- January – June 2024 Volume 28 Number 5
- Abstract
- Introduction
- Geological Setting
- Geographical Setting
- History of Mining and Production
- Materials and Methods
- Sample preparation
- Analysis
- Results and Discussion
- Gemmological properties
- Microscopy and inclusions
- Inner morphology
- Cut stones
- Chemical analysis
- Conclusions
- Acknowledgements
- References
THE AUSTRALIAN GEMMOLOGIST | Green Tourmaline from the Hofmeyr Mine, Eastern Zambia
Green Tourmaline from the Hofmeyr Mine, Eastern Zambia
Abstract
Zambia is recognised internationally as a major producer of copper, cobalt, and emerald from its Copperbelt province. In the east of Zambia, gem-bearing pegmatites contribute to the mineral wealth of the country producing deep blue aquamarine, morganite, tourmaline, and many other gem species. A few years ago, the Western Australian Division of the Gemmological Association of Australia received a donation of gem-quality green tourmalines from the pegmatites of the Hofmeyr mine, south of Nyimba, eastern Zambia.
The tourmalines were analysed using standard gemmological instruments, and their chemical composition was determined by scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS). The gemmological and spectroscopic properties of tourmaline f rom this locality are consistent with properties of tourmaline from other localities worldwide. The SEM-EDS analyses indicate that the tourmalines are the verdelite variety of elbaite.
Keywords: green tourmaline, pegmatite, Hofmeyr mine, eastern Zambia
Introduction
In late 2020, the Western Australian Division of the Gemmological Association of Australia (GAAWA) received a donation of gem-quality green tourmalines from Zambia by the Dr John Daniels Succession. This gift has initiated this study by the author, who had already been studying tourmalines (Payette and Klemm, 2011). The donation consisted of twenty-one rough samples that were added to the study collection of the GAAWA.
The green tourmalines from the Zambian locality occur mostly as hexagonal prisms, highly transparent and mostly free of inclusions. These attributes make the tourmalines well suited for faceting (Figure 1).
This paper will examine the history and geology of the deposit, and will document the composition and other properties of the green tourmalines.
Figure 1. Green tourmaline from Zambia which is highly transparent and mostly free of inclusions. Left: 44.60ct crystal; right: 9.62ct faceted tourmaline which were part of the study sample.
Geological Setting
The gem-bearing pegmatites of eastern Zambia developed during the latest stages of the Pan-African tectono-metamorphic event involving the Gondwana supercontinent (Porada, 1989). They are the result of tectonic and magmatic activity responsible for the formation of the Southern Irumide Belt and are related to the late stages of the intrusion of the Sinda Batholith at ~480 Ma (Johnson et al., 2006).
The main rock units in the belt are high-grade gneisses and granitic rocks of the 2680-2050 Ma Basement Complex, as well as metacarbonates, schists, metavolcanics, and quartzites of the 2050-1355 Ma Muva Supergroup (Kamona, 1994). The pegmatites intrude the paragneisses and schists of the Basement Complex and Muva Supergroup (Figure 2); they are commonly composed of quartz cores with outer zones of microcline feldspar. Pegmatites containing aquamarine and tourmaline are abundant and they are at the centre of small-scale mining in the region (Kamona, 1994).
Figure 2. Simplified geological sketch map of Zambia. Modified from Thieme and Johnson (1981).
Geographical Setting
Gem-quality tourmalines of almost all colours have been mined from many areas in eastern Zambia (Figure 3). Around Lundazi, at the northeast end of the Mozambique belt, multicolour (Thomas, 1982; Johnson et al., 1997) and yellow (Laurs et al., 2007) tourmaline have been described in the literature. The Chipata area, in the centre of the belt, is the source of yellow (Schmetzer and Bank, 1984) and red to brownish-red tourmaline (Koivula and Fryer, 1985). From the Nyimba area, in the south, red and green tourmalines have been sourced for many decades (Kamona,1994; Milisenda et al., 2000). More to the west, pegmatites in the area of Mkushi (Milisenda et al., 2000) produce pink, red and green specimens.
