 |
- by Bruce Cairncross |
INTRODUCTION
For
some time now, outstanding crystals of aegirine have been collected at “Zomba”,
Malawi (see Fig. 1). These specimens were reported in the early 1990s (Wilson
1991; Petersen and Grossman 1991; Robertson and King 1991). This aegirine was
associated with aesthetic smoky quartz, microcline, and zircon. However, with
further collecting in more recent years, several other rare species such as
parisite, epididymite, fergusonite, and eudyalite, amongst others, have also
been discovered. Although the occurrence of these minerals has previously been
published in some detail (Petersen and Grossman 1994), much new material has
since been collected and made available to the collecting market. Furthermore,
the exact locality of the pegmatites was never clearly described or recorded
until recently, and most, if not all, specimens have merely been labeled
“Zomba, Malawi.” Zomba is a village
located south of Zomba Mountain. A few South African collectors have made trips
to the locality to obtain specimens at the source and to determine the exact
locality of the pegmatites (Cairncross et al. 1999; Cairncross 2000).
HISTORY AND GEOLOGY
The
geology of the region that contains the aegirine-bearing pegmatites has been
fairly well documented. During the 1950s and early 1960s, the Geological Survey
of Nyasaland (now named Malawi) embarked on extensive mapping projects in the
Zomba area and several publications were written based on this work. The best
reference for this period is Bloomfield (1965) who gives a detailed
bibliography of all previous work up until 1965. Bloomfield’s publication deals
specifically with the Zomba region and he mentions the presence of pegmatites
containing large aegirine crystals. Two more readily accessible articles from
this era are those of Stillman and Cox (1960) and Vail and Monkman (1960).
During the mid-1980s to early 1990s more research took place, and this involved
detailed petrographical and geochemical studies on the syenites, alkaline, and
peralkaline rocks (Platt and Woolley 1986; Platt et al. 1987; Woolley
and Platt 1986). More recently, further detailed mineralogical research has
been undertaken (Woolley 1991: Woolley and Jones 1992; Jonsson and Högdahl
1999). There have also been overviews of the minerals and collectable minerals
found in the Zomba area (Petersen and Grossman 1994; Cairncross et al.
1999; Cairncross 2000).
The area of mineralogical interest falls into a
part of the Cretaceous Chilwa Alkaline Province which, as its name suggests,
consists predominantly of alkaline intrusives and extrusives associated with
the East African rift (Eby et al. 1995). The Chilwa province consists of
several granite, syenite and nepheline-syenite plutons that are associated with
extrusive carbonatites and agglomerates. The Zomba-Malosa Mountains are
pear-shaped and are geologically subdivided into two zones. The southern Zomba
area consists mainly of syenite surrounded by a periphery of alkaline granite.
The northerly-located Malosa section is composed of a mixture of syenite and
granite. North and northeast of the Zomba-Malosa Mountains are four other
geological complexes named, from west to east, Chinduzi, Mongolowe, Chaone and
Chikala (see Fig. 2). These are essentially nepheline syenite complexes. From
the collector’s standpoint, two important types of pegmatites are present in
the area. In the Chinduzi-Chikala range of mountains, nepheline-syenite
pegmatites occur. These contain large well developed aegirine crystals “….. up
to 5 by 2.5 cm in size…..” as reported by Bloomfield (1965). In contrast,
granitic pegmatites are found in the Zomba Mountain and Malosa Mountain. The
northwestern fault scarp in particular has an abundance of these pegmatites and
it is from these deposits that the best specimens have been collected. To the
south, the Zomba section of the complex has far fewer pegmatites.
MOUNT MALOSA
In order to get to
the pegmatites, one has to hike about 20 km from the nearest road. This
involves descending a steep slope down Zomba Mountain into a deep valley, and
then hiking up the opposite side onto the Malosa plateau. The exact site where
the crystals are excavated is dangerous. It is located on the northwestern side
of the mountain where a series of major faults cut through the mountain. These
have produced scarps with vertical cliff faces up to 800 meters high. The
pegmatites that contain the minerals strike across the mountain and down the
vertical cliffs so that some of these have to be reached by climbing down the
near-vertical rock face. Because the quartz-bearing pegmatites tend to be more
resistant to weathering, they form narrow ridges or “noses” along which one has
to balance and walk.
