Spectacular Sulfides from the Merelani Tanzanite Deposit, Lelatema Mountains, Manyara Region, Tanzania

Abstract

Outstanding specimens of several sulfide minerals from the Merelani tanzanite deposit have recently become available, including rare wurtzite in giant, deep red-brown, partially gemmy, well-formed crystals, lustrous black alabandite and form-rich pyrite with exceptional luster. Other sulfides identified include sphalerite, chalcopyrite, millerite, stannite, tennantite-tetrahedrite, sharp microcrystals of clausthalite and rare colusite-germanocolusite.

Full text

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The Mineralogical Record, volume 45, September–October, 2014
INTRODUCTION
When one thinks about the minerals of Merelani, Tanzania, the
wonderful gem species certainly come to mind. Tanzanite, the blue-
purple gem variety of zoisite, has so dominated the attention of
the miners and the gem and mineral markets since its discovery in
1967 (Keller 1992; Wilson et al., 2009), that, at least until relatively
recently, few other minerals from the mines have been preserved
for collection and study. Tsavorite, the mint-green gem variety of
grossular, and the intense green, gem-quality crystals of diopside
are the notable exceptions, and have been available to collectors for
Spectacular Suldes
from the
Merelani Tanzanite Deposit,
Manyara Region, Tanzania
Simon Harrison1
John A. Jaszczak2
Mike Keim3
Mike Rumsey4
Michael A. Wise5
Outstanding specimens of several sulfide minerals from the Merelani tanzanite
deposit have recently become available, including rare wurtzite in giant, deep
red-brown, partially gemmy, well-formed crystals, lustrous black alabandite
and form-rich pyrite with exceptional luster. Other sulfides identified include
sphalerite, chalcopyrite, millerite, stannite, tennantite-tetrahedrite, sharp
microcrystals of clausthalite and rare colusite-germanocolusite.
17–9 North Parade Buildings, Bath BA1 1NS, United Kingdom.
simon.harrison@porticodesigns.com
2Department of Physics and the A. E. Seaman Mineral Museum,
Michigan Technological University, 1400 Townsend Dr., Houghton,
Michigan 49931-1295. jaszczak@mtu.edu
3Marin Minerals, P.O. Box 5256, Larkspur, California, 94977.
4Mineral and Planetary Sciences Division, Earth Sciences Depart-
ment, The Natural History Museum, Cromwell Road London, SW7
5BD, United Kingdom. m.rumsey@nhm.ac.uk
5Department of Mineral Sciences, National Museum of Natural
History, Smithsonian Institution, Washington, DC 20560. wisem@si
.edu
several years. More recently, attractive clusters of yellow prehnite,
unusually large and gemmy crystals of axinite-(Mg), and vivid green
crystals of tremolite have also drawn attention. With the publication
of an entire issue of the Mineralogical Record devoted to Merelani
(Wilson et al., 2009), these and a surprising host of other minerals
from the mines have become better known. Close scrutiny of blue
prehnite specimens that became available in 2011 led to the iden-
tification of several previously overlooked yet common minerals
including rhodochrosite, fluorite and zircon (Simonoff and Wise,
2012a; 2012b). A series of articles in a single issue of Rocks and
Minerals (Americolo, 2013; Jaszczak and Trinchillo, 2013; Long
et al., 2013; Pohwat, 2013) has further highlighted the remarkable
occurrences of diopside, fluorapatite, graphite and several other
minerals. However, throughout all these publications, apart from a
few photographs of lustrous pyrite crystals, little attention has been
paid to the sulfides from Merelani . . . until now!
Armed with the aforementioned Merelani edition of the Mineral-
ogical Record as a companion, businessman and part-time mineral
dealer Simon Harrison travelled to the region in November 2010.
Seeing some of the mineral assemblages present, and suspecting
that perhaps pyrite wasn’t as rare at Merelani as was typically
thought, he started to question the miners. They confirmed that,
indeed, pyrite is not that rare, but because of both its high density

