Science Fair Project Encyclopedia
|Colour||Raspberry red, orange-red, pink|
|Habit||flattened, tabular, equant, aggregate|
|Fracture||Conchoidal to uneven|
|RI||1.601 to 1.620 (DR -0.008 to 0.011)|
Pezzottaite, marketed under the name raspberyl or raspberry beryl, is a newly identified mineral species, first recognized by the International Mineralogical Association in September 2003. Pezzottaite is a caesium analogue of beryl, a silicate of caesium, beryllium, lithium and aluminium, with the chemical formula Cs(Be2Li)Al2Si6O18. Named after Italian geologist and mineralogist Federico Pezzotta , pezzottaite was first thought to be either bixbite (red beryl) or a new variety of beryl ("caesium beryl"); unlike other beryls, however, pezzottaite contains lithium and crystallizes in the trigonal crystal system rather than the hexagonal system.
Remarkably vibrant colours, in shades of raspberry red to orange-red and pink, quickly garnered pezzottaite attention from gemmologists and gem dealers. Recovered from miarolitic cavities in the granitic pegmatite fields of Fianarantsoa province, southern Madagascar, the pezzottaite crystals were small—no more than about 7 cm in their widest dimension—and tabular or equant in habit, and few in number, most being heavily included with growth tubes and liquid feathers. However, their novelty, pleasing colours, and rarity made any material a valuable commodity: a total of 5 kg of rough was offered for sale at the 2003 Tuscon Gem & Mineral Show. Approximately 10 per cent of the rough would also exhibit chatoyancy when polished. Most cut pezzottaite gems are under one carat in weight and rarely exceed two carats: Even moderately included stones may fetch USD $1,000 or more per carat.
With the exception of hardness (8 on Mohs scale), the physical and optical properties of pezzottaite—i.e., specific gravity 3.10 (average), refractive index 1.601 to 1.620, birefringence 0.008 to 0.011 (uniaxial negative)—are all higher than typical beryl. Pezzottiate is brittle with a conchoidal to irregular fracture, and streaks white. Like beryl, it has an imperfect to fair basal cleavage. Pleochroism is moderate, from pink-orange or purplish pink to pinkish purple. Pezzottaite's absorption spectrum, as seen by a hand-held (direct vision) spectroscope, features a band at 485–500 nm with some specimens showing additional weak lines at 465 and 477 nm and a weak band at 550–580 nm.
Most (if not all) of the Madagascan deposits have since been exhausted. Pezzottaite has been found in at least one other locality, Afghanistan: this material was first thought to be caesium-rich morganite (pink beryl). Like morganite and bixbite, pezzottaite is believed to owe its colour to radiation-induced colour centres involving trivalent manganese. Pezzottaite will lose its colour if heated to 450°C for two hours, but the colour can be restored with gamma irradiation.
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