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Slickensides Galena & Other
Naturally Occurring Explosives

Siberian Marekanite, natural Prince Rupert`s drops.
Detonating Derbyshire Galena, Higginbotham narrowly escapes crackingwhole explosion.
Lanchester Witherite, collector`s dismay.

Both crystalline and vitreous minerals exist whose physical internal structure is so stressed as to cause them to occasionally explode. My own experience with such a mineral was with an apple sized specimen of witherite.

I`d collected the beautifully crystallised barium carbonate sample from mine spoil-heaps near Lanchester, Co. Durham in 1972. As I washed mud from the witherite in the kitchen sink, using a toothbrush and tepid water, it exploded. It completely shattered in my hands with a loud bang, much to the dismay of myself; a specimen lost, and to my Mother; her 12 year old son having just inundated her kitchen with toxic barium carbonate particles!

As explosions go, it was minor, however considerable stress energy is present in some minerals, such as Slickensides Galena -

A Lead Ore Explosion in Hay Cliff Mine, Derbyshire, 1738

This excerpt from White's History, Gazetteer and Directory of the County of Derby, 1857

"Hay Cliff Mine, in Eyam Edge, now no longer worked, was once the grand depository of that extraordinary phenomenon, in the mineral world, provincially called Slickensides. The external appearance of this curious species of galena is well known wherever mineralogy has been studied, At the present time, good specimens of it are extremely rare, and can only be met with in cabinets that have been long established. In those mines where it has most prevailed, it exhibits but little variety, either in form or character. An upright pillar of limestone-rock, intermixed with calcareous spar, contains the exploding ore; the surface is thinly coated over with lead, which resembles a covering of plumbago, and it is extremely smooth, bright, and even. The effects of this extraordinary mineral are not less singular than terrific. A blow with a hammer, a stroke or a scratch with a miner's pick, are sufficient to rend the rocks asunder with which it is united. The stroke is immediately succeeded by a crackling noise, accompanied with a sound not unlike the hum of a swarm of bees; shortly afterwards an explosion follows, so loud and appalling, that even the miners, though a hardy and daring race of men, turn pale and tremble at the shock. In the year 1738, an explosion took place in the Haycliff mine, when two hundred and fifty barrels of material wore blown out at one blast, each barrel containing 350 pounds weight. During the explosion, the earth had a tremulous motion, as if shook by an earthquake."

An earlier reference to Slickensides, from which the previous excerpt was presumably researched, from William Wood`s The History and Antiquities of Eyam, 1845 -

"The mines in Eyam Edge are very deep and the New Engine Mine is often stated to be the deepest in Derbyshire. Among the number in the Edge is the Hay-cliff, a mine distinguished for having contained in great abundance that extraordinary phenomenon in the mineral world, provincially called Slickensides. It is a species of Galena and is well known among mineralogists. This mine once had it in singular quantity and quality. The effects of this mineral are terrific: a blow with a hammer, a stroke or scratch with a miner's pick, is sufficient to blast asunder the massive rocks to which it is found attached. Of the nature of this mineral and its terrible power, there have been many but quite unsatisfactory solutions. Whitehurst, in his work on The Formation of the Earth, thus mentions its wonderful power:- In the year 1738 an explosion took place at the Hay-cliff Mine, Eyam, by the power of Slickensides. Two hundred barrels of materials were blown out at one blast- each barrel containing 350 lbs. weight. During the explosion the earth shook as by an earthquake. A person named Higginbotham once narrowly escaped with life by incautiously striking this substance in the above mine. Experienced miners can, however, work where it most abounds without much danger. It is also known by the name of Crackingwhole."

If anyone has any futher information on Slickensides Galena, I`d like to hear from you - email

Okhotsk Marekanite. Natural Prince Rupert`s Drops

If glass is melted and dripped into ice-cold water, the glass solidifies into drops with a thin tail. Tremendous stress between the surface and the interior is the result. If viewed between polaroid films, the stresses and strains can be seen, just as in toughened glass. The bulbous end of a drop can withstand being hit with a hammer, such is the toughness. If however, either the thin tail is snapped, or the body scratched with a file, Prince Rupert`s drops violently explode into glass powder.