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and its perceived lack of value, the miners rarely brought it above
ground. Thus Harrison began the process of persuading the miners
to bring these crystals out of the mines, and then to actually wrap
them rather than just piling them into carrier bags. Still unconvinced
of the value of the specimens, the miners nicknamed him “muzungu
kichaa,” which in Swahili translates to “crazy traveler,” in allusion
to his enthusiasm about this traditionally worthless material. Over
the next three years and a further 12 trips, the miners increasingly
brought him highly complex and mirror-bright pyrite crystals, along
with a few other surprises.
In March 2011, a miner brought Harrison a bag of pyrite shards
that was poured out onto a table. Among the shards was a red/brown
partial crystal (1 3 2.5 3 3 cm) of a relatively soft mineral with
an apparently hexagonal form. Unable to sight-identify the piece,
Harrison recognized its potential to be interesting and brought it
back to the UK. On his return the piece was submitted to the Natural
History Museum in London for identification. There it was initially
analyzed by the museum’s Curator of Minerals, Mike Rumsey, using
powder diffraction and energy dispersive X-ray spectroscopy (EDS)
techniques. Rumsey determined that it was likely to be wurtzite,
the rare zinc sulfide polymorph, and that it had a high manganese
content. However, since it was such an extremely large, fine and
colorful example of this rare species, far better than anything
already registered in the 2501 years of mineralogy preserved in
the Natural History Museum’s collection, the identification was not
truly confirmed until crystallographers at the Museum were able to
perform single-crystal diffraction analyses verifying its hexagonal
symmetry and its identity as one of the wurtzite polytypes.
Returning with the wurtzite specimen to Merelani in June of that
year, Harrison ascertained that the wurtzite was generally found in
association with tanzanite, and he asked the miners to save specimens
of it. Over the following two years, during which time Harrison
usually obtained one or two pieces per visit, he assembled a group
of equally fine and large wurtzite specimens, all of which likely
ranked among the best ever seen for the species. In February 2012
a broken crystal fragment, which was the largest specimen seen on
the market so far, was acquired; it measured approximately 3 3
3 3 6 cm. Interestingly, two months later on another visit, Harrison
acquired an additional crystal fragment of wurtzite of similar size
that had been held for him. Returning to the UK, Harrison suspected
that the two fragments might be related, and when he put them
together at the Natural History Museum they fit perfectly, resulting
in an extremely large, elongated wurtzite crystal 10.2 cm tall that
is now in the collection at the Natural History Museum in London,
registered as BM 2012,220 (Fig. 37). After this immediate success,
some time was spent trying to tessellate small and tiny fragments
to this large piece to enhance the specimen further, as it was still
clearly incomplete. Unfortunately this process was unsuccessful, so
the true size of the original, undamaged crystal remains unknown.
In late 2012 mining encountered a zone that was unusually
rich in wurtzite. As a result, pieces appeared on the market at an
increased rate. In addition, several excellent octahedral crystals of
an unidentified, sub-metallic black to dark gray mineral were found
in association with both wurtzite and sphalerite. At around the
same time, mineral dealer Mike Keim (Marin Minerals) acquired
a lustrous black octahedral crystal associated with diopside and
graphite from Merelani (Figs. 4b, c), and sent it to John Jaszczak
for identification. Using Raman spectroscopy, scanning electron
microscopy with associated energy dispersive X-ray spectroscopy
(SEM/EDS), as well as visual examination, Jaszczak determined
that the crystal was a remarkable example of the manganese sulfide
alabandite. Coincidentally, Jaszczak described this exciting material
to Harrison, who then realized that his own unidentified black octa-
hedral crystals were probably also alabandite, a hunch which was
Figure 2. Idealized Merelani
alabandite crystal forms
(a) octahedron, (b) octahedron
modified by cube, (c) octahedron
modified by dodecahedron, and
(d) octahedron modified by
trapezohedron {112}.
Figure 1. Location map.

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Figure 3. Alabandite crystals: (a) Lustrous cuboctahedral crystals, 4.2 cm; Cal Graeber (ex Simon Harrison) specimen. (b)
Octahedral crystal, 2.6 cm, with a partial overgrowth of second-generation alabandite; Steve Smith (ex Marin Minerals)
collection. (c) Flattened cubic crystal, 2.4 cm; Simon Harrison specimen. (d) Cuboctahedral crystal, 3.7 cm, showing slightly
hoppered growth on the cube faces and striations on the octahedral faces; Simon Harrison specimen. (e) Cuboctahedral
crystal, 2.5 cm; Dan Carlson collection (ex Marin Minerals specimen). (f) Well-formed pair of intergrown cuboctahedral
crystals, 5.5 cm; Cal Graeber (ex Simon Harrison) specimen. (g) Octahedral crystal, 1.8 cm on edge, showing minor
dodecahedron faces; Simon Harrison specimen. (h) Cuboctahedral crystal, 2.5 cm, with graphite-impregnated surfaces;
John A. Jaszczak (ex Simon Harrison) collection. (i) An extremely large (7.2 cm) and unusually lustrous alabandite crystal
with tanzanite and graphite. Crystal Classics (ex Simon Harrison) specimen. John A. Jaszczak photos.

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Figure 4. Alabandite crystals: (a) 2-cm crystal with overall octahedral form but with an unusual surface texture;
Marin Minerals specimen. (b, c) Two views of a 2-cm octahedral crystal associated with prismatic green diopside
and white laumontite, along with minor fluorapatite, titanite, pyrite and graphite; Marin Minerals specimen. (d)
A large parallel growth of cuboctahedral crystals, 8.7 cm; Marin Minerals specimen. (e) Parallel growth of sharp
octahedral crystals, 4.5 cm, associated with diopside and tanzanite; Marin Minerals specimen. (f) Octahedral
alabandite crystal, 8 mm, on a pedion face of wurtzite, 2.5 cm; Simon Harrison specimen. (g) Large octahedral
crystal, 5.5 cm, showing heavily pitted surfaces due to natural etching; Marin Minerals specimen. (h) Botryoidal
rhodochrosite, 2.5 cm, pseudomorphically replacing an octahedral alabandite crystal, associated with sphalerite,
wurtzite, and microcrystals of clausthalite; A. E. Seaman Mineral Museum collection (ex Simon Harrison specimen).
(i) Deeply etched octahedral crystal, 2.5 cm; Simon Harrison specimen. (j) Lustrous octahedral crystal, 6.5 cm;
Crystal Classics (ex Simon Harrison) specimen. John A. Jaszczak photos.

