Transactions of the Geological Society, 1st series, vol. 4/On Witherite found in Shropshire

From Wikisource
Jump to: navigation, search
On Witherite found in Shropshire
by Arthur Aikin




XXX. Notice concerning the Shrophire Witherite.


By ARTHUR AIKIN, Esq.


member of the geological society
and secretary to the society for the
encouragement of arts manufactures and commerce.


[Read 6th December, 1811.]


The Witherite or native carbonate of barytes, still continues to be one of the rarer productions of the mineral kingdom. The only thoroughly ascertained locality of this substance, according to Professor Jameson, (System of Mineralogy, I. 575,) is Anglesark in the county of Lancaster, where it was first discovered by Mr. James Watt. It is here found in veins traversing the independent coal formation, and accompanied by blende, galena, calamine and heavy-spar. To this locality however may be added, on the authority of Klaproth, (Analytical Essays, I. 389,) a mine near Neuberg in Upper Stiria, where this mineral occurs in a bed of spathose iron ore.

The mine at Anglesark is, I understand, abandoned: it may therefore be a matter of some interest to the members of the Geological Society, to state that my researches during the last summer in pursuance of my mineralogical survey of Shropshire, have made me acquainted with a mine, within the bounds of that county, in which witherite occurs very abundantly.

The most hilly district of Shropshire extends from the borders of Montgomeryshire to the town of Church Stretton, having the broad valley of the Severn for its northern boundary, and stretching as far south as the parallel of Bishopscastle. The general elevation of this tract above the Severn varies from 800 to above 1500 feet. It is completely intersected by two strait and simple vallies, the direction of which is about north-east and south-west, and several smaller ones parallel to the larger descend from the interior towards the north and south. The principal rock which presents itself is greywakke-slate, in beds which run north by east and south by west, rising at an angle of about 50° east by south. The colour of the rock where it has not undergone decomposition, is bluish or greyish black, probably from a mixture of carbon, for by exposure to the air it passes to a pale yellowish-grey, with a few spots of oxide of iron. Its cross fracture is dull, but its longitudinal fracture presents numerous small spangles of mica. The upper beds, or rather the superficial parts of many of the beds, are in the state of shale, that is, they are shattery, soft, and more or less decomposed. No true veins ever occur in this shale, and even veins of considerable size and regularity in the compact part of the rock, terminate almost immediately on coming in contact with the looser part.

It is in this compact greywakke-slate, and chiefly on the western side of one of the highest hills called the Stiperstones, that the principal lead mines of the county occur. Of these, that which is called the Snailbach mine, is the most important for its metallic produce, and is the only one in which witherite has hitherto been found.

The mine consists of one principal vein, with several strings proceeding from it. Its course is nearly east and west, descending at a high angle to the south: its greatest depth is about 180 yards, and there are no signs of its approaching to a termination: its general thickness is 10 or 12 feet, but in one particular part amounts to more than 30 feet. Cavities, or locks as the miners call them, are frequently met with of all sizes, from that of a walnut to that of a small room: many of the lesser cavities are filled with petroleum, the others are lined with crystals of heavy-spar, calcareous spar, and quartz, having their bottoms or floors covered with aggregated masses of the above crystals, with crystals and potatoe-shaped pieces of galena, with carbonate of lead, and with a black powder which is principally pulverulent galena. In some parts there is a saalbande or sticking of grey clay an inch or two in thickness, and here the vein is the least productive; in other parts the rock is hardened by an infiltration of quartz, and these are uniformly found to be the most productive. There is no regularity in the arrangement of the contents of the vein, but generally the galena occupies the sides, and the sparry veinstones the centre. Riders, or ribs of rock inclosed within the vein occur, but not frequently. The ore is for the most part foliated galena and striated galena, called by the miners pot ore and steel ore: iron pyrites is the next in quantity, and lastly blende, this latter being for the most part so intimately mixed with the galena as to be scarcely visible. The veinstones are calcareous spar, often approaching to schiefer spar, and foliated heavy-spar (called here water spar, because from the looseness of its aggregation most of the water drains through it into the mine.) Quartz is more rarely met with; and in the lower part of the mine, where the vein is very thick and sparry, the witherite is found in irregular masses, weighing from 40 lbs. to 2 or 3 cwt., imbedded in heavy spar. The name given to this substance by the miners is yellow spar, not that this is its real colour by day-light, but its transparency is so considerable that if a lighted candle be placed behind a mass of it the whole will glow with a yellowish light, a circumstance by which the miners distinguish it from heavy-spar; this latter from the looseness of its texture being in large masses quite opake. The colour of the witherite is white with the slightest possible, if any, tinge of yellow: its fracture is broad striated approaching to strait-foliated: it is for the most part massive. I have seen only a single specimen that presented any indications of a regular crystalline form. In other particulars it agrees with the usual descriptions of this substance.

The Anglesark witherite has been analysed by Klaproth, and besides carbonate of barytes appears to contain above two per cent. of carbonate of strontites, and a scarcely appreciable quantity of oxide of copper. The Stirian witherite, on examination by the same chemist, appears to be a pure carbonate of barytes. The presence of carbonate of strontites being a circumstance of some interest, I was induced to examine the Shropshire Witherite for the purpose of ascertaining whether in this particular it agreed with that from Lancashire.

For this purpose 200 grains were dissolved in muriatic acid, and left behind 1.8 gr. of a white powder which was sulphate of barytes.

The muriatic solution being supersaturated with ammonia was evaporated to dryness and ignited till the muriate of ammonia was driven off: the residue was redissolved in water, but left behind 1.5 gr. of a brown sediment, which on digestion with sulphuric acid was separated into 1. gr. of silex insoluble in the acid, and 0.5 gr. of alumine coloured by oxide of iron, soluble in the acid, and which deposited crystals of alum on the addition of sulphate of potash.

The purified muriatic solution was brought to the crystallizing point, and the muriate of barytes which fell down was removed: the mother-liquor was mixed with alcohol and heated, the fluid (after standing a minute to clear) was poured off, and on cooling deposited long needled crystals of muriate of strontites. Water was then added, the crystals were redissolved, and carbonate of ammonia was poured in till it occasioned no further precipitate. The carbonate of strontites thus obtained weighed on ignition 2.2 gr. and being redissolved in muriatic acid and mixed with alcohol communicated to the flame of this last the red colour characteristic of strontites.

The muriate of barytes was in like manner decomposed at a boiling heat by carbonate of ammonia, and the carbonate of barytes thus obtained weighed, after edulcoration and ignition, 192.6 gr. Hence 100 parts of this witherite contain

96.30 carbonate of barytes
1.10 ──────── of strontites
0.90 sulphate of barytes
0.50 silex
0.25 alumine and oxide of iron
─────
99.05
0.95 loss