Greenockite

Greenockite

This very rare mineral is something of a Glasgow speciality. Most of the world's best specimens were found near here over a century ago. The Hunterian Museum has over 30 specimens, many of superb quality.

What is greenockite?

Greenockite is the mineralogical name for naturally occurring cadmium sulphide with a hexagonal crystal structure. The chemical formula is CdS. Synthetic forms of this material are used as pigments in some yellow paints, although due to their toxicity, this is rare nowadays.

What does it look like?

The colour can be various shades of yellow or orange. On fresh surfaces, it is very bright and lustrous, but becomes duller when weathered - much greenockite from the Glasgow area has a dull greenish-yellow surface tinge. The very largest crystals are around 1.5 cm across, but most are much smaller: 1-3mm would be more typical. Normally the crystals are not well formed, but are entirely enclosed in other minerals, especially prehnite or calcite, and are only seen when the material is broken. Good, well-formed crystals are found in cavities: these are usually hexagonal pyramids with a flat base, although flatter hexagonal plates also occur.

Click for larger image.	Click for larger image.
M374. Greenockite, on prehnite from Bishopton, Renfrewshire. The large tabular crystal on the left is about 5mm across. Acquired from unknown source in the 19th century.	100024. Greenockite (1cm crystal) on altered basalt from Erskine, Renfrewshire. Pre-1850 specimen from Thomas Brown's collection.

How was it discovered?

Greenockite was first described by Jameson, and Connell, in 1840, who examined specimens found in a railway cutting at Bishopton, on the south bank of the River Clyde, a few miles west of Glasgow. However, it had been noticed earlier: a specimen had been collected in 1816, by Thomas Brown, but identified as sphalerite.

Is it named after the town of Greenock?

No. It was named after Lord Greenock, who owned the land on which it was found. Greenockite is one of the select band of minerals named after Scots or Scottish localities.Others include strontianite, caledonite, lanarkite, susannite, scotlandite, macphersonite, leadhillite, edingtonite, thomsonite, mattheddleite, and mullite.

What's the geology of this area?

Greenockite is found in Lower Carboniferous (Visean) basaltic lavas of the Strathclyde Group. These are mildly to moderately silica-undersaturated rocks, erupted around 340 million years ago. Ankaramitic rocks ("Craiglockhart Type" basalts) are a major component of the lavas in the Bishopton area. These lavas show extensive hydrothermal alteration, caused by hot circulating groundwaters during cooling of the lava flows, , and it was during this alteration that the greenockite was formed.

Is it valuable?

Soon after the first descriptions in 1840, specimens became highly sought after, and high prices were soon being asked for greenockites from this area. There must have been something of a roaring trade in the stuff, as most of the great mineral museums across the world have specimens of greenockite from Bishopton. A few specimens have also been found at other localities nearby, although Bishopton, seems to have been the only locality which produced large quantities of material. Crystallized greenockite remains an extremely scarce material, and good specimens are now rarely seen on the market.

Is it poisonous?

Cadmium is an extremely toxic metal, and eating large pieces of greenockite would have very serious, if not fatal effects. However, greenockite is also an extremley insoluble mineral under normal conditions, and so very little cadmium escapes into the environment through dissolution of greenockite. Although data on this are lacking, it is quite likely that the rocks around Bishopton are not especially rich in cadmium. Greenockite occurs there because special processes have concentrated cadmium which is normally dispersed through large volumes of rock at very low levels, into a few large grains. The formation of greenockite crystals may well have resulted in abnormal cadmium-depletion in the surrounding rocks.

Higher environmental levels of cadmium are found associated with some black shales, and with zinc orebodies. The latter almost always contain significant cadmium, and may give rise to serious pollution problems.

Where else is greenockite found?

Other Scottish localities inlcude:

Erskine; probably from an old quarry in the grounds of the current hospital (Meikle, 199*). This is only a mile or so from Bishopton. Only known from old specimens, some very fine, and all in the Hunterian Museum. The matrix of these specimens is rather different from the Bishopton material, being "ashier" and more porous.

Kilpatrick, on the north side of the clyde, opposite Erskine and Bishopton. This is where Thomas Brown found his specimen in 1816, almost 30 years before the species was formally described. This specimen is in the Hunterian Museum, together with much of the rest of his collection. No other specimens have been found since, to my knowlege.

Loanhead quarry, near Beith in Ayrshire. This active quarry is about 15 miles to the south of the other localities. Excellent specimens of thomsonite, prehnite and analcime are easily collected, as well as more exotic species such as harmotome, edingtonite and greenockite, and many more. Greenockite is extremely scarce, and the specimens are not as good as the Bishopton/Erskine/Kilpatrick ones.

Although the original Bishopton railway tunnel/cutting locality is not accessible now, greenockite has turned up in fair quantities recently, in spoil produced during the construction of the M8 motorway, very close to the original locality. Some large grains, and even some well-crystallised material has been found, as well as the exceedingly scarce cadmium carbonate mineral, otavite (Faithfull and Hubbard, in press).

greenockite is fairly common as a bright yellow powdery stain on weathered zinc-rich ores at Wanlockhead in Dumfriesshire. This type of occurrence is quite common else where in the world as well, but the crystals are sub-microscopic in size.

You can see Scottish greenockite specimens on display at the Hunterian Museum, and at the National Museums of Scotland, in Edinburgh.

Anywhere else?

Yellow powdery crusts of greenockite (and the similar cubic polymorph of CdS, hawleyite), are not uncommon where sphalerite is weathered, but macroscopically crystallised greenockite is very rare.

Localities inlclude:

Paterson area, New Jersey, USA

Arlington quarry, Loudon Co., Virginia, USA

Asunta, in Potosi, and other localities in Bolivia.

Silver Standard Mine, British Columbia, Canada

These have been lifted from the literature, and I haven't spent much time chasing up better info so I'd be very grateful for any information about these, or other sources of good greenockite specimens. Email me at:

J.Faithfull@museum.gla.ac.uk.

I want to know more...

For a good source of general mineral data, including greenockite, try

The Mineralogy Database
or
The Mineral Gallery

That's all folks!