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The Mineralogical Record, volume 45, September–October, 2014
subsequently confirmed. Miners had been encountering alabandite
crystals between late December 2012 and early January 2013, but
had not recognized their identity or significance, and as a result
specimens were not typically saved. With Harrison’s encouragement,
miners have now been preserving the alabandite crystals that, like
the wurtzite, rank alongside some of the world’s finest and largest
examples of the species.
MINERALS
The preservation of pyrite, wurtzite, alabandite and what seems
to be an increasing number of matrix specimens is making it
possible to document a more complete descriptive mineralogy of
Merelani’s unique deposits. The sulfide minerals described in this
section have been identified by visual examination, and typically
have been verified using one or more of the following techniques:
SEM/EDS (Scanning Electron Microscopy/Energy Dispersive
Spectroscopy), Raman spectroscopy and X-ray diffraction. Miner-
als followed by (?) have been identified only tentatively, and are
subjects of further research.
Alabandite MnS
Alabandite occurs as millimeter- to centimeter-sized crystals
and grains intimately associated with wurtzite and sphalerite,
as rounded masses, and as well-formed crystals—sometimes of
exceptional size and luster. Although a few matrix specimens are
known, most specimens that have been available on the market are
Figure 5. Alabandite crystals: (a) Sharp octahedral crystal with cube modifications, epitactic chalcopyrite, and tennantite-
tetrahedrite, 3.5 cm; A. E. Seaman Mineral Museum collection (ex Simon Harrison). (b) Octahedral crystal, 3.8 cm, with a
trapezohedron corner modification; Carl Francis collection (ex Simon Harrison specimen). (c) Spinel-law twin of alabandite
(upper right) on a wurtzite crystal, 1.5 cm; a small broken crystal of clausthalite sits at the left side of the alabandite twin; John
A. Jaszczak (ex Simon Harrison) specimen. (d, g) Extremely large parallel growth of alabandite crystals, 11 cm, associated with
chalcopyrite and minor laumontite, stilbite, tanzanite, wurtzite and quartz; private collection (ex Cal Graeber, ex Simon Harrison).
The cavity at the left was rich in crystals of graphite and silvery whiskers of an as-yet incompletely characterized sulfide. (e)
Octahedral crystal, 3.3 cm, with raised triangular hillocks; Marin Minerals specimen. (f) Thin golden chalcopyrite crystals, in
probable epitactic orientation, on a cuboctahedral alabandite crystal, 2.8 cm; Marin Minerals specimen. John A. Jaszczak photos.

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loose, partial crystals and crystal clusters without matrix. While
small crystals can be sharp and euhedral, larger crystals tend to be
lumpy to subhedral. Nevertheless, well-formed crystals to 11 cm
across have been found. Crystals are dominantly of octahedral form,
although typically distorted, and commonly have cube modifica-
tions. Less commonly, crystals can be more dominantly or even
purely cubic. Octahedral crystals with modifying dodecahedral and
trapezohedral faces are also known. Some octahedral crystals are
twinned on (111). Later-stage alabandite overgrowths on alabandite
give some crystals a rough texture, and raised triangular hillocks
have also been observed on some octahedral faces. Many crystals
show various degrees of natural etching on their surfaces, often
to the point of leaving the crystals deeply pitted or even skeletal.
The surfaces of altered alabandite are sometimes associated with
botryoidal rhodochrosite, and one particularly unusual example
(Fig. 4h) shows what appears to be a complete replacement of the
alabandite by rhodochrosite. In addition to wurtzite and sphalerite,
alabandite can be associated with almost all of the well-known
Merelani minerals, including graphite, diopside, fluorapatite,
calcite, quartz, laumontite, tremolite and pyrite; rarer associations
include gemmy tanzanite, rhodochrosite, mesolite, chabazite-Ca,
millerite, chalcopyrite, clausthalite and even whiskers of an as-yet
incompletely described sulfide (see below).
Semi-quantitative SEM/EDS analyses of Merelani alabandite
indicate only Mn and S, with no detectable Zn. Alabandite has a
pronounced green streak, and gives off a strong rotten-egg smell
(H2S) when scratched or broken.
Chalcopyrite CuFeS2
Chalcopyrite occurs as golden, perhaps epitactic, thin coatings
on alabandite, and also as tiny disphenoidal crystals on wurtzite,
sphalerite, and alabandite.
Figure 8. Cuboctahedral clausthalite, showing
twinning on (111), from a vug in transparent
calcite. Associated minerals also include
diopside, titanite, graphite, microcrystals of
wurtzite, chabazite-Ca, and silvery whiskers of
an as-yet unidentified sulfide. John A. Jaszczak
(ex Zdene=k Prokopec) collection. SEM image by
John A. Jaszczak.
Figure 6. Octahedral clausthalite crystals to
0.7 mm in a vug in wurtzite (reverse side of the
specimen of rhodochrosite pseudomorphically
replacing alabandite, shown in Fig. 4h). John
A. Jaszczak photomicrograph.
Figure 7. Octahedral clausthalite crystals to
0.7 mm in a vug in wurtzite. SEM Image by
John A. Jaszczak.
Clausthalite PbSe
Highly lustrous, silvery octahedral and cuboctahedral micro-
crystals of the rare lead selenide clausthalite occur associated with
wurtzite, sphalerite, alabandite and rhodochrosite pseudomorphs
after alabandite. Twinning on {111} has also been observed.
Clausthalite has thus far been identified on only a few specimens;
the largest crystals found to date are smaller than 0.75 mm.
Colusite-Germanocolusite
Cu26V2(As,Sn,Sb)6S32—Cu26V2(Ge,As)6S32
Several small (up to approximately 2 mm), silvery tetrahedral
crystals and a single, larger broken mass about 8 mm across were
determined on the basis of semi-quantitative SEM/EDS analysis
and Raman spectra to be consistent with colusite-group minerals,
with the amounts of Ge, As, Sn and Sb being somewhat variable
among the crystals tested. The crystals show a metallic luster on
their broken surfaces and were found partially embedded in, and
perhaps even epitactically oriented on, wurtzite crystals. Associated
species include laumontite, calcite, sphalerite and a coating of minute
pale green acicular crystals which have been tentatively identified
as a vanadium-bearing member of the pumpellyite group, but are
too small for identification with complete confidence.

