Editing Bytownite

{{Short description|Mineral: intermediate member of a solid solution series (70 to 90 % anorthite and albite)}}
{{infobox mineral
| name     = Bytownite
| category = [[Tectosilicate]] [[minerals]], [[feldspar]] [[Mineral group|group]], [[plagioclase]] series
| image       = Bytownite-mrz296a.jpg
| imagesize   = 170px
| alt         = 
| caption     = A somewhat rounded, alluvial crystal of bytownite from the Dorado Mine, [[Casas Grandes]], [[Chihuahua (state)|Chihuahua]], [[Mexico]] (size 3.7 × 2 × 1.3 cm)
| formula     = (Ca,Na)[Al(Al,Si)Si<sub>2</sub>O<sub>8</sub>]	
| strunz      = 9.FA.35
| dana        = 76.01.03.05
| system      = [[Triclinic]]
| class       = Pinacoidal ({{overline|1}}) <br/><small>(same [[H-M symbol]])</small>
| unit cell   = a = 8.178&nbsp;[[Ångström|Å]], b = 12.870&nbsp;Å, <br/>c = 14.187&nbsp;Å; α = 93:5°, <br/>β = 115:9°, γ = 90:63°; Z&nbsp;=&nbsp;8
| color       = Colorless, white, gray
| colour      = 
| 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        = 6 – 6.5
| luster      = Vitreous, pearly on cleavages
| streak      = White
| diaphaneity = Transparent to translucent
| gravity     = 2.72 – 2.74
| density     = 
| polish      = 
| opticalprop = Biaxial (+/−)
| refractive  = n<sub>α</sub> = 1.563 – 1.572 n<sub>β</sub> = 1.568 – 1.578 n<sub>γ</sub> = 1.573 – 1.583
| birefringence = δ = 0.010 – 0.011
| pleochroism = 
| 2V          = Measured: 86°, Calculated: 80° to 88°
| dispersion  = r > v strong
| extinction  = 
| length fast/slow =
| fluorescence = 
| absorption  = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| impurities  = 
| alteration  = 
| other       = 
| prop1       = 
| prop1text   = 
| references  = <ref name=Mindat>[http://www.mindat.org/min-815.html Mindat with locations]</ref><ref name=Handbook>[http://rruff.geo.arizona.edu/doclib/hom/bytownite.pdf Handbook of Mineralogy]</ref><ref>[http://webmineral.com/data/Bytownite.shtml Webmineral data]</ref>
}}
'''Bytownite''' is a [[calcium]] rich member of the [[plagioclase]] [[solid solution]] series of [[feldspar]] [[mineral]]s with composition between [[anorthite]] and [[labradorite]]. It is usually defined as having between 70 and 90%[[Anorthite|An]] (formula: {{Chem2|(Ca_{0.7-0.9}Na_{0.3-0.1})[Al(Al,Si)Si2O8]}}). Like others of the series, bytownite forms grey to white [[triclinic]] crystals commonly exhibiting the typical plagioclase [[crystal twinning|twinning]] and associated fine striations.

The [[specific gravity]] of bytownite varies between 2.74 and 2.75. The [[refractive index|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 [[Petrography|petrographic]] analysis are required for identification.

==Occurrence==
[[Image:Feldspar - Bytownite Sodium calcium aluminum silicate Crystal Bay Minnesota 2689.jpg|thumb|left|Bytownite from [[Crystal Bay Township, Minnesota|Crystal Bay]], [[Minnesota]]]]
Bytownite is a rock forming mineral occurring in [[mafic]] [[igneous rock]]s such as [[gabbro]]s and [[anorthosite]]s. It also occurs as [[phenocryst]]s in mafic [[volcanic rock]]s. It is rare in [[metamorphic rock]]s. It is typically associated with [[pyroxene]]s and [[olivine]].<ref name=Handbook/>

The mineral was first described in 1836<ref>[[Thomas Thomson (chemist)|Thomas Thomson]], ''Outlines of Mineralogy, Geology, and Mineral Analysis'', vol. I, Baldwin & Cradock, Londres, 1836, [https://books.google.com/books?id=F8jJ77vvJXsC&pg=PA372 p. 372]</ref> and named for an occurrence at [[Bytown]] (now [[Ottawa]]), Canada.<ref name="Mindat" /> Other noted occurrences in Canada include the Shawmere anorthosite in [[Foleyet Township]], [[Ontario]], and on [[Yamaska Mountain]],
near [[Abbotsford, Quebec|Abbotsford]], [[Quebec]]. It occurs on [[Rùm]] island, [[Scotland]] and Eycott Hill, near [[Keswick, Cumbria|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]].<ref name="Handbook" />

In the US 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]].<ref name="Handbook" />

==References==
{{Reflist}}
* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., Wiley, {{ISBN|0-471-80580-7}}

==External links==
{{Cite EB1911|wstitle=Bytownite}}

[[Category:Tectosilicates]]
[[Category:Calcium minerals]]
[[Category:Sodium minerals]]
[[Category:Feldspar]]
[[Category:Triclinic minerals]]
[[Category:Gemstones]]
[[Category:Minerals in space group 2]]

[[fr:Bytownite]]