BERTRANDITE
Bertrandite – Be4Si2O7(OH)2 – is a rarer (or less recognized!) sorosilicate, an alteration of beryl in Be-rich granitic pegmatites. It is the second most important Be ore, after beryl. It forms crystals with a maximum length of 5 cm.
It may contain Fe, Ca, and Al. There is a colloidal vitreous variety. It is pyroelectric.
1. Characteristics
Crystal system: Orthorhombic pyramidal.
Color: Colorless, white to pale yellow.
Habit: Thin tabular, prismatic, acicular, generally forming radial aggregates.
Cleavage: {001} perfect, {100} distinct, {010} distinct, {110} distinct.
Tenacity: No information available.
Twinning:Twinning is common in {011} and {021}, forming “V”-shaped twins, with the axes intersecting at angles of 60º and 120º.
Fracture: Irregular to conchoidal.
Mohs Hardness: 6 – 7
Parting: No.
Streak: White to gray.
Lustre: Vitreous, pearly in cleavage planes {001}.
Diaphaneity: Transparent to translucent.
Density (g/cm³): 2.59 – 2.60
2. Geology and Deposits
Bertrandite is found in Be-rich pegmatites, usually as a hydrothermal alteration product of beryl. It occurs in fissures in granites and associated pegmatites, as well as in miarolitic cavities of greisens.
Frequently, bertrandite occurs as crystals grown on beryl or as a pseudomorphic replacement for beryl. It rarely occurs as a primary mineral.
It is found in small quantities in many localities, but in the United States (USA) it is the most important Be ore (Spor Mountain and Brush Beryllium Mine, Juab County, Utah).
3. Mineral Associations
It is associated with minerals typical of granitic pegmatites:
– quartz (including smoky quartz and chalcedony varieties),
– feldspars (albite, orthoclase, microcline),
– micas (muscovite, biotite, chlorite),
– carbonates (calcite, rhodochrosite, siderite)
– other alteration products of beryl, such as bavenite, phenakite, epididymite, and milarite.
– fluorapatite, beryl (including aquamarine variety), fluorite, tourmaline, garnet (spessartine), columbite, pyrochlore, sphalerite, hematite, pyrite and Uranium minerals.
– rarer minerals such as herderite, anatase, brookite, analcime, cheralite, bastnäsite-(Ce), xenotime-(Y), danalite, and aegirine.
4. Transmitted Light Microscopy
Refraction indices: nα: 1,591 nβ: 1,605 nγ: 1,614
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: No information available.
Relief: Moderate.
Cleavage: {001} perfect, {100}, {010} and {110} distinct.
Habits: Fine tabular habit, generally prismatic to acicular, in radial aggregates.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.023: colors ranging from gray, yellow, orange, and red to first-order blue.
Extinction: Probably parallel in longitudinal sections.
Elongation sign: No information available.
Twins: Common, in a “V” shape, with angles of 60º and 120º between the crystals that form the twin.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(-)
2V angle: 73 a 81º
Alterations: It is an alteration product of beryl.
May be confused with: No information available.
5. Reflected Light Microscopy
Since it is a transparent, light-colored mineral found in ores composed of other transparent, light-colored minerals, reflected light microscopy is not the appropriate analytical method for its identification.
Its habit is somewhat diagnostic. However, considering that it is generally colorless, transparent, massive, and has the hardness and density of silicates, it is very easy to confuse it with other minerals.