Chrysoprase and Prase Opal from Haneti, Central Tanzania
Total Page：16
File Type：pdf, Size：1020Kb
NOTES & NEW TECHNIQUES CHRYSOPRASE AND PRASE OPAL FROM HANETI, CENTRAL TANZANIA James E. Shigley, Brendan M. Laurs, and Nathan D. Renfro This article briefly describes the geology, mining, Commercial quantities of gem-quality chryso- and gemological characteristics of high-quality prase and green prase opal (nickel-bearing Tanzanian chrysoprase and prase opal from the chalcedony and common opal, respectively) Iyobo Mountain mine near Haneti (figure 1). This have been recovered from altered serpentinite mine is operated by Dimitri Mantheakis, who host- deposits near Haneti, Tanzania. Material stud- ed two of the authors (JES and BML) there in May ied for this report came from the largest mine, 2008. Although both materials have also been found located on top of Iyobo Mountain, which has in several other parts of the world (table 1), Tanzania been actively exploited for two decades. is probably the most important source of chryso- Although somewhat similar in color and prase after Australia. appearance, chrysoprase and prase opal can easily be distinguished from one another by dif- ferences in refractive index (~1.55 vs. ~1.45) BACKGROUND and specific gravity (~2.60 vs. ~2.11). Chryso- The green color of chrysoprase has long been prase makes up the vast majority of the output, thought to be caused by one or more submicroscopic 2+ and the mine shows good potential for contin- green nickel compounds (as Ni ). While this idea ued production. has been discussed for more than two centuries, the exact nature of these nickel compounds continues to be debated (Natkaniec-Nowak et al., 1989). Some hrysoprase and prase opal are nickel-contain- researchers suggested it was a nickel oxide (possibly ing green varieties of chalcedony and non- the mineral bunsenite; see Heflik et al., 1989), but play-of-color (common) opal, respectively. subsequent spectroscopic and high-magnification C imaging studies disproved this idea (see Rossman, Both have been used as gem materials for thousands of years. Chrysoprase was described as being the 1994; Gawel´ et al., 1997). Most others have ascribed “most prized” type of chalcedony by Webster (1994, the coloring agent to layered or framework hydrated p. 233). Central Tanzania is an important contempo- nickel silicates (such as kerolite) or nickel-contain- rary source of both these gems. The Tanzanian mate- ing clay minerals (such as garnierite, lizardite, or rials were first briefly described by Gübelin (1975, pp. saponite; see Rossman, 1994, pp. 458–459; Nagase et 76–78) and Schmetzer et al. (1976). These reports al., 1997; Dyrek et al., 2001; Sachanbin´ski et al., were followed by a characterization of the green prase 2001; Sojka et al., 2004). opal by Koivula and Fryer (1984), and a more detailed The nickel compound(s) occur as tiny, colloidal study of the chrysoprase by Kinnunen and Malisa particles that are dispersed as inclusions throughout (1990). the host silica matrix. The presence of a small amount of iron (as Fe3+) can modify the color of chrysoprase to be more yellowish green, whereas See end of article for About the Authors and Acknowledgments. light scattering from microdefects or small particles GEMS & GEMOLOGY, Vol. 45, No. 4, pp. 271–279. in the translucent material (the “Tyndall Effect”) has © 2009 Gemological Institute of America been suggested as the cause of the more bluish green NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY WINTER 2009 271 Figure 1. Tanzanian chryso- prase (left, cabochons rang- ing from 3.72 to 18.42 ct) and prase opal (inset, 35.82 ct) are commercially mined from one deposit near Haneti. All but the pieces of rough were examined for this study. Gift of Dimitri Mantheakis; GIA Collection nos. 37910–37914 (right) and 37909 (inset). Photos by Robert Weldon. appearance seen in reflected light (Sachanbin´ski et sold under the name “lemon chrysoprase,” has been al., 2001). Gawel´ et al. (1997) studied chrysoprase described as a rock consisting of magnesite and from several localities, and concluded that those quartz (Johnson and Koivula, 1996) or of both miner- samples with a lower degree of crystallinity tended als along with chrysoprase (Henn and Milisenda, to have higher nickel contents and a more intense 1997). All these other materials have gemological green color. Presumably, dispersed particles of nick- properties that allow them to be distinguished from el-containing minerals also color the green prase chrysoprase and prase opal. opal from Tanzania, since the visible spectra of both materials are similar (see Results section). Schmetzer et al. (1976) described this material as OCCURRENCE AND DESCRIPTION being opal-CT. OF THE TANZANIAN MATERIAL Because of their opaque-to-translucent green Location. Chrysoprase has been found on three adja- appearance and ability to take a good polish, both cent hills that lie ~12 km southeast of the village of chrysoprase and prase opal have been used as substi- Haneti and 12 km north of the