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| These cut stones (2.06 ct and 5.52 ct) and crystal (14 g) of herderite are from northern Pakistan. Courtesy of Farooq Hashmi; photo by C.D. Mengason. |
Herderite–hydroxyl-herderite [CaBePO
4 (F,OH)] has been known from granitic pegmatites in
Pakistan’s Northern Areas for several years. Well-formed crystal specimens of herderite from this region are sought after by mineral collectors, but the material is rarely transparent enough to be faceted. In June 2006, Farooq Hashmi (Intimate Gems,
Jamaica,
New York) loaned GIA some rough and cut examples of “lime” green herderite that he obtained during the past two years in the mineral market of
Peshawar, Pakistan. The material was represented as coming from the
Shigar Valley, as well as from the more general localities of Skardu and Gilgit, which are common gem and mineral trading areas in northern
Pakistan. The most likely source of the gem herderite is the Kandahar mine, Baha, Braldu Valley (about 35 km north of Skardu), according to Dudley Blauwet (Dudley Blauwet Gems, Louisville, Colorado), who has extensive experience traveling to the gem and mineral localities of northern Pakistan. Mr. Blauwet also indicated that he recently obtained cuttable herderite from two additional locations in the
Shigar Valley area: near Chhappu in the
Braldu Valley, and at a new mine near Doko in the
Basha Valley.
The two faceted herderites (2.06 and 5.52 ct; figure 14) were characterized by one of us (EPQ) for this report; the properties of the smaller stone are listed first, as appropriate: color—light green and medium-light green; pleochroism—both stones showed weak-to-moderate green and yellowish green (a third color was not observed); diaphaneity—transparent; R.I.—1.587–1.616 and 1.586–1.616; birefringence—0.029 and 0.030; hydrostatic S.G.—3.02 and 3.04; Chelsea filter reaction—none; fluorescence—weak-to-moderate blue and moderate-to-strong blue to long-wave UV radiation, weak violet and moderate violet to short-wave UV radiation; and a weak 585 nm absorption feature was observed with the desk-model spectroscope for both stones. Microscopic examination of both samples revealed moderate-to-strong doubling and numerous partially healed fractures with negative crystals, some of which contained both a liquid and a gas. EDXRF spectroscopy of the larger stone showed major amounts of Ca, P, and Sr, and minor Cr, Mn, Y, and Pb. The instrument cannot detect Be or F.
The properties of these Pakistani samples are comparable to those previously reported for herderite (e.g., Spring 2004 Lab Notes, pp. 61–62), although their refractive indices were somewhat higher. Based on the work of P. B. Leavens et al. (“Compositional and refractive index variations of the herderite–hydroxyl-herderite series,” American Mineralogist, Vol. 63, No. 9–10, 1978, pp. 913–917), the R.I. values of the Pakistani herderite indicate that the composition is near the midpoint between the OH- and F-dominant end-members. Such a composition was also inferred from an R.I. value (nβ = 1.610) of a herderite from the Shigar Valley area by A. H. Kazmi et al. (“Gem pegmatites of the Shingus-Dusso area, Gilgit, Pakistan,” Mineralogical Record, Vol. 16, No. 5, 1985, pp. 393–411). Although we have referred to the material as herderite in this entry for simplicity, the correct mineralogical designation is herderite–hydroxyl-herderite.
Brendan M. Laurs and Elizabeth P. Quinn
