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Boracite
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<p>Boracite is a <a href="/facts/Magnesium/XyIhd2FG">magnesium</a> <a href="/facts/Borate_mineral/dWx4jogd">borate mineral</a> with formula: <a href="/facts/Magnesium/XyIhd2FG">Mg</a>3<a href="/facts/Boron/Pmwgd7Mf">B</a>7<a href="/facts/Oxygen/acyn6o8r">O</a>13<a href="/facts/Chlorine/y27UwYBM">Cl</a>. It occurs as blue green, colorless, gray, yellow to white crystals in the <a href="/facts/Orthorhombic/3zo4F8P0">orthorhombic</a> - pyramidal <a href="/facts/Crystal_system/dMQLk1q4">crystal system</a>. Boracite also shows pseudo-isometric cubical and <a href="/facts/Octahedral/G38q92xW">octahedral</a> forms. These are thought to be the result of transition from an unstable high temperature <a href="/facts/Cubic_(crystal_system)/92Co7BKM">isometric</a> form on cooling. Penetration <a href="/facts/Crystal_twinning/7xU6CKfR">twins</a> are not unusual. It occurs as well formed crystals and dispersed grains often embedded within <a href="/facts/Gypsum/MItYU70n">gypsum</a> and <a href="/facts/Anhydrite/66QvVbXV">anhydrite</a> crystals. It has a <a href="/facts/Mohs_hardness/wXBMcVIm">Mohs hardness</a> of 7 to 7.5 and a <a href="/facts/Specific_gravity/ySquGd2x">specific gravity</a> of 2.9. <a href="/facts/Refractive_index/yjkzvgeE">Refractive index</a> values are nα = 1.658 - 1.662, nβ = 1.662 - 1.667 and nγ = 1.668 - 1.673. It has a <a href="/facts/Conchoidal_fracture/2tg82TPw">conchoidal fracture</a> and does not show <a href="/facts/Cleavage_(mineralogy)/p4YHe01X">cleavage</a>. It is insoluble in water (not to be confused with borax, which is soluble in water). </p><p>Boracite is typically found in <a href="/facts/Evaporite/vNW8fcEy">evaporite</a> sequences associated with <a href="/facts/Gypsum/MItYU70n">gypsum</a>, <a href="/facts/Anhydrite/66QvVbXV">anhydrite</a>, <a href="/facts/Halite/9xEEKKT8">halite</a>, <a href="/facts/Sylvite/1q1ym0bQ">sylvite</a>, <a href="/facts/Carnallite/N7RQQwRd">carnallite</a>, <a href="/facts/Kainite/TsVQwb7C">kainite</a> and <a href="/facts/Hilgardite/keAoTRo2">hilgardite</a>. It was first described in 1789 for specimens from its <a href="/facts/Type_locality_(geology)/ky6QjASt">type locality</a> of <a href="/facts/L%25C3%25BCneburg_Kalkberg/okQD2tww">Kalkberg</a> hill, Lüneburg, Lower Saxony, Germany. It is also found near <a href="/facts/Sussex%2c_New_Brunswick/eQkWkcST">Sussex, New Brunswick</a>. </p><p>The name is derived from its <a href="/facts/Boron/Pmwgd7Mf">boron</a> content (19 to 20% boron by mass). </p>