village of Itiso. tutes for jade. Dyed green agate and green glass have Haneti is 75 km north of Dodoma along Highway occasionally been used to imitate chrysoprase A104 (figure 2). The mine operated by Mr. (O’Donoghue, 2006). Also known is a chromium- Mantheakis is located near the top of the southern- colored green chalcedony found in Australia, Bolivia, most of the three hills (known as Iyobo Mountain; and Zimbabwe (material from the last has been figure 3), at 5° 31.90′ S, 35° 59.33′ E and at an eleva- referred to as mtorolite; see Phillips and Brown, tion of 1,452 m. The area is accessible during the dry 1989; Hyr`´sl and Petrov, 1998; Hyr`´sl, 1999; Willing season (which runs April through November) by a and Stocklmayer, 2003). An opaque-to-semitranslu- dirt road from Dodoma to Haneti, and from there to cent yellow-green carving material from Australia, the mine site by a rough dirt track. 272 NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY WINTER 2009 Moshi AFRICA A104 KENYA Arusha TANZANIA Geology. According to Kinnunen and Malisa (1990), 0 100 km rocks in the area of the Haneti deposit are of Archean Mombasa 0 100 miles age and consist of metamorphosed ultramafics that B1 form the three chrysoprase-bearing hills. These are TANZANIA aligned in a northwesterly direction, which marks the A104 Tanga orientation of regional fracture/shear zones. They A14 described the dominant rock type hosting the chryso- Chrysoprase and Haneti prase opal mine INDIAN prase as a silicified and ferruginized (silica- and iron- B127 OCEAN Itiso A14 altered) serpentinite. Dodoma Zanzibar B129 TABLE 1. Other world sources of chrysoprase and Morogoro Dar es prase opal. Salaam Locality Geologic setting Reference Figure 2. The Haneti chrysoprase and prase opal mine is located in central Tanzania, between Chrysoprase Dodoma and Arusha. Marlborough, Queensland, Weathered and laterized Brooks (1965), Krosch Australia serpentinites or other ultra- (1990), Jones (1992), mafic rocks O'Brien (1997), Brown Both the chrysoprase and prase opal occur in dis- (2000), Downing (2007), Osmond and Baker (2009) continuous veins up to several centimeters thick Warrawanda, Weathered and altered Nagase et al. (1997) and several meters long in the weathered host rock. Western Australia serpentinites associated According to Mr. Mantheakis, the chrysoprase veins with granites appear to be more abundant on the western side and Yerilla Station, Veins in weathered Jones (1994a,b), upper portions of Iyobo Mountain; some areas have Western Australia siliceous ironstones Brown (2000) Niquelandia, Goiás, Brazil A galena mine (?) Kammerling et al. almost no such mineralization. Within a vein and (1990) between different veins, the material can vary from Saxony, Germany Not reported Wittern (2001) high-quality translucent to low-quality opaque. The Niigata Prefecture, Honshu, Not reported Mindat.org (2009) best chrysoprase occurs in those areas of a vein that Japan are surrounded by red clay or soil, while poorer-qual- Sarykul Boldy, Kazakhstan Nickel deposit in Sachanbin´ski et al. ity material is found where the vein is enclosed by weathered serpentinite (2001) yellowish material. The best-quality prase opal is Ambatondrazaka, Not reported Behier (1963) Madagascar found in veins within very brittle host rock. Szklary, Lower Silesia, Nickel deposit in Drzymal´a and Serkies Poland weathered serpentinite (1973), Nis´kiewicz (1982), Mining. Although at one time some Ni-rich magne- Heflik et al. (1989), site veins were prospected in the region, the only Sachanbin´ski et al. (2001) mining in the Haneti area has been for chrysoprase- Southern Urals, Russia Not reported Mikhailov (2000) Tulare County, California, Not reported O'Donoghue (1995) United States Various sites in Arizona, Not reported Mindat.org (2009) NEED TO KNOW California, Colorado, • Chrysoprase and green prase opal are nickel- Massachusetts, New York, bearing green chalcedony and common opal, North Carolina, Oregon, Rhode Island, and Vermont respectively. • They have been mined from Iyobo Mountain Prase opal near Haneti, Tanzania, for more than two decades. South Bohemia, Not reported Duda et al. (1991) • They are found in veins hosted by altered Czech Republic serpentinite. Silesia, Poland Not reported Drzymal´a and Serkies • Typically, 7–10 tonnes of “mine-run” chrysoprase (1973), Webster (1994) is produced annually (3% suitable for cabochons). Southern Urals, Russia Small vein at a “chrome Mikhailov (2000) • In the past decade, ~1.5 tonnes of “mine-run” mine” prase opal was produced. Napa County, California, Not reported “Prase opal in California,” United States 1936 NOTES AND NEW TECHNIQUES GEMS & GEMOLOGY WINTER 2009 273 Mining is currently carried out in an open pit (figure 4) at the top of Iyobo Mountain. As many as 35 local people are employed during the dry season. So far, 28 pits have been mined on the mountain, ranging from ~20 to ~250 m2. Mechanized equip- ment is used to expose areas containing the gem- bearing veins, which are then excavated with hand tools (figure 5).