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Bruce Cairncross Bruce Cairncross East Africa is famous for its world-class gemstones and mineral specimens, most notably the highly sought-after blue tanzanite, found only in Tanzania in a very small area in the northeast. Many other minerals and gemstones are mined in the region and this is the first full-colour publication to showcase and examine those occurring in the five countries that make up East Africa – Kenya, Tanzania, Rwanda, Uganda and Burundi. The book includes: An introduction to the fascinating geology of the region, including a map depicting the simplified geology and mineral occurrences. Detailed descriptions of minerals, including their gemological properties, history and occurrence. Multiple full-colour photographs of each mineral and gem specimen. Diopside CaMgSi 2O6 Including variety chromian diopside can be faceted into fancy gemstones (Figure 139) and crystals are prized among collectors. Named in 1806 from Greek dis, ‘two’, and opsis, ‘face’. The name refers to the aspects or views in which Chromian diopside is known from Kajiado County in Kenya, where specimens consist of gemmy green the crystal can be oriented. diopside in a blue calcite matrix. The most prized specimens and best gem-quality Crystal system material come from the Merelani Hills in Tanzania Hardness monoclinic where it was first found in 1996. Since then, pockets of crystals have periodically been discovered. The Specific gravity 5.5–6.5 3.22–3.38 chromian diopside is found in layered metasomatic Streak rock in graphite-kyanite gneiss. Crystals can be loose, white, grey Refractive index single or clustered together. Size varies from a few Birefringence 1.664–1.730 millimetres to prismatic crystals of 5–6 cm. One of Pleochroism 0.024–0.030 the discoveries, made in 2007, yielded gem-quality FOR ORDERS PLEASE CONTACT: Lustre n/a diopside on coarsely crystalline graphite, the latter vitreous, dull acknowledged as the finest graphite specimens known (Figures 57 and 84. The contrast of the mint-green Sasha Nicolas Diopside is a clinopyroxene, the magnesium end member of the diopside on black-grey tabular graphite matrix makes diopside-hedenbergite (Fe endmember) to johannsenite (Mn Figure 83 Diopside crystals attached to a tanzanite crystal, for visually stunning specimens (Jaszczak & Trinchillo, 2.3 cm. The tanzanite is a natural colour and has not been endmember) solid solution series. It is characteristically pale green 2013). The Samax mine has also produced excellent heat-treated. Merelani Hills, Lelatema Mountains, Simanjiro to colourless, white, grey, brown or black. Crystals tend to be quality chromian diopside. District, Manyara Region, Tanzania. philip short to long prismatic, but can also be massive to columnar. It is hitge specimen, bruce [email protected] cairncross photo found in a great variety of igneous and metamorphic rocks, such as peridotite, kimberlite, basalt, andesite, carbonatite, marble, dolomite and certain skarn deposits. A chrome-rich variety of or +27 (0)11 3273550 diopside occurs in kimberlitic xenoliths and is used as an indicator Figure 76 A 0.66-carat faceted chromian mineral when prospecting for diamond-bearing kimberlites. The diopside gemstone. Merelani Hills, Lelatema bright green chromian diopside variety occurring in East Africa Mountains, Simanjiro District, Manyara Region, Tanzania. warren taylor rainbow of africa gem collection , mark mauthner photo RRP: R220.00 ISBN: 9781775845560 Figure 78 An unusually shaped faceted diopside stone of 2.63 carats. Figure 77 A 0.88-carat faceted chrome diopside Merelani Hills, Lelatema Mountains, Simanjiro District, Manyara Region, gemstone. Merelani Hills, Lelatema Mountains, Tanzania. warren taylor rainbow Simanjiro District, Manyara Region, Tanzania. of africa gem collection , mark mauthner photo bruce cairncross specimen www.penguinrandomhouse.co.za and photo Figure 79 A selection of chromian diopside crystals, largest crystal 3.5 cm. Merelani Hills, Lelatema Mountains, Simanjiro 40 diopside District, Manyara Region, Tanzania. patrick meyer collection , jeff scovil photo Minerals & Gemstones_A-Z_Actinolite-Fluorapatite.indd 40-41 diopside 41 2019/04/29 13:32 Scapolite Na4Al Including marialite and meionite 3Si9O24Cl to Ca 4Al6Si O 6 24CO3 & Wilk, 1976). ‘Petschite’ is a commercial name for Named from Greek skapos, ‘rod, shaft’, and lithos, ‘stone’, alluding to the elongate, prismatic habit purple scapolite from Tanzania. of the crystals. The Dodoma scapolite is one of the older known in the early-2000s. Some Morogoro scapolite that is localities in Tanzania, dating back to specimens collected yellow under daylight fluoresces blue and red under Crystal system during the 1980s, although earlier discoveries from ultraviolet light. Both localities