{{Short description|Hydrated potassium uranyl silicate mineral}} {{Infobox mineral | name = Boltwoodite | category = Uranium silicate mineral | image = Boltwoodite-69288.jpg | imagesize = 260px | alt = | caption = Acicular crystals of umber-yellow boltwoodite from Namibia (size: 1.8 × 1.7 × 1.4 cm) | formula = (K<sub>0.56</sub>Na<sub>0.42</sub>)[(UO<sub>2</sub>)(SiO<sub>3</sub>OH)]·1.5(H<sub>2</sub>O) | IMAsymbol=Bdw<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 9.AK.15 | dana = | system = Monoclinic | class = Prismatic (2/m) <br/><small>(same H-M symbol)</small> | symmetry = ''P2''<sub>1</sub>/m | unit cell = a = 7.0772(8)&nbsp;Å, <br/>b = 7.0597(8)&nbsp;Å, <br/>c = 6.6479(7)&nbsp;Å; <br/>β = 104.982(2)°; Z&nbsp;=&nbsp;2 | color = Pale yellow, orange yellow | colour = | habit = Elongated crystals, acicular to fibrous | twinning = | cleavage = Perfect on {010}, imperfect on {001} | fracture = | tenacity = Brittle | mohs = 3.5–4 | luster = Silky to vitreous, dull or earthy in aggregates | streak = White | diaphaneity = Transparent to translucent | gravity = 4.7 | density = | polish = | opticalprop = Biaxial (−) | refractive = n<sub>α</sub> = 1.668 – 1.670 n<sub>β</sub> = 1.695 – 1.696 n<sub>γ</sub> = 1.698 – 1.703 | birefringence = δ = 0.030 – 0.033 | pleochroism = Weak, X = colorless, Y = Z = yellow | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= Fluoresces dull green in both SW and LW UV | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = 25px Radioactive | alteration = | references = <ref>[https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Boltwoodite Mineralienatlas]</ref><ref name=HBM>[http://rruff.geo.arizona.edu/doclib/hom/boltwoodite.pdf Handbook of Mineralogy]</ref><ref name=Mindat>[http://www.mindat.org/min-716.html Mindat.org]</ref><ref name=Webmin>[http://www.webmineral.com/data/Boltwoodite.shtml Webmineral data]</ref><ref>{{cite journal |last1=Burns |first1=Peter |title=The Structure of Boltwoodite and Implications of Solid Solution Toward Sodium Boltwoodite |journal=The Canadian Mineralogist |date=1998 |volume=36 |page=1069-75}}</ref> }} '''Boltwoodite''' is a hydrated uranyl silicate mineral with formula (K<sub>0.56</sub>Na<sub>0.42</sub>)[(UO<sub>2</sub>)(SiO<sub>3</sub>OH)]·1.5(H<sub>2</sub>O), distinct in crystal structure from sodium boltwoodite, which has an orthorhombic structure rather than monoclinic.<ref>{{cite journal |last1=Burns |first1=Peter |title=The Structure of Boltwoodite and Implications of Solid Solution Toward Sodium Boltwoodite |journal=The Canadian Mineralogist |date=1998 |volume=36 |page=1069-75}}</ref> It is formed from the oxidation and alteration of primary uranium ores. It takes the form of a crust on some sandstones that bear uranium. These crusts tend to be yellowish with a silky or vitreous luster.<ref name=Webmin/><ref>{{Cite web|url=http://www.galleries.com/minerals/silicate/boltwood/boltwood.htm|title = BOLTWOODITE (Hydrated Potassium Uranyl Silicate Hydroxide)}}</ref>

==Discovery and occurrence== thumb|left|upright|Orange to yellow boltwoodite on dark calcite matrix from Namibia (size: 5.2 x 3 x 2cm) It was first described in 1956 for an occurrence in Pick's Delta Mine, Delta, San Rafael District (San Rafael Swell), Emery County, Utah, US.<ref name=Mindat/> It is named after Bertram Boltwood (1870–1927) an American pioneer of radiochemistry.

Boltwoodite occurs as secondary silicate alteration crusts surrounding uraninite and as fracture fillings. It is found in pegmatites and sandstone uranium deposits of the Colorado Plateau-type. It occurs associated with uraninite, becquerelite, fourmarierite, phosphouranylite, gypsum and fluorite.<ref name=HBM/>

==References== {{Reflist}}

{{Commons category|Boltwoodite|position=left}}

Category:Uranium(VI) minerals Category:Monoclinic minerals Category:Minerals in space group 11

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