A Prince Rupert Drop The drops were introduced to England in the 1640's by Prince Rupert of Bavaria (1619-1682). His uncle, King Charles the Second, thought them a jolly jape and used them in a practical manner; he had guests hold the drops tight in their hands, then snapped off the tail.

Obsidian nodules found within perlitic rock near the Marekanka River, Okhotsk, Siberia, Russia are internally stressed, and show similar properties to Prince Rupert`s drops.
The following article on perlite is from the 1911 Encyclopaedia Britannica -

"Perlite is a glassy volcanic rock which, when struck with a hammer, breaks up into small rounded masses that often have a pearly lustre. The reason for this peculiarity is obvious in microscopic sections of the rock, for many small cracks may be seen traversing the glassy substance. These mostly take a circular course, and often occur in groups, one within another. The circular cracks bound the little spheres into which the rock falls when it is struck, and the concentric fissures are the cause of the pearly lustre, by the reflection of light from enclosed films of air.

It is easy to imitate perlitic structure by taking a little Canada balsam and heating it on a slip of glass till most of the volatile matters are driven out; then drop it in a basin of cold water and typical perlitic structure will be produced. The reason is apparently the sudden contraction when the mass is chilled.

In mineralogical collections rounded nodules of brown glass varying from the size of a pea to that of an orange may often be seen labelled Marekanite. They have long been known to geologists and are found at Okhotsk, Siberia, in association with a large mass of perlitic obsidian. These globular bodies are, in fact, the more coherent portions of a perlite; the rest of the rock falls down in a fine powder, setting free the glassy spheres. They are subject to considerable internal strain, as is shown by the fact that when struck with a hammer or sliced with a lapidary`s saw they often burst into fragments. Their behaviour in this respect closely resembles the balls of rapidly cooled, unannealed glass which are called Prince Ruperts drops. In their natural condition the marekanite spheres are doubly refracting, but when they have been heated and very slowly cooled they lose this property and no longer exhibit any tendency to sudden disintegration."

Notes on Witherite

The witherite sample which exploded in my hands in the early `70s was not representative of the mineral as a whole. The sample was pure witherite, mostly massive, but with numerous small cavities within which were visible the hexagonal pyramidally terminated crystals I had been attracted to. Witherite is deposited hydrothermally, so perhaps a cavity within the sample held enclosed and entrapped, some of the original solution of genesis, with dissolved gasses exerting internal pressure. My use of tepid water to clean the sample could have caused enough thermal shock to trigger the explosive release of this pressure.

each individual witherite crystal in this group is actually a triplet Witherite itself is a rather interesting mineral, in that it always crystallises as a twinned form called a trilling. The crystals are actually triplets, and form dipyramidally terminated pseudo-hexagonal prisms, although the mineral is orthorhombic. In 1783, W. Withering discovered witherite, determining that a fibrous mineral found at Anglesark in Lancashire was actually the carbonate of barium BaCO3, not the sulphate (barytes). each individual witherite crystal in this group is actually a triplet

For those interested in mining, there follows an excerpt from a 1938 article by G. Trestrail "Witherite in Northumberland". Originally published in Mine & Quarry Engineering, Mr. Trestrail`s account of Withering`s discovery is at odds with my own researches. Also, bear in mind that the "slickensiding" mentioned in the Settlingstones mine is a well known geological phenomenon, caused by relative movement of rock faces along a fault, and is not directly connected with explosive slickensides galena.

Witherite in Northumberland, by G. Trestrail 1938

"Witherite or natural barium carbonate is a relatively rare mineral. It was first identified by Dr. Withering, a physician of Birmingham, while examining a collection of minerals from an old lead mine on Alston Moor, and the mineral was named in his honour. It crystallises in the orthorhombic system in the form of hexagonal pyramids, which have the appearance of a simple crystal, but in reality are repeated twins. The faces of the crystals are usually rough and horizontally striated. The deposit contains granular, crystalline masses which, in the process of mining, disclose small cavities or vugs lined with shining twinned crystals of pure witherite ; also more rarely it occurs in globular, tuberous and botryoidal forms showing columnar or amorphous structure. Fracturing is uneven and brittle, the hardness ranges from 3 to 3.75, and it has a specific gravity of 4.29 to 4.35. The colour is variable from white, often yellowish or grayish, and occasionally stained with intrusive salts.