have produced gem- Mpwapwa were known from the mid-1970s. Marble quality yellow and mauve-purple scapolite (Moore, Hardness tetragonal units in the Uluguru Mountains in Morogoro Region 2016). The Tanzanian gem-quality scapolite is known for Specific gravity 5–6 have also produced scapolite, with specimens surfacing its chatoyancy and Fritz (2007) describes orange-brown An invaluable reference Streak 2.5–2.7 chatoyant ‘cat’s-eye’ scapolite from Tanzania that closely Refractive index resembles similar-coloured alkali-feldspar. white Birefringence 1.712–1.721 Pleochroism 0.005 Lustre for collectors, gemologists, n/a vitreous Scapolite currently refers to the solid solution series of which marialite Na 4(Al3Si O the end-members; marialite9 24)Cl and is the meionite sodium-rich Ca end-member and students, those working in 4(Al Si 6 9O )(CO meionite is the calcium-rich end-member. Scapolite can24 be opaque3) are to transparent and varies from colourless-white, to yellow, grey and purple; green, pink and red are less common colours. Transparent the gemstone and mineral scapolite is used as a gemstone. The crystals are typically elongate, prismatic with pyramidal terminations, but can also be stubby in A habit. Scapolite crystals fluoresce under short-wave and long-wave ultraviolet light. The mineral is commonly found in metamorphosed industry in East Africa, and evaporite sedimentary rocks, notably in marbles. A Two alluvial specimens of yellow scapolite, one of which showed distinct dichroism from pale yellow to colourless, were reported from northeastern Umba Region, Tanzania in 1967 (Zwaan, anyone with an interest in 1971). Dunn et al. (1978) subsequently analysed several Tanzanian scapolites and found them to be marialite, although Krupp and Schmetzer (1975) classified them as intermediate members of the the earth sciences. marialite-meionite group. Some specimens are highly fluorescent, from bright yellow under short-wave ultraviolet light to intense strong yellow under long-wave ultraviolet light. The Umba scapolites are known to have inclusions of other minerals such as pyrrhotite and lepidocrocite, as well as internal growth tubules (Graziani & Gübelin, 1981). The scapolite is found in contact- metamorphosed limestones affected by the intrusion of mafic and ultramafic rocks. Scapolite of various attractive colours including yellow, golden- B yellow, mauve and violet come from the alluvial deposits in the Figure 216 Umba area in Tanga Region, as well as in Dodoma Region, central Two very large and fine scapolite gemstones: A 70.68 carats, cut from rough Tanzania (Schmetzer et al., 1976). The violet-coloured scapolite material found in 1990; B 63.45 carats, cut B from Dodoma Region has been considered the finest known (Strunz from material found in 1999. Mpwapwa District, Figure 215 A terminated scapolite crystal Dodoma Region, Tanzania. under A daylight light and B fluorescing under 92 scapolite africa warren taylor gem collection rainbow of 350 nm ultraviolet light, 1.6 cm. Uluguru , mark mauthner photo Mountains, Morogoro Region, Tanzania. Figure 214 cairncross specimen bruce An exceptional gem-quality scapolite (marialite) crystal, and photo 2.5 cm. Rubeho Mountain, Mpapwa District, Dodoma Region, Tanzania. Minerals & Gemstones_A-Z_Garnet.indd 92-93 schauss specimen , jeff scovil photo INTRODUCTION alex scapolite 93 CONTENTS 2019/04/29 13:46 Foreword 4 Introduction 5 6 Minerals and gemstones 7 Geology of East Africa 7 East African resources Burundi 10 Kenya 11 gemstones and collectable minerals than others. Rwanda 13 EAST AFRICA, defined here as Tanzania, Kenya, Uganda, Tanzania and Kenya have the necessary geological Rwanda and Burundi, is famous for its world-class environments to have produced excellent gemstones Tanzania 14 gemstones and mineral specimens, although not all of the and some mineral species that normally do not occur countries have an equal abundance of these resources. as gemstones. Uganda 16 Tanzania, in particular, is famous for its coloured Uganda, Rwanda and Burundi are not that well- 17 gemstones, and several newly discovered gems and endowed with gemstones, but they do have economic Gemological properties minerals have entered the market during the early part deposits that, from time to time, produce interesting 18 of the twenty-first century. Kenya, too, has its share of mineral specimens, and gemstones such as tourmaline A to Z of minerals and gemstones famous gems, such as tsavorite. and aquamarine. 130 A number of important geological criteria must be Acknowledgements met for gemstones to form, and certain geological Type minerals 131 environments are more conducive to producing 132 References and further reading Figure 1 A selection of natural and rough gems and minerals from East
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