The scarp involves a
descent down a 60º - 70º slope and the pegmatite veins average approximately 1
– 1.5 meters wide. The local miners have been digging and collecting here for
several years and have moved many tons of overburden soil and waste rock. They
do this by using the most primitive equipment such as machetes, iron rods and
hoes. Some dig only with their bare hands. There are no mines operating in the
region; all the specimens are dug from outcrop. The task of extracting minerals
therefore requires a lot of physical digging and clearing away of unwanted soil
and overburden. Red, oxidized soil overburden tumbles down the steep slope and
tails off into the valley below. These red scars mark sites where excavation
has already reached an advanced stage. Climbing equipment such as ropes and
pitons are unheard of, so the fact that many fine crystalline specimens are
collected under such dangerous conditions is quite remarkable. The aegirine-bearing
pegmatite host-rock consists mainly of quartz and potassium feldspar
(microcline). The enclosing country rock is composed of very weathered syenite.
Most of the weathered material is very soft and easy to move away. The better
quality and larger specimens are reported to come from further down the 600 –
800 meter cliff faces.
The local "miners" who dig out the specimens often set up an informal
market in the bush, miles away from any town or village. Here, minerals can be
bought. Approximately 40 – 50 miners are usually present, all with their
respective specimens, ready to trade. The local Malawians are miners in the
truest sense, because the Malawi government has granted them mining
certificates to prospect for minerals and several of them also have legal
claims to the sites where the minerals are excavated. Specimens range in size
from thumbnail, single crystals, to museum-sized matrix specimens of aegirine,
quartz and feldspar that can weigh over 15 kg.
DESCRIPTION OF MINERALS
Table 1 provides
a list of the minerals that are reported from the region. Those species that
are of collector interest are described below. Detailed descriptions of the
other minerals can be found in Peterson and Grossman (1994) and Cairncross et
al. (1999).
Aegirine, occurs abundantly
in very aesthetic crystals. These vary considerably in their crystal form, size
and association with other mineral species. Extremely lustrous coal-black,
prismatic, single crystals up to 24 cm are known but composite crystals up to
40-50 cm have also been collected. Crystal terminations can be a simple
pinacoid, but often, very steep hemipyramids impart distinctive tapering ends
to the crystals, resulting in terminations in sharp points. Groups of elongate,
prismatic aegirine crystals can form haystack-like aggregates. Some aegirine
crystals are studded with white microcline crystals. Others are intergrown with
prismatic, doubly terminated, beige zircons. Some quartz crystals have
inclusions of very acicular, hair-like microscopic crystals of dark-green to
black aegirine that impart black phantoms to the otherwise clear quartz. Other
included quartz crystals have orange-brown hairlike inclusions. These have been
analyzed and are also fibrous aegirine that has been partially oxidized.
Arfvedsonite, occurring
as, sharp prismatic crystals is one of the most interesting minerals obtained.
These are black and display typical aligned striations on the prism faces, parallel to the
c-axis. Stubby crystals 4 –5 cm long are common but some are over
10 cm, although these were weathered and corroded. Associated species include
microcline, zircon, aegirine, and quartz. Arfvedsonite is also found as fine
hair-like crystals included in quartz, giving the quartz a dark-green color.
Incipient alteration of arfvedsonite to riebeckite is relatively common
(Petersen et al. 1994).
Caysichite-(Y),
known from two specimens have been identified (Massanek 1999). These were
originally thought to be natrolite but have been analyzed and shown to be
caysichite -(Y). The crystals are up to 1.1 cm long, pseudotetragonal and have
a milky outer layer and transparent inner core. The overall color is pale
rose-pink. The mineral occurs together with smoky quartz, aegirine and
orthoclase.
Epididymite, was
known several years ago, from a few small (millimeter-sized) epididymite
crystals, observed on some specimens and as inclusions of white, acicular
crystals in smoky quartz. However, larger epididymite crystals are now also
known. A terminated hexagonal crystal 4.5 cm long and 4 cm in diameter, on a
matrix of aegirine and smoky quartz is in the Farquharson collection in
Johannesburg. Similar large crystals are in the Messner collection in
Johannesburg (now in Austria, 2004 – Editor). These crystals are colorless to
white, pale-cream. Other specimens of epididymite are more typically elongate,
thin (<1 mm) prismatic crystals, very commonly included within quartz
crystals.
