Boracite

{{Short description|Magnesium borate mineral}}

{{Infobox mineral

| name = Boracite

| boxwidth =

| boxbgcolor =

| image = Boracite-169813.jpg

| imagesize = 260px

| alt =

| caption =

| category = Tektoborates

| formula = Mg₃B₇O₁₃Cl

| IMAsymbol=Boc{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|url=https://www.cambridge.org/core/journals/mineralogical-magazine/article/imacnmnc-approved-mineral-symbols/62311F45ED37831D78603C6E6B25EE0A|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}}

| molweight = 392.03 g/mol

| strunz = 6.GA.05

| dana = 25.06.01.01

| system = Orthorhombic

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

| symmetry = Pca2₁

| unit cell = a = 8.577(6) Å,
b = 8.553(8) Å,
c = 12.09(1) Å; Z = 4

| color = Colorless, white, gray, brown, orange, yellow, pale green, dark green, blue-green, or blue; colorless in transmitted light

| colour =

| habit = Crystalline, disseminated (pseudocubic)

| twinning = Rarely as penetration twins

| cleavage = None

| fracture = Irregular/uneven, conchoidal

| tenacity = Brittle

| mohs = 7 - 7.5

| luster = Vitreous - adamantine

| streak = White

| diaphaneity = Subtransparent to translucent

| gravity = 2.95

| density = 2.91 - 3.10

| polish =

| opticalprop = Biaxial (+)

| refractive = n_α=1.658 - 1.662,
n_β=1.662 - 1.667,
n_γ=1.668 - 1.673

| birefringence = δ = 0.010–0.011

| pleochroism = None

| 2V = 82°

| dispersion = 0.024 (weak)

| extinction =

| length fast/slow =

| fluorescence = None

| absorption =

| melt =

| fusibility =

| diagnostic =

| solubility = Very slowly soluble in H₂O; Slowly but completely soluble in HCl

| impurities =

| alteration =

| other =

| prop1 =

| prop1text =

| references = {{cite web|title=Boracite Mineral Data|url=http://webmineral.com/data/Boracite.shtml|publisher=Webmineral.com|accessdate=2011-10-27}}{{cite web|title=Boracite Gems|url=http://www.classicgems.net/gem_boracite.htm|publisher=ClassicGems.net|accessdate=2011-10-27}}{{cite web|title=Boracite mineral information and data|url=http://www.mindat.org/min-721.html|publisher=Mindat.org|accessdate=2011-10-27}}{{cite web|title=Boracite|url=http://rruff.geo.arizona.edu/doclib/hom/boracite.pdf|publisher=Mineral Data Publishing|accessdate=2011-10-27}}

}}

Boracite is a magnesium borate mineral with formula: Mg₃B₇O₁₃Cl. It occurs as blue green, colorless, gray, yellow to white crystals in the orthorhombic - pyramidal crystal system. Boracite also shows pseudo-isometric cubical and octahedral forms. These are thought to be the result of transition from an unstable high temperature isometric form on cooling. Penetration twins are not unusual. It occurs as well formed crystals and dispersed grains often embedded within gypsum and anhydrite crystals. It has a Mohs hardness of 7 to 7.5 and a specific gravity of 2.9. Refractive index values are nα = 1.658 - 1.662, nβ = 1.662 - 1.667 and nγ = 1.668 - 1.673. It has a conchoidal fracture and does not show cleavage. It is insoluble in water (not to be confused with borax, which is soluble in water).

Boracite is typically found in evaporite sequences associated with gypsum, anhydrite, halite, sylvite, carnallite, kainite and hilgardite. It was first described in 1789 for specimens from its type locality of Kalkberg hill, Lüneburg, Lower Saxony, Germany. It is also found near Sussex, New Brunswick."Phase transitions in the series boracite-trembathite-congolite; an infrared spectroscopic study" Peter C. Burns ; Michael A. Carpenter. The Canadian Mineralogist (1997) 35 (1): 189–202

The name is derived from its boron content (19 to 20% boron by mass).