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Marcasite FeS2
Raman spectroscopy and SEM/EDS confirms that marcasite
occurs as small (1 mm) golden hemispheres on the surfaces of a
few small wurtzite crystal fragments. These resemble pyrite micro-
spheres but the crystallites composing the marcasite aggregates are
smaller and form more symmetrical spheres with smoother surfaces.
Millerite NiS
Millerite is found rarely as radiating sprays of golden acicular
crystals up to 8 mm long on the surfaces of hummocky or naturally
broken alabandite. Minor associated species include crystals of
diopside, calcite, and graphite.
Molybdenite MoS2
Molybdenite has been confirmed on two specimens, as foliated
crystallites exposed from enclosing pyrite in one, and in massive
pyrrhotite in the other. The molybdenite has a slightly bluish luster
compared to the much more common graphite. Associated species
also include fluorapatite, graphite, quartz, blocky anorthite and
phlogopite (containing V and Ti).
Pyrite FeS2
Pyrite crystals with a mirror-bright luster occur in sizes ranging
from less than 1 mm to well over 20 cm across. At their best they
Figures 9-10. Germanocolusite crystals (tentative identification) on wurtzite. The
crystal at the left is coated by a crust of possible poppiite. The crystal on the right
appears to be epitactically oriented on wurtzite. SEM images by John A. Jaszczak.
Figure 11. Marcasite in small spherules to
0.2 mm on irridescent wurtzite, partly coated
by a white magnesium silicate mineral. John
A. Jaszczak collection and photomicrograph.
Figure 12. Millerite needles to 2.5 mm long
on alabandite. John A. Jaszczak (ex Simon
Harrison) collection and photograph.
are some of the finest pyrites in the world. Crystals are commonly
bounded by pyritohedron, cube, octahedron and diploid faces with
varying degrees of relative development. Some crystals have the
shape of an icosahedron, which results from the equal development
of the pyritohedron and octahedron. Cube faces can be smooth or
slightly striated. A small number of crystals show a distorted habit,
being rather flattened. Such crystals may be twinned, but further
study is necessary.
Although these beautiful crystals are often damaged as a result
of their brittleness and the explosive nature of mining in the area,
close examination of what appears to be damage to crystals shows
some of the affected areas to actually be growth contacts with other
crystals, or natural breaks that are now partly covered by other
minerals. Some pyrite crystals have surfaces with small pock marks
which apparently are due to the crystal’s growth up against flaky
graphite crystals, some of which may still be partly embedded on
the pyrite surfaces.
Some pyrite crystals have highly rounded surfaces and a frosty
metallic luster. These appear to have been partly resorbed at some
time after growth. Some crystals are partly to completely altered
and replaced by a brownish material that appears to be a mixture
containing both iron oxide and silica. Crystals are rarely recovered

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Figure 13. Anorthite crystal, 2.4 cm tall,
with titanium and vanadium-bearing
phlogopite, fluorapatite, pyrrhotite,
pyrite, and molybdenite (not visible in
the photo). Simon Harrison specimen;
John A. Jaszczak photo.
Figure 15. Pyrite crystal cluster,
5.4 cm tall, showing pyritohedron,
octahedron, and striated cube faces.
A. E. Seaman Mineral Museum
collection (ex Simon Harrison).
John A. Jaszczak photo.
Figure 16. Pyrite crystal, 5 cm, with exceptionally
lustrous faces. Cal Graeber (ex Simon Harrison)
specimen. John A. Jaszczak photo.
Figure 14. Pyrite crystals to
1 cm on drusy graphite, 4.5 cm.
John A. Jaszczak collection (ex
Simon Harrison) and photo.