The green tourmalines described in this paper come from the Hofmeyr mine near the town of Nyimba, Eastern Zambia, very close to the Mozambique border (see Figure 3). The mine was named after the nearby Hofmeyr mission.
Figure 3. Some areas in eastern Zambia where tourmaline has been mined.
The pegmatites in this area formed as km-long lenticular, elongated bodies. They are very thick (20-50m) with inner core zones that are 3-6m wide and hydrothermally altered (Milisenda et al., 2000).
The tourmaline at Hofmeyr mine has been mined from both eluvial and primary (pegmatite) deposits (L. Klemm, personal communication, 24 November 2023). The large pegmatite vein has been worked by benching, and deep pits and shafts testify to intense mining activities (Figure 4). Countless small holes have been dug into the soil below the pegmatite body, mining for alluvial stones (Figures 5 and 6).
Figure 4. Recent working on the pegmatite. Photo courtesy of Leonhard Klemm.
Figure 5. Mining the soil for alluvial stones below the pegmatite. Photo courtesy of Leonhard Klemm.
Figure 6. One man, one pit. Photo courtesy of Leonhard Klemm.
History of Mining and Production
It is well known that the Hofmeyr mine has produced very fine tourmaline of all colours for decades. The colour of the tourmaline from this mine varies from pink to deep red; dark blue, blue, blue-grey and turquoise; green, olive-green and bottle-green (Lukusuzi River, 2017; Milisenda et al., 2000). The Hofmeyr pegmatite is also host to very fine aquamarine (Chadukwa, 2018).
However, very little information is available on the production of tourmaline from the mine. The first available report, by Jourdan (1990), mentions that tourmaline at Nyimba was mined by the small-scale producers Mindeco Small Mines. It appears that at one point in time, illegal miners mined black and green tourmaline, and pink quartz at the abandoned site that used to belong to the Mindeco Small Mines, (Chadukwa, 2018).
According to reports from U.S. Geological Survey (USGS) Minerals Yearbook – The Mineral Industry of Zambia (Coakley, 1999, 2000, 2003; Table 1), tourmaline was recovered regularly between 1995 and 2003 from the mine.
Table 1. Tourmaline production from various artisanal operations at Kalunga Wbeba Mine, Eastern Province, and Hofmeyr Mine near Nyimba (Coakley, 1999, 2000, 2003).
Even if the USGS reports do not give the specific production of green tourmaline from the Hofmeyr mine, it appears that tourmaline production from the area was quite important during the years 2001-2003.
Large production of green tourmaline from the Hofmeyr mine is mentioned in a report from the Zambian partner of Dr John Daniels:
“At that time nearly all the tourmaline product in Zambia came from a mine known locally as Hofmeiers’ [sic] Mine which is located nearby a small town called Nyimba in the eastern part of Zambia very close to the Mozambique border. Some very fine material came from there. The green tourmaline I sent over also came from there as did much of the pink/cranberry material.
“Production at this location has fallen to a mere trickle as opposed to the flow we had before.
“Interestingly, there was very little separation in the variety of colours that were found in single veins of material. A large variety often came from a single pit that was dug.” (Extracts from an email to Dr John Daniels from his Zambian contact, 2004).
It would seem that the mine had been dormant for a while and started production again in 2010 under a new licence held by a cooperative run by the local communities. In 2013, Lukusuzi River was founded as a private initiative to support local Zambian artisanal miners (Lukusuzi River, 2017).
It appears that the town of Nyimba economic backbone is mainly based on mining at the nearby Hofmeyr mine (Tracks4africa, 2019).
Materials and Methods
A lot of 390 well-formed crystals and crystals fragments (locally referred to as “gem balls”) of green tourmalines from the Hofmeyr mine was available for examination. From that lot, five specimens were selected for faceting. The five faceted stones (5.50ct to 9.62ct) and eighteen crystals (12.62ct to 56.00ct) were selected for further analysis; these were chosen to represent the variety of material on hand (Figure 7).