Fergusonite-(Y), is
rare, and not many fergusonite-(Y) crystals are known; the species remains a
rarity from the locality. Crystals usually are grouped together in bundles less
than 1 cm. These display the typical tapering habit and are pale yellow. All of
the fergusonite-(Y) crystals are associated with aegirine.
Galena, was
until recently, never been reported from the Zomba region, but found in amongst
many hundreds of aegirines was a small galena specimen. At first it was thought
that the galena was collected at some other locality in Malawi and merely added
into the Zomba material for sale. However, on cleaning the specimen and
examining it, the presence of small, attached aegirine crystals confirmed its
source as indeed originating from Zomba.
Goethite, is prevalent on most of the specimens that
come from the area. It forms a brown rust-colored coating to most specimens. An
interesting occurrence of goethite was found on a few specimens - goethite
pseudomorphs after what is believed to be parisite. The crystals that are being
replaced are flat, tabular, and hexagonal in outline, but are now composed
entirely of iron hydroxides.
Another
relatively new discovery is attractive, sharp cubic crystals of pyrite, up to 1
cm on edge, now altered to goethite. These occur as groups of crystals, some
nucleated onto aegirine crystals.
Parisite-(Ce), has been reported from the Zomba massif. One
of the largest crystals to date is a terminated, 3.2 cm X 2.1 cm hexagonal
crystal associated with aegirine and a second, smaller naturally etched
parisite. The parisite displays a common feature of the Zomba specimens, having
a tapering, prismatic profile towards the flat, pinacoidal termination. Most of
the Zomba parisite crystals are an attractive yellow color, and have
distinctive, naturally etched and corroded surfaces. Small (< 2 mm), highly
corroded, almost formless parisite crystals can be relatively common, scattered
on a matrix of aegirine, quartz and potassium feldspar. Rarely, some crystals
are transparent.
Potassium feldspar, is common at Mount Malosa. Although no definitive
analyses were carried out, a few random feldspars have been examined and proved
to be microcline. One of the largest
crystals known measures 25 cm on edge. Groups of microcline are common
and often associated with quartz, smoky quartz, aegirine and arfvedsonite. The
feldspar is white to very pale-cream and Manebach twinning is very common. Some
K-feldspar crystals have pitted or corroded crystal faces.
Quartz, is
typical within any batch of Zomba material. The crystal sizes range from
thumbnail to over 35 cm. The quartz is usually clear and colorless. However,
inclusions of acicular aegirine produce almost black crystals. Smoky quartz is
also common. The habits of some of the quartz specimens are somewhat unusual.
Most have the typical prismatic hexagonal shape, but some crystals are
extremely flattened while others display steep, pyramidal tapering towards the
termination. Many quartz crystals have overgrown aegirine. A few quartz
specimens have included yellow to dull-orange acicular crystals, identified
quantitatively as aegirine. Quartz is associated most commonly with aegirine
and potassium feldspar, as well as zircon. A few crystals of pale-purple
amethyst are also known. Recently (mid-2000), interesting pseudomorphs of
quartz after another hexagonal mineral have been found. Drusy quartz has
replaced what is believed to be large apatite crystals, some over 15 cm long,
although unreplaced apatite has not yet been reported from the deposit, so this
identification remains unproven.
Scheelite, as
two loose, subhedral crystals, was collected in 1999. These are an attractive
orange color (similar to those from China). To date, these remain the only
known scheelite specimens from Zomba.
Siderite, occurs as a few specimens of rhombohedral,
tan-colored siderite crystals, forming the matrix to quartz and feldspar.
Zircon, in crystals up to 4 cm have been collected. Some are
opaque and light brown, but others are gemmy, transparent orange crystals. The
abundance of zircon, occurring as loose crystals as well as composite
aggregates, and as zircon associated with most of the other minerals found at
Zomba, attest to it being relatively common.
RECENT DISCOVERIES
During late-1998 and
1999, trips were made to the Zomba massif that produced some new and
interesting minerals. There are at least a dozen or more, small (1-2 mm)
well-crystallized minerals that are as yet unidentified and awaiting
quantitative analyses. However, what appear to be small 2-3 mm cubic crystals
of pale-blue fluorite have been positively identified. There are no doubt many
interesting micromount minerals that have been overlooked in the past because
the local Malawi digger’s tend to only concentrate on the larger, more
commercial quartz and aegirine specimens and are not even aware of the rare
micromount potential of the minerals of Zomba.