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The Mineralogical Record, volume 45, September–October, 2014
Figure 21. Twinned (?) pyrite crystal, 3.5 cm.
John A. Jaszczak collection (ex Cal Graeber
and Simon Harrison) and photograph.
Figure 17. Pyrite, 4 cm, showing
pyritohedron, octahedron, and striated
cube faces, on a matrix of calcite, graphite,
diopside, and tiny whiskers of white
tremolite. John A. Jaszczak specimen
(ex Simon Harrison) and photo.
Figure 18. Pyrite crystal, 10 cm, associated with
minor graphite. Cal Graeber (ex Simon Harrison)
specimen; John A. Jaszczak photo.
Figure 19. Pyrite, 6.5 cm with lustrous cube,
pyritohedron, and octahedron faces. Marin
Minerals specimen; John A. Jaszczak photo.
Figure 20. Large pyrite crystal, 9 cm, that is
slightly elongated but notably flattened, and
exhibiting dominant pyritohedron and cube
faces. Cal Graeber (ex Simon Harrison)
specimen; John A. Jaszczak photo.

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Figure 22. Pyrite, 1.6 cm, on graphite-
included diopside. Marin Minerals
specimen; John A. Jaszczak photograph.
Figure 23. Rounded pyrite crystal, 4 cm, with both
lustrous and frosty surfaces. John A. Jaszczak
collection (ex Simon Harrison) and photo.
Figure 24. Pyrite crystal, 1.2 cm, showing equal
development of the pyritohedron and octahedron giving the
appearance of an icosahedron. An edge between adjacent
pyritohedron faces is modified by a cube face. Associated
minerals include calcite, laumontite and diopside. John A.
Jaszczak (ex Zdene=k Prokopec) specimen and photo.
Figure 26. Pyrite crystal, 1 cm, replaced by iron
oxide. Nearly equal development of the pyritohedron
and octahedron faces give the crystal an overall
icosahedral shape. John A. Jaszczak collection (ex
Marin Minerals) and photo.
Figure 25. Idealized crystal drawings of pyrite (left) showing equal
development of octahedron o{111} and pyritohedron e{120} resulting in
the shape of an (irregular) icosahedron. (right) A similar crystal with
cube faces modifying the edges between adjacent pyritohedron faces.

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The Mineralogical Record, volume 45, September–October, 2014
on matrix, but those that are may be associated with platy white
calcite crystals, gemmy green diopside crystals, or purple to tan-
colored zoisite (tanzanite) crystals, as well as microcrystals of other
species including graphite and laumontite, and whiskers of tremolite.
Pyrite is also a common accessory mineral with alabandite and
grossular (variety tsavorite).
Sub-millimeter-size octahedral crystals and spheroidal aggregates
of pyrite crystals also occur on wurtzite crystals. Jaszczak and
Trinchillo (2013) reported late-stage pyrite crystals (10 to 20 μm)
on graphite crystals from the December 2007 find of exceptional
diopside and graphite crystals from the Karo mine.
Pyrrhotite Fe1–xS
Pyrrhotite does not seem to occur as well-formed crystals at
Merelani, but is described here for the sake of completeness.
Most specimens are massive lumps to several centimeters across
with bronze color, associated with distinctly more golden-metallic
pyrite, quartz, zoisite and, rarely, molybdenite. One specimen
studied has bronze-colored, massive pyrrhotite in association with
massive golden pyrite, botryoidal graphite and prismatic crystals of
meionite to 2 cm long that fluoresce brilliant yellow in longwave
and shortwave ultraviolet light. SEM/EDS analysis of a pyrrhotite
grain from this specimen revealed small grains of an unidentified
nickel-iron sulfide (possibly pentlandite?). SEM/EDS analysis of
a polished section of another phyrrhotite sample revealed grains
of a nickel-iron-arsenic sulfide (possibly ferroan gersdorffite?)
associated with graphite.
Giuliani et al. (2008) have described samples of pyrrhotite from
Block B that contain vanadium, nickel and, to a lesser extent,
chromium in variable concentrations. This pyrrhotite occurs in
association with pyrite, graphite, quartz, tanzanite, vanadium-rich
phlogopite, the rare vanadium oxide karelianite and, rarely, inclu-
sions of chalcopyrite.
Sphalerite ZnS
Sphalerite commonly occurs in intimate association with wurtzite,
either intergrown with it or as epitactic growths on the surfaces
of wurtzite crystals. In the latter case the sphalerite is typically
darker colored than the wurtzite and shows more complex striation
patterns. It also occurs as dark brown-red, sometimes translucent,
Figure 29. Sphalerite crystal, 2 cm, showing
complex striation patterns. John A. Jaszczak (ex
Simon Harrison) specimen; John A. Jaszczak
photo.
Figure 27. Thin section of
intergrown sphalerite and
wurtzite, 10 3 8 mm. Right:
Image taken in transmitted
linearly polarized light without
a second polarizer acting as
an analyzer. Left: Image taken
with the sample between
crossed polarizers. Note that
the triangular region that is
extinct (dark) along with the
dark surrounding epoxy. The
extinct sectors are isotropic and
are therefore sphalerite. The
bright section is anisotropic and
is wurtzite. John A. Jaszczak
specimen and photos.
Figure 28. Multiply-twinned sphalerite, 2 cm, backlit to
highlight its translucency and color. John A. Jaszczak
collection (ex Simon Harrison specimen) and photo.

