Amblygonite

Amblygonite (/æmˈblɪɡəˌnaɪt/) is a fluorophosphate mineral, (Li,Na)AlPO₄(F,OH), composed of lithium, sodium, aluminium, phosphate, fluoride and hydroxide. The mineral occurs in pegmatite deposits and is easily mistaken for albite and other feldspars. Its density, cleavage and flame test for lithium are diagnostic. Amblygonite forms a series with montebrasite, the low fluorine endmember. Geologic occurrence is in granite pegmatites, high-temperature tin veins, and greisens. Amblygonite occurs with spodumene, apatite, lepidolite, tourmaline, and other lithium-bearing minerals in pegmatite veins. It contains about 10% lithium, and has been utilized as a source of lithium. The chief commercial sources have historically been the deposits of California and France.

Amblygonite
General
Category	Phosphate minerals
Formula	(Li,Na)AlPO4(F,OH)
IMA symbol	Aby[1]
Strunz classification	8.BB.05
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	C1
Identification
Color	Generally white or creamy, but can also be colorless or pale yellow, green, blue, beige, gray, brown or pink.
Crystal habit	Prismatic to columnar form
Twinning	Microscopic polysynthetic twinning common
Cleavage	[100] Perfect, [110] good, [011] distinct
Fracture	Irregular/ineven, sub-Conchoidal
Mohs scale hardness	5.5–6[2]
Luster	Vitreous to pearly[2]
Specific gravity	2.98–3.11
Polish luster	greasy to vitreous (in gem material)[2]
Optical properties	Double refractive, biaxial, may be either positive or negative[2]
Refractive index	na=1.577 – 1.591, nb=1.592 – 1.605, nc=1.596 – 1.613
Birefringence	.020 – .027[2]
Pleochroism	weak to none[2]
Ultraviolet fluorescence	very weak green in long wave, light blue phosphorescence in long wave and short wave[2]

History

The mineral was first discovered in Saxony by August Breithaupt in 1817, and named by him from the Greek amblus, blunt, and gonia, angle, because of the obtuse angle between the cleavages. Later it was found at Montebras, Creuse, France, and at Hebron in Maine; and because of slight differences in optical character and chemical composition the names montebrasite and hebronite have been applied to the mineral from these localities.The term amblygonite has been used interchangeably in mining, whether this mineral or montebrasite was extracted. In fact, montebrasite is much more common than amblygonite, which is a rare mineral.^[3] It has been discovered in considerable quantity at Pala in San Diego county, California; Caceres, Spain; and the Black Hills of South Dakota. A blue form of amblygonite-montebrasite has been described from Rwanda. The largest documented single crystal of amblygonite measured 7.62 × 2.44 × 1.83 m and weighed about 102 tons.^[4]

Gemology

Transparent amblygonite has been faceted and used as a gemstone. As a gemstone set into jewelry it is vulnerable to breakage and abrasion from general wear, as its hardness and toughness are poor.^[2] The main sources for gem material are Brazil and the United States. Australia, France, Germany, Namibia, and Norway, and Spain have also produced gem quality amblygonite.^[2]

Amblygonite from Taquaral, Itinga, Minas Gerais, Brazil. Scale at bottom is one inch, with a vertical line at one cm.

See also

• List of minerals