsford

Abbotsford may refer to a place in:Australia* Abbotsford, New South Wales, a suburb of Sydney, Australia* Abbotsford, Victoria, a suburb of Melbourne, AustraliaCanada* Abbotsford, British Columbia, a city in Canada...

, Quebec

Quebec

Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....

. It occirs on Rùm

Rùm

Rùm , a Scottish Gaelic name often anglicised to Rum) is one of the Small Isles of the Inner Hebrides, in the district of Lochaber, Scotland...

 island, Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 and Eycott Hill, near Keswick

Keswick, Cumbria

Keswick is a market town and civil parish within the Borough of Allerdale in Cumbria, England. It had a population of 4,984, according to the 2001 census, and is situated just north of Derwent Water, and a short distance from Bassenthwaite Lake, both in the Lake District National Park...

, Cumberland

Cumberland

Cumberland is a historic county of North West England, on the border with Scotland, from the 12th century until 1974. It formed an administrative county from 1889 to 1974 and now forms part of Cumbria....

, England. It is reported from Naaraodal, Norway

Norway

Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

 and in the Bushveld complex of South Africa

South Africa

The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

. It is also found in Isa Valley, Western Australia

Western Australia

Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

.

In the USA it is found in the Stillwater igneous complex

Stillwater igneous complex

The Stillwater igneous complex is a large layered intrusion located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of...

 of Montana; from near Lakeview, Lake County, Oregon

Lake County, Oregon

Lake County is a county in the high desert south central region of the U.S. state of Oregon, so named for the many lakes found within its boundaries, including Lake Abert, Hart Lake Reservoir, and Goose Lake. While Lake is among Oregon's largest counties, it is sparsely populated with 7,895...

. It occurs in the Lucky Cuss mine, Tombstone, Arizona

Arizona

Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

; and from the Grants district, McKinley County, New Mexico

McKinley County, New Mexico

-2010:Whereas according to the 2010 U.S. Census Bureau:*15.2% White*0.5% Black*75.5% Native American*0.8% Asian*0.0% Native Hawaiian or Pacific Islander*3.1% Two or more races*4.6% Other races*13.3% Hispanic or Latino -2000:...

. In the eastern US it occurs at Cornwall, Lebanon County, Pennsylvania

Lebanon County, Pennsylvania

As of the census of 2000, there were 120,327 people and 32,771 families residing in the county. The population density was 332 people per square mile . There were 49,320 housing units at an average density of 136 per square mile...

 and Phoenixville, Chester County, Pennsylvania

Chester County, Pennsylvania

-State parks:*French Creek State Park*Marsh Creek State Park*White Clay Creek Preserve-Demographics:As of the 2010 census, the county was 85.5% White, 6.1% Black or African American, 0.2% Native American or Alaskan Native, 3.9% Asian, 0.0% Native Hawaiian, 1.8% were two or more races, and 2.4% were...

.

Uses

Bytownite is occasionally used in jewelry. As a gemstone

Gemstone

A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...

, bytownite is usually facet

Facet

Facets are flat faces on geometric shapes. The organization of naturally occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure...

ed 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

Sunstone

Sunstone is a plagioclase feldspar, which when viewed from certain directions exhibits a brilliant spangled appearance; this has led to its use as a gemstone. It has been found in Southern Norway, and in some United States localities...

gemstone varieties).