Summer 2009

Featured Gem News International: Colorless Petalite and Pollucite from Laghman, Afghanistan

Farooq Hashmi (Intimate Gems, Jamaica, New York) recently loaned GIA three colorless stones (figure 1), which he believed to be petalite (14.98 ct) and pollucite (11.30 and 12.10 ct) from Laghman Province, Afghanistan. He purchased the rough in Peshawar, Pakistan, between 2007 and 2008. The stones were reportedly byproducts of pegmatite mining for tourmaline and other gems. Mr. Hashmi said he saw several kilograms of both gem materials in the Peshawar market, but he understands from local gem dealers that neither of them is being actively mined.

Petalite and pollucite are both known to be hosted in lithium-rich granitic pegmatites, and they have a Mohs hardness of 6.5. Petalite (also known as castorite) is a lithium aluminum tectosilicate (LiAlSi₄O₁₀) and a member of the feldspathoid group. An important lithium ore, it occurs with spodumene, lepidolite, and tourmaline in tabular crystals and columnar masses that range from colorless to gray and yellow. Pollucite, (Cs,Na)(AlSi₂O₆)•nH₂O, is a zeolite that forms a solid-solution series with analcime and commonly occurs with quartz, spodumene, petalite, and tourmaline, among other pegmatite minerals. Isometric crystals ranging from colorless to white and occasionally pale pink can be found, though well-formed examples are rare.

Standard gemological testing produced the following properties (with those of petalite listed first, then the 11.30 and 12.10 ct pollucites, respectively): RI—1.505–1.515, and 1.518–1.519 or 1.517–1.518; birefringence—0.010 and 0.001 (pollucite can be weakly anisotropic); SG—2.40 and 2.90; UV fluorescence—all three samples were inert to both long- and short-wave UV radiation. No bands or lines were observed with the desk-model spectroscope. Microscopic examination of the petalite only revealed two feathers. No inclusions were observed in the 12.10 ct pollucite, but the 11.30 ct stone contained colorless inclusions of pollucite (identified by Raman spectroscopy) and a plane of crystals in the pavilion (figure 2) that had a Raman pattern similar to that of muscovite.

EDXRF analyses of the petalite showed major amounts of Si and Al, and traces of Fe, Ge, Cs, and Sm. Analyses of the pollucite samples revealed major Si, Al, and Cs, low amounts of Rb, Rh, Yb, and La, and traces of Ti.

We performed infrared (see G&G Data Depository) and Raman spectroscopy to further characterize these unusual stones. The IR spectra for the petalite showed bands at ~3358, 3270.6, 3037, and 2591 cm⁻¹, and broad absorption below ~2390 cm⁻¹. The IR spectra for the pollucite samples showed a band at ~4720 cm⁻¹ and broad absorption between ~5321 and 5162, ~4141 and 3140, and below ~2325 cm⁻¹. The Raman spectra matched those of petalite and pollucite in our Raman database.

A pink cat’s-eye petalite from South Africa (Winter 1986 Lab Notes, pp. 239–240) had properties similar to the colorless petalite from Afghanistan, except for its dull red fluorescence to UV radiation and its lower SG (2.34). Both the petalite and pollucite we studied had properties analogous to those reported by M. O’Donoghue (Gems, 6th ed., Butterworth-Heinemann, Oxford, UK, 2006, pp. 436–438), although the birefringence of petalite was slightly higher (0.013) in that publication.

Erica Emerson (eemerson@gia.edu) and Paul Johnson
GIA Laboratory, New York

Note: A description of the faceting of these stones is available here.