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sometimes lustrous, crystals to several centimeters across that are
invariably twinned. Complex twinning can make the identification
of sphalerite crystal forms a challenge. The crystals typically show
a tetrahedral morphology when epitactically oriented on wurtzite.
Stannite (?) Cu2FeSnS4
Microcrystals (up to about 10 μm across) of a copper-iron-tin
sulfide occurring epitactically on the pyramid faces of wurtzite
were analysed using SEM/EDS and are tentatively identified as
stannite. Although stannite has thus far been identified on only a
single wurtzite crystal, it is plausible that examination of additional
wurtzite crystals may reveal more. Associated minerals include
graphite and the acicular green microcrystals mentioned above.
Tennantite-Tetrahedrite
Cu6[Cu4(Fe,Zn)2]As4S13—Cu6[Cu4(Fe,Zn)2]Sb4S13
Tetrahedral microcrystals have been identified on the back of an
octahedral alabandite crystal, in association with chalcopyrite and
graphite. SEM/EDS indicates that the crystals contain elements Cu,
As, Sb, S, Zn and Mn. Raman spectra are in good agreement with
the tennantite-tetrahedrite series, giving a dominant peak between
the peaks representative of the series’ two end-members.
Wurtzite ZnS
Crystals of wurtzite were recognized as early as 1999 by Reyno
Scheepers (personal communication, 2013), but appear to have
remained otherwise unknown until recently. Some wurtzite speci-
mens that reached the market were mislabeled as sphalerite. The
best wurtzite crystals from Merelani are lustrous, unusually large
(up to 10 cm in the longest direction), hexagonal pyramids with
a gemmy transparency and red-orange color. Unfortunately, many
Figure 33. SEM image of tennantite-tetrahedrite crystals in
parallel growth on the stepped alabandite specimen shown
in Figure 5a. A. E. Seaman Mineral Museum collection (ex
Simon Harrison specimen); John A. Jaszczak SEM.
Figures 30-31.
Front and
back views
of a lustrous,
twinned black
sphalerite
crystal with red
internal flashes,
4.8 cm, showing
terraced
surfaces on
the back side.
Simon Harrison
specimen; John
A. Jaszczak
photo.
Figure 32. Stannite (tentative identification) crystals epitactic
on wurtzite. Simon Harrison specimen; John A. Jaszczak SEM.

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The Mineralogical Record, volume 45, September–October, 2014
wurtzite specimens which have been poorly handled are broken
and/or badly abraded on their surfaces. As in the case of alabandite,
matrix specimens are extremely rare; the best one known to us
has a tabular 1 3 2.3-cm crystal resting on a cluster of low-angle
rhombohedral calcite crystals that have white frosty surfaces but
are colorless and transparent in their interiors.
Wurtzite’s pyramid faces are typically striated perpendicular to
the c-axis, sometimes so finely that they show subtle interference
colors. More commonly, crystals are deeply striated or grooved.
Transparent crystals commonly show interesting polygonal internal
voids. Pedion faces of wurtzite commonly show growth steps and
hexagonal or trigonal growth hillocks. Single-crystal X-ray dif-
fraction studies indicate that the wurtzite is the 4H polytype (Allan
Pring, personal communication, 2013). The RRUFF project (Downs,
Figure 36. Wurtzite crystal
(back lit), 5.5 cm. A. E.
Seaman Mineral Museum
collection (ex Cal Graeber,
ex Simon Harrison); John
A. Jaszczak photo.
Figure 34. Wurtzite crystal,
4.3 cm, showing steps and
hexagonal hillocks. Crystal
Classics (ex Simon Harrison)
specimen and photo.
Figure 35. Wurtzite with
epitaxic darker colored
sphalerite, 3.5 cm. Crystal
Classics (ex Simon Harrison)
specimen and photo.