CONCLUSION
Access
to the area from where the collectable minerals are found is not easy. One has
to carry in all provisions, camping, and digging equipment because no
facilities or shops exist on the mountains. In order to spend a few days
collecting, one has to camp on the summit of the mountain – it is not possible
to hike in and out every day. The mineral specimens are all collected from
outcropping pegmatite veins. Collecting the crystals is hazardous in the
extreme. Most of the more easily accessible pegmatites have been excavated and
depleted of minerals. The remaining pegmatites are those that crop out on the
very steep scarp faces of the northwestern part of Malosa Mountain. Local
Malawians climb these treacherous slopes barefoot, hanging precipitously above
drops of several hundred meters to the valley below. Recently (late-2000), the
Malawi Police have begun patrolling the area, asking any foreigner for the
compulsory permits and documentation that are required by Malawi law to collect
and deal in mineral specimens. If one does not possess these documents, the
resulting penalties can be severe, even resulting in imprisonment.
Table 1: List of minerals reported from the Zomba-Malosa region.
| Aegirine* | NaFe3+Si2O6 |
| Albite | NaAlSi3O8 |
| Arfvedsonite* | Na2(Fe2+,Mg)4Fe3+Si8O22(OH)2 |
| Bastnäsite - (Ce) | (Ce,La)(CO3)F |
| Biotite | K(Mg,Fe2+)3(Al,Fe3+)Si3O10(OH,F)2 |
| Calcite | CaCo3 |
| Caysichite - (Y)* | Y4(Ca3REE)(OH)(H2O)5(Si8O20)(CO3)6•2H2O |
| Epididymite* | NaBeSi3O7(OH) |
| Epidote | Ca2(Fe3+,Al)3(SiO4)(OH) |
| Eudialyte | Na2(Ca,Ce)2(Fe2+,Mn2+,Y)ZrSi8O22(OH,Cl)2 |
| Eudidymite | NaBeSi3O7(OH) |
| Fergusonite - (Y)* | YNbO4 |
| Galena* | PbS |
| Goethite* | Fe3+O(OH) |
| Hematite | Fe2O3 |
| Hingganite - (Y) | (Y,Yb,Er)BeSiO4(OH) |
| Hingganite-(Yb) | (Yb,Y)BeSiO4(OH) |
| Ilmenite | Fe2+TiO3 |
| Mangan-neptunite | KNa2Li(Mn2+,Fe2+)2Ti2Si8O24 |
| Monazite | Ce,La,Nd,Th)PO4 |
| Neptunite | KNa2Li(Fe2+,Mn2+)2Ti2Si8O24 |
| Orthoclase-microcline* | KAlSi3O8 |
| Parisite-(Ce)* | Ca(Ce,La)2(CO)2F2 |
| Polycrase-(Y) | (Y,Ca,Ce,U,Th)(Ti,Nh,Ta)2O6 |
| Polylithionite | KLi2AlSi4O10(F,OH)2 |
| Pyrochlore | (Ca,Na)Nb2O6(OH,F) |
| Quartz* | SiO2 |
| Riebeckite | Na2(Fe2+,Mg)3Fe33+Si8O22(OH)2 |
| Scheelite | CaWO4 |
| Siderite* | Fe2+CO3 |
| Sphene | CaTiSiO5 |
| Thorite | (Th,U)SiO4 |
| Xenotime | YPO4 |
| Zircon* | ZrSiO4 |
* = Species discussed in the text.
(List compiled from
Bloomfield 1965; Petersen and Grossman 1994; Petersen et al. 1994;
Cairncross et al. 1999 and Johnsen et al. 1999 and personal
observations).
Note: rutile has been
deleted from the list on Zomba minerals. The original, visual identification of
this species was made by Petersen and Grossman (1994), who reported on
reddish-brown hairlike crystals included in quartz. Analyses have since shown
these inclusions to be aegirine.
Prof. Bruce Cairncross,
Department of Geology, Rand Afrikaans University,
P.O. Box 524, Auckland Park, 2006, South Africa.
Email: bc@na.rau.ac.za
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