New specimens from the Erongo Mountains

19 September 2012

In mid 2012, there was new find in the Erongo Mountains of Namibia, of a single pocket producing superb specimens of Schorl Tourmaline, purplish Fluorite, Quartz, and Goshenite Beryl. Crystal Classics and Kristalle were very pleased to have been offered the majority of this pocket, and are now premiering the best specimens as an update on our websites. A second separate find, also made around the same time within the Erongo Mountains but at a different location, has produced stunning specimens with green Fluorite, Aquamarine Beryl, Schorl Tourmaline on Feldspar. Similarly we were pleased to cherry pick the best selection of specimens at the time they were found. The Erongo region is famed for its rich wealth of specimens - dominantly coming from the miarolitic cavities located in the Erongo Granite - and these specimens maintain the high standards set by this famous specimen-producing locality.

Green Fluorite, Aquamarine and Schorl on Feldspar, a spectacular specimen from the second find.

The Erongo area is a complex sequence formed from several volcanic episodes, with lava and pyroclastic flows. This was followed by the instrusion of granodiorite and ignimbrites into a central volcanic area, and further pyroclastic formations. The deeply layered structure of the lavas and pyroclastics was then intruded by the Erongo Granite. The Erongo Granite intruded as a ring-like body at possibly no more than a depth of 1-2 km, and was the major heat source for the circulation of the mineral rich solutions. The igneous complex as a whole has been dated at around 133 Ma, forming part of the Damaraland Alkaline Province. The dominant mineralizing fluids were Boron rich allowing the formation of Schorl Tourmaline, with accessory Beryl, Fluorite etc. The collectible minerals are found within miarolitic cavities largely filled with nests of Schorl crystals which are quite common to find with no other species with them. The formation of miarolitic voids is a function of the emplacement of the Erongo Granite at a relatively high horizon in the Earth's crust, allowing the Erongo Granite to bubble under the lower lithostatic pressure particularly in the roof area of the Granite, creating rounded voids from the gas bubbles. Other pegmatitic occurrences are located around the fringe of the complex in the host schist rock. Erosion over the following epocs has denuded the Erongo Granite, allowing us to eventually collect some of the outstanding minerals from this famous locality.

Observing the specimens that we acquired, each find has it own unique features, which we have summarised here. 

1) First Find - Schorl, purplish Fluorite, Quartz and Goshenite:

Schorl, Fluorite, Smoky Quartz and Goshenite

The Schorl appears to be one of the first formed minerals in this pocket (which would common from a Boron rich fluid) as it often pierces the Fluorite and Quartz crystals, where they have grown around needle-like crystals. The Schorl typically forms crystals which are well formed with lustrous pyramidal terminations at one end, becoming more heavily striated down the length grading into bundles of parallel needle-like crystals at the other – possibly due to an increase in the rate of growth, as it would be quicker/easier for multiple parallel terminations to grow than one large termination. The Schorl ranges in size, often intergrown at angles, and its 'sprays' of crystals can look like it is exploding out of the specimen.

Sprays of Schorl - the whitish thin line can be seen above which the Schorl crystals become complex with multiple terminations.

The Schorls when viewed under magnification are often translucent with yellowish-brown, green and purple to pinkish tints. Colour banding can be seen along the length of the crystal, with sometimes a fine whiteish divisional line between the well formed end and the complex terminations - possibly due to the changing in the conditions of formation. Whilst Schorl is normally thought of as black, most Tourmalines do show colour zoning, and colour zoned Schorl is found at Erongo. 

A large Schorl crystal showing the well formed lustrous pyramidal termination at one end.

Fluorite was found at the top of the pocket of this single find, becoming rarer as the pocket was excavated downwards. The purplish Fluorites are quite complex crystals, of varying shapes and shades. The Fluorite may have formed initially as cubes, with deep purple phantoms parallel to the octahedral planes. The corners of the cubes have the appearance of being etched away to the octahedral plane so that many Fluorite crystals are heavily modified cubo-octahedrons with tiny stepped surface growth features. The purple phantoms appear at surface as many pinpoints and veins of colour, particularly on these octahedral faces, which gives a lovely effect over the pale bluish internal body colour.

Modified cubo-octahedral Fluorite crystal, with stepped growth features, and near surface dark purple colour concentrations, often in 'veins'

Goshenites are the uncommon colourless variety of beryl. The crystals found on these specimens  are very glassy and water clear, with internal bright yellow phantoms caused by inclusions. They have sharp hexagonal form, measuring up to over 1cm, and stand out against the other minerals with their glassiness.

A glassy Goshenite with yellowish inclusions defining a phantom. The Fluorite also shows the concentration of colour on the octahedral planes.

The Quartz tends to be smoky, forming elongate prismatic crystals. They often have naturally etched faces, giving them a matt finish, but are very gemmy internally.

2) Second find - green Fluorite on Feldspar, with Schorl and Aquamarine:

Fluorite on Feldspar

The second separate find was made around the same time, but in a different location. It has produced specimens of Fluorites growing on Feldspar. These Fluorites are a bright gorgeous green with deep purple near surface colouration. Their crystal shape is different from the first find, appearing more cubic in form with domed or undulating crystal faces. The Feldspars on which they have grown have been naturally corroded so that they are finely striated, often with a honeycomb texture which provides a bright white to yellow backdrop for the Fluorite colour.

Green Fluorite with purple outerzones, on honeycombed Feldspar

Well formed Schorls may accompany these specimens, showing similar form to the first find with large well formed lustrous terminations at one and and complex multiple terminations at the other. The crystals here however, tend to have a stocky shape (at least in the specimens we have acquired), rather than the elongated sprays. In this find the Schorl appears to have formed possibly after the Fluorite. Bubbly Hylophane Opal is present on one specimen on a naturally fractured surface of a Schorl crystal.

This find also produced specimens with the association of Aquamarine - the blue variety of Beryl, creating a stunning combination with the green Fluorite, biscuit coloured Feldspar and lustrous black Schorl.

ref: CAINCROSS, B. and BAHMANN, U. (2006) Famous Mineral Localities: The Erongo Mountains, Namibia. Mineralogical Record, Vol 37, Sept-Oct 2006, p361-470

Author: Robin and Steve
Categories: Collection Acquisitions , News & Information