BERTRANDITE
Bertrandite – Be4Si2O7(OH)2 – is a rarer (or less recognized!) sorosilicate, an alteration product of beryl found in Be-rich granitic pegmatites. It is the second most important beryllium ore, ranking only behind beryl.
It may contain Fe, Ca, and Al. A vitreous, colloidal variety exists. It is pyroelectric.
1. Characteristics
Crystal system: Orthorhombic pyramidal.
Color: Colorless, white to pale yellow.
Habit: Bertrandite forms crystals up to 5 cm in length. Thinly tabular, prismatic, acicular; generally forms radial aggregates.
Cleavage: {001} perfect, {100} distinct, {010} distinct, {110} distinct.
Tenacity: No information available.
Twinning: Twinning on {011} and {021} is common, producing V-shaped twins with axes intersecting at angles of 60° and 120°.
Fracture: Irregular, conchoidal.
Mohs Hardness: 6 – 7
Parting: No.
Streak: White to gray.
Lustre: Vitreous, pearly on {001} cleavage planes.
Diaphaneity: Transparent.
Density (g/cm³): 2.59 – 2.6
2. Geology and Deposits
Bertrandite is found in Be-rich pegmatites, typically as a hydrothermal alteration product of beryl. It occurs in fissures within granites and associated pegmatites, as well as in miarolitic cavities in greisens.
Bertrandite frequently occurs as crystals growing on beryl or as a pseudomorph replacing beryl. It rarely occurs as a primary mineral.
It is found in small quantities in many localities, but in the United States, it is the most important ore of Be (Spor Mountain and Brush Beryllium Mine, Juab County, Utah).
3. Mineral Associations
It occurs associated with minerals typical of granitic pegmatites: quartz (including smoky quartz and chalcedony varieties), feldspars (albite, orthoclase, microcline), micas (muscovite, biotite, chlorite), fluorapatite, beryl (including the aquamarine variety), fluorite, tourmaline, garnet (spessartine), columbite, pyrochlore, sphalerite, hematite, and pyrite.
It also occurs with carbonates (calcite, rhodochrosite, siderite), uranium minerals, and rarer minerals such as herderite, anatase, brookite, analcime, cheralite, bastnäsite-(Ce), xenotime-(Y), danalite, and aegirine.
It is associated with other beryl alteration products, such as bavenite, phenakite, epididymite, and milarite.
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, and {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 the longitudinal sections.
Elongation sign: No information available.
Twins: Common, V-shaped, with angles of 60° and 120° between the crystals forming the twin.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(-)
2V angle: 73 to 81º
Alterations: It is an alteration of beryl.
May be confused with: No information available.
5. Reflected Light Microscopy
As 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 a diagnostic feature. However, given that it is generally colorless, transparent, and massive, and possesses the hardness and density typical of silicates, it is very easily confused with other minerals.