Settlingstones vein is clearly one of hydrothermal origin, and is enclosed in a fissure accompanied by a complexity of faulting. It lies in a mass of alternating sandstones, shales and limestones named the Bernician series after Bernicia the ancient name of Northumberland. This series contains the great whin sill, an intrusive quartz dolerite, which is one of the interesting features of local geology. Generally the strata incline eastwards, veering around in a southerly direction toward the Tyne valley. A fault at the west of the mine extensively dislocates the strata vertically and laterally to the south, and a further complication is the presence of a southeast anticlinal axis by which the lower beds are brought up into the belt at the eastern extremity of the mine. Erosion along the crest of the anticline has partly denuded the whin sill and glacier deposits, clay with boulders, cover the strata which in places attain a depth of 150 feet. Throughout the belt the rock mass is disturbed and displaced by major faults and, to a less degree, by minor faulting. The whin sill averages 130 feet thick throughout the mine and has, with the strata, apart from the faulting, a uniform inclination of approximately 1 : 30 to the south west. Above and below the whin sill are beds of whetstone which are apparently metamorphosed shale, altered at the time of the injection of the igneous rock. The interesting feature of the mine is the change in the character of mineral deposition which occurred at the cross vein at the Winter shaft. Here the vein fissure and strata are faulted abnormally and the lead ore is entirely cut off at the fault plane. The vein then becomes a barren stringer for a distance of some 800 ft. in a westerly direction, and from this point the fissure becomes charged with witherite.

The witherite vein is practically vertical and varies in width from 4 to 12 ft. Exceptionally, a 30 ft. width has been exposed, but the average is something like 8 ft. Of the vein characteristics, a stringer of zinc blende intercalated in the witherite, known as the "lead" is a feature of interest. This stringer is well defined and continuous throughout the mine and when the vein is impoverished, as happens at times in the softer strata, the lead continues and is followed advantageously. In the wide parts of the vein it shows ramification, and at times mineralisation changes occur between the relative divisions. Gangue minerals consist of lime in the form of calcite and baryto-calcite which frequently fill the capillary interstices of the witherite mass. Cavities are a common occurrence, sometimes of large dimensions, and invariably crystal lined. Iron pyrites is not prevalent, but it occasionally impregnates the vein locally. Galena occurs remotely in pockets or lenses. Barytes is rarely present in the amorphous state its occurrence is associated with the vein cavities which it encrusts with a coral like deposit, and it is inherent in the zeolite Harmotome which permeates the voids of the vein. Rapid changes in the vein matrix constitute a deposition enigma; contrasting features are pure transparent witherite, witherite in granular crystalline form, witherite with a varying disposition of gangue mineral, brecciated country rock and the presence of argillaceous shale are admixtures of frequent occurrence. Of the hydrothermal influence on the walls of the enclosing rocks metamorphic changes are widespread ; the "white whin" is a feature of seritic alteration whilst the witherite contacts frequently show chloritic inclusions ; voids, in the swells of the vein, are often chloritised.

Except in one or two instances the cross veins are barren. They vary in width from a few inches to one or two feet, and are generally open-jointed fissures filled with a matrix of fault breccia and permeated with ferruginous solutions. They make a clean intersection across the vein and their occurrence invariably displaces the vein laterally to a small extent. Post mineral displacement is in evidence, the shear zones on the fissure walls indicate intense movement and slickensiding within the mineral mass has been observed.

The formation of the enclosing rock has an effect on the mineral deposition. The lead ore showed enrichment in the whin sill and became impoverished in the lower shales at a comparatively shallow depth. The witherite exhibits similar fertile features in the whin sill and the corresponding wall of the harder sedimentary rock while the structure of the wide open fissure has lent itself to the environment of ore deposition. In behaviour, the vein in the shale beds at the higher part of the mine is erratic. Swells of pure witherite rapidly contract or break into stringers."

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