The Mineralogical Record, volume 45, September–October, 2014
14
Figure 39. A rare example of a wurtzite crystal, 2.3 cm, on
a matrix of rhombohedral calcite crystals with pale green
diopside, graphite flakes, naturally broken chips of wurtzite,
a cluster of pale green prehnite, twinned pseudocubic
chabazite-Ca crystals, and silvery cylindrical whiskers of
an as-yet incompletely characterized molybdenum-lead
sulfide. Simon Harrison specimen; John A. Jaszczak photo.
Figure 37. Wurtzite crystal (10.2 cm;
repaired). Natural History Museum,
London collection (BM 2012,220) (ex
Simon Harrison specimen); Harry Taylor
photo. Copyright Natural History Museum
London.
Figure 38. Wurtzite with calcite,
4.3 cm. Crystal Classics (ex Simon
Harrison) specimen and photo.
2006) has also determined the wurtzite to be of the 4H polytype
(Marcus Origlieri, personal communication, 2013; http://rruff
.info/wurtzite/display=default/R130069 last accessed 3/25/2014).
Wurtzite’s finer-scale striations perpendicular to the c-axis on its
pyramid faces and lack of parallel striations on the pedion faces
help to distinguish it from sphalerite.
The pedion faces of wurtzite commonly have epitactic over-
growths of sphalerite. Wurtzite’s pyramid faces have epitactic
sphalerite overgrowths as well, but less commonly than on the
pedion. Epitactic sphalerite typically shows a dominant tetrahedral
morphology with a [111] axis parallel to the wurtzite [001] (c-axis).
Figure 40. Wurtzite, 6 cm, with diopside and
graphite. Private collection (ex Cal Graeber and ex
Simon Harrison specimen); Joe Budd photo.

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The Mineralogical Record, volume 45, September–October, 2014
Figure 41. Wurtzite gemstones (1.95 (top left), 1.15 (top right),
and 0.67 ct. (center bottom)) faceted by Joe Law. Cal Graeber
(ex Simon Harrison) specimens; John A. Jaszczak photo.
Figure 42. Lustrous, metallic whiskers of an as-yet
incompletely characterized molybdenum-lead sulfide
on the alabandite specimen shown in Fig. 5d. John A.
Jaszczak photo; field of view ca. 1.5 cm.
Figure 44. Group of molybdenum-lead-sulfide
whiskers (field of view 1.5 mm) impaling transparent
crystals of stilbite. Also associated are crystals of
tabular graphite. Note the undulating diameter of
the upper-most whisker. John A. Jaszczak photo.
Figure 43. Broken end of a molybdenum-
lead-sulfide whisker showing the concentric
arrangement of lamellae around the whisker
axis. John A. Jaszczak SEM.
Figure 45. Tapered tip of a molybdenum-lead-
sulfide whisker. John A. Jaszczak SEM.
The sphalerite is typically darker and more opaque than the wurtzite
host crystals, the latter being distinctly more orange-red in most
cases. The isotropic optical properties of the sphalerite, in contrast
to the anisotropic optical properties of wurtzite, are evident in thin
section. Wurtzite microcrystals with a prismatic habit have also
been observed.
Common associations of wurtzite include diopside, graphite,
calcite, laumontite, stilbite, tremolite and zoisite. Drusy golden
chalcopyrite occasionally can be seen as epitactic coatings on
sphalerite tetrahedra and as triangular growths on wurtzite pedion
faces. Less commonly, marcasite, alabandite, fluorapatite, prehnite
and the acicular green mineral mentioned above may be associated.
Wurtzite crystals have been faceted as rather exotic gemstones
up to 76 ct. (Hyršl, 2013).

The Mineralogical Record, volume 45, September–October, 2014
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Figure 47. Fabricated specimen, 3.5 cm, of
pyrite, diopside, laumontite, graphite and
minor tanzanite, obtained from Merelani
in 2013. John A. Jaszczak photo.
Figure 46. Phase diagram for the MnS-ZnS system at
50 atomic percent S showing phases for alabandite (MnS rt),
wurtzite (ZnS ht), sphalerite (ZnS rt), and a liquid phase (L).
The colored regions are single-phase regions with continuous
variation in the relative Zn and Mn content, whereas the
white regions correspond to regions of two-phase co-existence.
Below 340° C, for example, equilibrium systems with between
about 1 and 47 atomic percent Zn must be composed of
mostly alabandite (with about 1 percent Zn) and sphalerite
(with about 3 percent Mn). Wurtzite can coexist with either
alabandite or sphalerite at temperatures between 340° and
1203° C depending on the Zn-to-Mn ratio of the system. Phase
diagram taken from Knitter (1999, 2006).
Incompletely characterized sulfide whiskers
Cylindrical whiskers with a silvery metallic luster of an as-yet
incompletely characterized phase were also found associated with a
large variety of minerals, including calcite, chabazite-Ca, diopside,
graphite, prehnite, quartz, stilbite, tanzanite and wurtzite. On one
particularly large alabandite specimen (Fig. 5d, e; Fig. 42), many
such whiskers occur loosely packed with lustrous millimeter-sized
graphite crystals in a centimeter-wide crevice, as well as lying in
random orientations on part of the alabandite’s surface. The lengths
of the whiskers typically range from tens of microns to a few mil-
limeters, but rarely can reach to over 1 cm, while their diameter
ranges from 2 to 100 μm. Broken whiskers show a lamellar structure
concentric around the cylinder axis. It is likely that these are the same
phase as the whiskers first identified as molybdenite by Simonoff
and Wise (2012a; 2012b); however, additional studies suggest that
this might not be the whole story, as SEM/EDS and electron micro-
probe analyses show the whiskers contain major elements Mo, Pb
and S, with additional minor elements Se, Sb and V. Moreover, a
satisfactory match of Raman spectra from the whiskers could not
be made with any spectrum in the RRUFF Raman library. Further
research is currently under way to fully characterize this material.
DISCUSSION
What is so special about the Merelani district that common
minerals such as zoisite, diopside, and grossular, among others,
form in exceptional gem-quality crystals? Good answers to this
question will likely be available only once the specific conditions

