bertrandite

{{Infobox mineral

| name = Bertrandite

| image = Bertrandite-38545.jpg

| alt =

| caption = Bertrandite from the Golconda pegmatite, Minas Gerais, Brazil

| category = Sorosilicate

| formula = Be₄Si₂O₇(OH)₂

| IMAsymbol=Btd{{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}}

| strunz = 9.BD.05

| dana =

| system = Orthorhombic

| class = Pyramidal (mm2)
H-M symbol: (mm2)

| symmetry = Ccm2₁

| unit cell = a = 8.7135(4) Å,
b = 15.268(1) Å,
c = 4.5683(3) Å; Z = 4

| color = Colorless to pale yellow

| habit = Thin tabular, prismatic to needle-like crystals commonly in radial clusters

| twinning = Common on {011} or {021} forming heart- or V-shaped twins

| cleavage = Perfect on {001}; distinct on {100}, {010} and {110}

| fracture =

| tenacity =

| mohs = 6–7

| luster = Vitreous, pearly on cleavage surfaces

| streak =

| diaphaneity = Transparent

| gravity = 2.59–2.60

| density =

| polish =

| opticalprop = Biaxial (−)

| refractive = n_α = 1.591 n_β = 1.605 n_γ = 1.614

| birefringence = δ = 0.023

| pleochroism =

| 2V = Measured: 73° to 81°

| dispersion =

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| references = [http://rruff.geo.arizona.edu/doclib/hom/bertrandite.pdf Handbook of Mineralogy][http://www.mindat.org/min-642.html Bertrandite on Mindat.org][http://webmineral.com/data/Bertrandite.shtml Bertrandite on Webmineral]

}}

Bertrandite is a beryllium sorosilicate hydroxide mineral with composition: Be₄Si₂O₇(OH)₂. Bertrandite is a colorless to pale yellow orthorhombic mineral with a hardness of 6–7.

It is commonly found in beryllium rich pegmatites and is in part an alteration of beryl. Bertrandite often occurs as a pseudomorphic replacement of beryl. Associated minerals include beryl, phenakite, herderite, tourmaline, muscovite, fluorite and quartz.

It, with beryl, are ores of beryllium.

It was discovered near Nantes, France in 1883 and named after French mineralogist, Emile Bertrand (1844–1909).

One of the world's largest deposits of bertrandite is Spor Mountain, Thomas Range, Utah which is currently the source of most of the world's beryllium production.{{cite web|last1=Fact Sheet 2016–3081|title=Beryllium—A Critical Mineral Commodity—Resources, Production, and Supply Chain|url=https://pubs.usgs.gov/fs/2016/3081/fs20163081.pdf|website=usgs.gov/|publisher=USGS|accessdate=16 May 2017|location=pubs.usgs.gov|pages=4|language=English|format=Article|date=October 2016}}{{cite book |last1=Shawe |first1=Daniel |editor1-last=Ridge |editor1-first=John |title=Geology of the Spor mountain Beryllium District, Utah, in Ore deposits of the United States, 1933–1967 |date=1968 |publisher=The American Institute of Mining, Metallurgical, and Petroleum engineers, Inc. |location=New York |pages=1148–1161}}