16.4.12: Pezzottaite

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gemology
Gemology Project

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Pezzotaite
Chemical composition	Cesium Beryllium Lithium Aluminium Silicate Cs(Be₂Li)Al₂Si₆O₁₈
Crystal system	Trigonal
Habit	subhedral-to-euhedral hexagonal tabular
Cleavage	imperfect (basal)
Fracture	conchoidal to irregular
Hardness	8
Optic nature	uniaxial -
Refractive index	1.601-1.620
Birefringence	0.011
Dispersion	unknown
Specific gravity	3.04-3.14
Lustre	vitreous
Pleochroism	moderate to strong

Pezzottaite image gallery

Pezzottaite is a raspberry-red to pink member of the Beryl Group named after Dr. Federico Pezzotta. Discovered in Madagascar and IMA approved in 2003 as a new mineral.

Although related to Beryl it is not a variety of Beryl, it is a gem quality member of the Beryl Group to which also Beryl belongs.

Crystallography

Pezzottaite has trigonal symmetry. Pezzottaite with Cs and Li is not isostructural with beryl but the arrangement is close. Pinacoid, prism, pyramid are common forms. Etching and corrosion is seen.

Chemical composition

Composition with trace elements: Pezzottaite is Cs(Be₂Li)Al₂Si₆O₁₈ ; i.e. Cs, Li rich beryl. In Beryl three Be atoms as in formula occupy three symmetrical positions but in pezzottaite, the three positions are occupied in an ordered pattern by Be₂Li rather than Be₃. Thus these positions are not related by symmetry and changes to 3-fold symmetry rather than hexagonal and larger unit cells. This increase has an effect on the properties to a great extent. Other important traces include Rb, Na, Mn, K, Sc, Ti, Fe also.

Beryl: Be₃Al₂Si₆O₁₈
Pezzottaite: Cs(Be₂Li)Al₂Si₆O₁₈

Diagnostics

Color

Due to radiation induced color center involving Mn3+; Purplish pink, sometimes orangish tinge present (Madagascar).

Magnification

Inclusions: Growth tubes, fractures, fingerprint and fluid inclusions with phase. Transparent near colorless crystals. Also transparent green crystals of tourmaline. Rarely color zoning.

Phenomena

Cat's eye

Treatments

Fracture filling with oil or resin.
Heat may lighten color and irradiation darken it.

Simulants

Morganite (lower heft, lower R.I)
Tourmaline (Doubling of inclusions & Back facet edges, Higher birefringence)
Tourmaline cat's eye is very close to pezzottaite cat's eye (Birefringence blink and inclusions)

Occurrence

Geological occurrence: Granitic pegmatites having Li, Cs, Ta and other rare Earth elements. Mainly crystallized from the fluids in the cavities as late-stage crystals.

Geographical occurrence: Sakavalana pegmatite in Madagascar; little in Afghanistan too and sometimes in other granitic pegmatites.

Sources

Gems and Gemology 2003 - GIA
Journal of Gemmology - Gem-A

External links

Pezzottaite
Pezzottaïte: "Béryl" rose-pourpre atypique riche en Césium de Madagascar (in French)