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The Mineralogical Record, volume 45, September–October, 2014
that have produced the other exceptional colored gemstones in the
regions associated with the Gondwana supercontinent, including
northeastern Africa, Mozambique, southern India and Sri Lanka,
have been determined. This is a complex question that is still the
subject of extensive study (Malisa and Muhongo 1990; Feneyrol
et al. 2013; Giuliani 2013; Saul 2014); the answer to it is sure to
involve multiple variables, including temperature, pressure, fluid
compositions and natural fluxes, alongside complex geological
histories. It is well beyond the scope of this paper to address this
larger question, but it seems reasonable to speculate that those
same conditions that favored the formation of colored gemstones
in the Merelani deposits also made possible the formation of such
remarkable crystals of pyrite, alabandite and wurtzite.
It would be noteworthy if the fine crystals of either wurtzite or
alabandite alone occurred at the Merelani gem deposits, but the
occurrence of both species in exceptional crystals seems extraor-
dinary. However, their occurrence in the same deposit, indeed even
in the same specimens, might be expected where mineral-forming
fluids rich in zinc, manganese and sulfur are present. As noted above,
even before the existence of alabandite was recognized, chemical
analysis of the wurtzite revealed a significant presence of manganese
substituting for zinc. Indeed, the MnS-ZnS phase diagram suggests
that the substitution of Mn for Zn would promote the chances of
forming wurtzite over sphalerite at lower temperatures than would
otherwise be possible in the absence of Mn. The phase diagram also
shows that wurtzite is stable over a wide compositional range of Mn
substituting for Zn. Detailed microprobe analyses yield an average
composition for Merelani wurtzite of Zn0.82Mn0.16S, as compared
to the sphalerite average composition of Zn0.84Mn0.14Cu0.01S (Allan
Pring, personal communication, 2013). These analyses show that
sphalerite has somewhat less Mn than does wurtzite, but more than
would be suggested by the phase diagram, which indicates a maxi-
mum of only 4 atomic percent Mn (0.08 Mn atoms per S atom). The
phase diagram also suggests that alabandite is less receptive to Zn
substitutions for Mn in its structure, except at higher temperatures.
Interestingly, up to the detection limits of our SEM/EDS analysis,
no Zn has so far been detected as substituting for Mn in alabandite.
While it is suggestive, we note that the phase diagram shown is
only for the pure MnS-ZnS system (at 50% S), and does not take
into account other important factors that would be of importance
in the geological system. Such factors as hydrothermal fluids and
possible mineral fluxes are likely to extend the phase boundaries to
lower temperatures (Allan Pring, personal communication, 2013).
CONCLUSIONS
No longer are tanzanite, tsavorite and diopside the only minerals
getting attention at Merelani. Alabandite, wurtzite and pyrite will
surely now be among those minerals that also come to a collector’s
mind at the mention of this famous locality. After so many years of
mining there, one can only imagine the treasures that were unearthed
in the past, only to be damaged and thrown onto the rock piles. The
increasing recognition of Merelani’s rich mineralogical diversity,
as well as the high value of so many well-crystallized species to
collectors, is good news for the local miners, who traditionally only
focused on finding a single mineral. Unfortunately, with increasing
appreciation of the kinds of specimens that collectors are looking
for, especially crystals on matrix, some faked specimens from the
Merelani area are now seen on the market. We have found crystals
of tanzanite, pyrite, and diopside glued with cyanoacrylate to a
matrix composed of various combinations of brown zoisite, graphite,
and laumontite. Nevertheless, now that the miners are recognizing
and preserving other species for collectors it seems only a matter
of time before more wonderful surprises come to light from one
of the world’s greatest mineral localities.
ACKNOWLEDGMENTS
We thank Mark Welch of the Natural History Museum in London
for performing the single crystal diffraction analysis on the first
wurtzite specimen we studied. We also are grateful to Allan Pring
for generously sharing X-ray and microprobe analyses of wurtzite
and sphalerite. We thank Cal Graeber for making several specimens
available for photography, and Ian Bruce (Crystal Classics) and
Jaroslav Hyršl for supplying photographs. Thanks also to Zdene¨k
Prokopec for donating the specimens for study. Crystal drawings
were made using the crystal drawing program Euhedra©2012 by
W. H. Tislar. Staffmembers of the A. E. Seaman Mineral Museum
are grateful to Bill Shelton for first bringing Merelani’s wurtzite to
their attention. Thanks to Jessica and Robert Simonoff for sharing
information about the sulfide whiskers and their associations. The
work at Michigan Technological University was supported in part
by the Edith Dunn and E. W. Heinrich Mineralogical Research
Trust, for whose support we are most grateful.
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