Figure 7. A sample of 140 rough tourmaline crystals examined in this study. They rest on an A4 white sheet of paper and are only partially lit. They appear almost black except when viewed with a bright light and represent the larger portion of the lot available (second crystal in top row is about 17mm x 19mm).
Sample preparation
Three flat surfaces had been polished on twelve of the samples as follows: one surface was polished in a direction parallel to the c-axis, the second surface was polished in a direction perpendicular to the c-axis, and the third in a random direction.
Two specimens were sliced to allow for the observation of internal growth features. From one specimen, two slices were cut in a direction parallel to the c-axis, and from the other, two slices were cut in a direction perpendicular to the c-axis.
Finally, three specimens were selected for the chemical analysis. One of the specimens had a flat face polished in a direction parallel to the c-axis, and two specimens had a flat face polished in a direction perpendicular to the c-axis. The specimens were then coated with carbon; carbon coating is valuable to make non-conductive samples amenable to scanning electron microscopy energy-dispersive spectroscopy analysis (SEM-EDS).
Flat surface preparation and polishing was conducted in the workshop of the Western Australian Lapidary and Rock Hunting Club Inc., and the carbon coating was performed at the Centre for Microscopy, Characterisation and Analysis (CMCA), University of Western Australia (UWA).
Analysis
The tourmalines were analysed using standard gemmological instruments. Specific gravity was calculated by hydrostatic weighing for the twenty-three specimens selected. Refractive indices were measured for the five faceted stones and for twelve of the available crystals on which three flat surfaces had been polished. Polarisation behaviour was noted using a standard polariscope, and the pleochroism was noted using a dichroscope.
Furthermore, the five faceted stones were examined with a Chelsea filter, a short-wave/long-wave ultraviolet lamp, and a desk-model spectroscope. Internal features were observed using a standard binocular gemmological microscope.
Chemical analyses were conducted by SEM-EDS on a JEOL JCM-7000 desktop Neoscope at the CMCA, UWA.
Desktop SEM-EDS system, even with a minimal configuration, can be used to accurately quantify most common major and minor elements in rock materials (Chen et al., 2023). With adequate standards and calibration, desktop SEM-EDS provides accurate analyses for tourmaline species classification (Beckett-Brown et al., 2023). However, for the purposes of this study, qualitative analyses only were performed.
Figure 8. The green tourmalines occur as hexagonal prisms, often with longitudinal parallel striations. Red laterite soil is still attached to the surface of many specimens.
A: stubby crystal, 17 x 19mm, 52.73ct
B: stubby crystal, 17 x 17mm, 44.61ct
C: thin prism, 9 x 14mm, 12.62ct
D: short nodule, 14 x 9.25mm, 15.25ct
Results and Discussion
Appearance of rough material
The rough samples (some of which are illustrated in Figure 8) occur mostly as prismatic crystals with a hexagonal cross-section. From the lot of 390 specimens examined about 45% are stubby and large (Figure 8, A and B); 10% occur as thin prisms (Figure 8, C); 40% are short with rounded top and bottom surfaces showing conchoidal fracture, creating an overall nodular shape (Figure 8, D); and about 5% are misshaped, probably because some crystals have broken apart during transport from the pegmatite to the alluvial ground. Many specimens show more or less worn surfaces and some have remnants of the red laterite soil from which they were mined (Figure 8, C and D).
Striations parallel to the c-axis are evident on the sides of most of the specimens, which is a characteristic feature of tourmaline. In tourmaline, the prism faces consist of a combination of hexagonal, trigonal and ditrigonal prisms. The striations are caused by oscillatory combination of the trigonal prism (1010) and the hexagonal prism (1120) (Rutley, 1970; Figure 9).
On some of the specimens, the striations were lined with colourless to white material with a vitreous lustre, which contained fine, long prisms of black minerals (Figure 10). These minerals are likely to be massive quartz associated with slender crystals of black tourmaline. The presence of quartz and tourmaline as a coating on the green tourmaline suggests that the evolution of the Hofmeyr pegmatite involved more than one stage of tourmaline crystallisation.
Figure 9. Vertical striations on prism faces parallel to the c-axis are caused by oscillatory growth between the trigonal prism and the hexagonal prism.
