Bertrandite

(mineralogy) Be₄Si₂O₇(OH)₂ A colorless or pale-yellow mineral consisting of a beryllium silicate occurring in prismatic crystals; hardness is 6-7 on Mohs scale, and specific gravity is 2.59-2.60.

Environment

A secondary mineral mostly formed from late post-deposition hydrothermal attack on pegmatitic beryl. Can also be primary from lower-temperature gas deposition (sublimate) causing rock alteration.

Recognizable only when in distinct crystals, mostly small, and commonly tabular parallel to the base. The pearly luster of the base is quite characteristic and is probably due to a lamellar, parallel growth of individual crystals, possibly in a polysynthetic twinning sequence. Often twinned into "heart-shaped" twins, which then are not tabular in habit but very typical of the mineral.

Colorless, flesh color. Luster pearly on base, glassy otherwise; hardness 6; specific gravity 2.6; fracture flaky; cleavage perfect basal, good prismatic. Transparent to translucent; strongly responsive electrically to temperature changes.

Alkaline beryllium silicate (42.1% BeO, 50.3% SiO ₂ , 7.6% H ₂ O).

Whitens but will hardly fuse on charcoal, insoluble in acid. Turns blue with cobalt nitrate test.

Specimens found almost exclusively in complex pegmatites with corrosion-pitted beryls, eliminating any similar mineral. It is less fusible than the feldspars, but untwinned crystals may be difficult to distinguish from later albite feldspar growths in pockets. Usually the later albite will have grown on an earlier feldspar surface and aligned with them so that a group of prominences will reflect light simultaneously, in contrast to randomly oriented bertrandites that each reflect as individuals. The distinctive twins are easily recognized. Similar zeolites (resembling stilbite) fuse easily.

Bertrandite is usually considered rare, but actually is far more common than realized. It is likely to be encountered in any beryl pegmatite that hints of a history of secondary mineral formation, such as etching of the beryl, the growth of albite -- especially the cleavelandite variety -- or sulfide, fluorite, and calcite deposition in late cavities. Until recently, the best crystals came from Mt. Antero, Colorado, up to Ɖ in. (1 cm) long. Excellent small crystals are found in pockets in the cleavelandite feldspar at Portland, Connecticut. Small crystals were found coating beryl of Bedford, Westchester Co., New York. It is associated with apatite at Stoneham, Maine.

Pseudomorphs of masses of bertrandite plates after beryl are found in a pegmatite in Jefferson Co., Colorado, and similarly in the state of Rio Grande do Norte in Brazil. Small primary crystals have been found on cassiterite-mica seams in Portugal and on beryl at Raade, Norway. Most European examples are inconspicuous, occupying cavities where beryl once grew. Simultaneously, in 1993, 1-in. (2.5-cm), slightly concave, and flattened stilbite-like, pectolite-like betrandite crystal bundles turned up with dealers from sources as distant as Pakistan and Brazil.

Though hemimorphic, like hemimorphite, the low symmetry is less apparent in this mineral because the crystals are so commonly flattened parallel to the basal pinacoid. Hence, the slight difference in truncations on upper and lower edges is not especially noticeable, and, as suggested, it seems likely that there may be repetitive twinning.

Bertrandite from Brazil

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. It, with beryl, are ores of beryllium.

It was discovered near Nantes, France in 1883 and named after French mineralogist, Emile Bertrand (1844-1909).^[1][2][3]

See also

• List of minerals
• List of minerals named after people

References

1. ^ http://rruff.geo.arizona.edu/doclib/hom/bertrandite.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-642.html Mindat
3. ^ http://webmineral.com/data/Bertrandite.shtml Webmineral

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