Figure 10. Left: on the surface of this specimen, a thin coating of whitish quartz is observed in one striation. Fine long prisms of black tourmaline are included in the quartz (the arrow points towards one of the inclusions of tourmaline). Height of specimen is 19.31mm. Right: a closer look at some of the fine long prisms of black tourmaline included in the quartz (top arrow points to the same tourmaline inclusion as the one in the left image). Height of field of view is about 3.5mm.
Gemmological properties
The gemmological properties of the green tourmalines from this locality are consistent with properties of tourmalines from other localities worldwide (Table 2).
Table 2. Gemmological properties of the green tourmalines.
Microscopy and inclusions
Internal features of the five faceted stones, the twelve rough specimens on which three flat surfaces had been polished, and the two sliced specimens were observed using a standard binocular gemmological microscope. All faceted stones show strong doubling of the back facet edges, a common characteristic of tourmaline, regardless of its source.
All specimens appear clean internally, with the exception of a few stress fractures observed in some of the twelve crystals; these fractures are associated with the lapidary work.
Unexpected discoveries can sometimes reward careful observation. In one slice, growth tubes aligned parallel to the c-axis were observed (Figure 11), some of which contain fluids and/or foreign material. These tubes do not originate from the base of the host tourmaline; instead, they are initiated by guest minerals present inside the host (Figure 12).
Figure 11. Left: a set of growth tubes (arrowed) aligned parallel to the c-axis was observed in only one of the sliced specimens (height of specimen is 15.4mm). Centre a closer look at the growth tubes (height of field of view is 7.5mm). Right: cluster of guest minerals that initiated the growth of the tubes (width of field of view is 0.8mm).
Figure 12. The growth tubes do not always start at the foot of the host tourmaline. They are often initiated by some obstacle such as guest minerals. (a) first growth step; (b) inclusions attached to the crystal; (c) final growth step.
It is clear from Figure 12 that after a first growth step (a), inclusions (cluster of minerals in this case, as illustrated in Figure 11) became attached to the growing crystal (b). In the next growth step (c), the presence of these inclusions prevented tourmaline material being deposited above the cluster of minerals, in the direction of growth. As a result, elongated tubes formed in the direction of the crystal growth. These tubes may end within the host mineral; they may also reach the surface at the end of the growth, as seen here.
Figure 13. Pronounced dichroism, green and brownish-orange, seen with the unaided eye. Specimen is approximately 17mm x 17mm x 19mm.
Inner morphology
At first glance, besides their pronounced dichroism visible with the unaided eye, the tourmalines looked quite uniform in colour. However, when tourmaline crystals are sliced and ground to an optimum thickness, coloured zones and other fine features become visible (Rustemeyer, 2003, 2021, 2022).
Two specimens in the lot were selected and cut as thin slices in order to study their internal growth features (Figure 14). From one specimen, two slices were cut in a direction parallel to the c-axis. In these slices a dark root was observed, overgrown by gem tourmaline that shows simple pyramidal colour zoning, of different shades of green (Figure 14, left).
In the slices cut perpendicular to the c-axis, different geometric patterns appear (Figure 14, right). These features are mostly triangular in shape. This type of colour zoning was called ‘delta structure’ by Rustemeyer (2003) and are within the face triangular pyramid. A green-coloured rim is seen in the sector zone of the prism layer.
These variations in colour and pleochroism are common in strongly coloured tourmalines and highlight internal changes in composition. The chemical composition of the delta features and the pyramidal sector zone of the green tourmaline from the Hofmeyr mine remain to be analysed.
Figure 14. Left: slice cut parallel to the c-axis shows simple pyramidal colour zoning in shades of green in transmitted light. The dark root is not a common feature of the green tourmaline from the locality; specimen is 18 x 14 x 2.5mm.
Right: slice cut perpendicular to the c-axis showing delta-shaped features in shades of orange inside the triangular growth zone, with green-coloured rim in the sector zone of the prism layer in transmitted light. The delta features (arrowed) in this specimen are mostly a truncated triangular shape; specimen is 15 x 10 x 1.5mm.
Figure 15. Left: this 9.62ct emerald cut verdelite is faceted with its table parallel to the c-axis. Right: the 7.03ct oval stone was cut with its table perpendicular to the c-axis. It is clear that the emerald cut stone nicely displays the green colour (a-axis colour) in the face-up position while its oval companion shows the dark and brownish-orange colour down the c-axis.
Figure 16. The pavilion facets return views from various directions through the stone; pavilion facets that reflect a view in the direction of the a-axis will show that colour (green in our specimens). Thus, even in a specimen cut with its table perpendicular to the c-axis, by observing the faceted stone from different directions, the attractive green a-axis colour can be seen through the crown facets, as observed here in the oval specimen (arrowed).
Cut stones
In tourmaline, the colour is darker looking down the c-axis (the ω-ray is usually the darker of the two colours). Thus, dark-coloured rough material is oriented with the table parallel to the c-axis to produce lighter coloured gems. In paler coloured tourmalines, the opposite orientation is used with the table perpendicular to the c-axis to maximize the colour of the cut stone.
The green tourmalines of this study display a pronounced pleochroism of two colours of different hues. In order to assess which orientation produces a gem with the most pleasing face-up colour, some rough was cut with the table perpendicular to the c-axis and other rough was cut with the table parallel to the c-axis. The rough material which was cut with its table parallel to the c-axis produced a gem which nicely displays the green colour in the face-up position (Figure 15).
Chemical analysis
The chemical composition was determined by qualitative analysis using energy-dispersive x-ray fluorescence. Major amounts of alumina and silica were detected; there were minor amounts of sodium, iron and magnesium; and traces of manganese and titanium were recorded. Based on these results, it is reasonable to determine that the green tourmalines are the verdelite variety of elbaite (colour-based varietal name; Henry et al., 2011). Elbaite, Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)3OH, is well known in shades of green and red. Interactions between
Fe2+ and Ti4+ (intervalence charge transfer) generate the green colour of the verdelite variety of elbaite (London, 2016; Pezzota and Laurs, 2011). The green tourmaline at Hofmeyr was in the past referred to as ‘emeralite’ (Kamona, 1994).
Conclusions
Hofmeyr pegmatite are worthy of attention as gem material. These attractive gems are notable for their high transparency, pronounced dichroism and relatively large sizes.
Also notable is the importance of the mining activity at Hofmeyr mine for the small town of Nyimba. The green tourmalines of the Hofmeyr mine represent only part of the mine’s mineral wealth. The town’s economic backbone is based on the mining of all gem material at the mine. The exploitation of the mine by local operators contributes to the development of a sustainable approach for the benefit of the local community.
The Zambian government’s efforts to promote the country’s mineral wealth are also noteworthy. In December 2019, the Zambian Embassy in Washington DC hosted the Jewel of Africa Gem and Jewelry Exhibition, with a view to increasing the country’s share of the multi-billion euro American and global jewellery trade (Mwanza, 2019). In addition to emeralds, Zambian gemstones exhibited included tourmalines and aquamarine from the Nyimba area.
Acknowledgements
The author is grateful to the Succession of Dr John Daniels for the donation of the green tourmalines used in this study. Dr Daniels was a geologist and, at one point, Supervisor for the Regional Geological Mapping Division of the Geological Survey of Western Australia. A supergene sulfide mineral, danielsite, was named in honour of Dr Daniels who collected the material in which the new mineral was found (near Coppin Pool, Ashburton Shire, Western Australia).
The author would like to thank the following people for their assistance: Leonhard Klemm for his photographs and update information about the deposit; Murray Thompson, Desert Fire Design, and Angus Sim, Australian Faceting Guild, for the fashioning of five specimens; members of the Western Australian Lapidary and Rock Hunting Club for providing useful comments for the surface preparation of samples; and David Vaughan for cutting the thin slices.
The author acknowledges the facilities, and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments. Dr Alexandra Sukorova is also thanked for providing quality training on the Neoscope to the author.
All images courtesy of the author unless otherwise stated.
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