BARYTES  , a widely distributed

mineral composed of barium sulphate (BaSO4) . Its most striking feature and the one from which it derives its name barytes, barite (from the Greek Oapvs, heavy) or heavy spar, is its weight . Its specific gravity of 4.5 is about twice as great as that of salt and of many other colourless, transparent and glassy minerals not unlike barytes in general appearance . The mineral is usually found in a state of consider-able chemical purity, though small amounts of strontium and calcium sulphates may isomorphously replace the barium sulphate: ammonium sulphate is also sometimes present, whilstclay, silica, bituminous matter, &c., may be enclosed as impurities . Crystals of barytes are orthorhombic and isomorphous with the strontium and lead sulphates (celestite and anglesite) ; they are usually very perfectly developed and present great variety of form . The simplest are rhomb-shaped tables (fig . 1) bounded by the two faces of the basal pinacoid (c) and the four faces of the prism (m); the angle between the prism-faces (mm) is 78° 23', whilst that between c and m is 9o° . The mineral has a very perfect cleavage parallel to the faces c and m, and the cleavage surfaces are perfectly smooth and bright . The crystals of prismatic habit represented in figs . 2 and 3 are bounded by the domes d and f and the basal pinacoid c; fig . 4 is a plan of a still A%:: ~ .~ Gw --------------------- more complex crystal . Twinning is represented only by twin-lamellae, which are parallel to the planes m and f and are of secondary origin, having been produced by pressure .

Crystals of barytes may be transparent and colourless, or

white and opaque, or of a yellow, brown, bluish or greenish colour . Well developed crystals are extremely common, but the mineral occurs also in a granular, earthy, or stalactitic condition . It is known as cawk in the Derbyshire lead mines . The " crested " or " cock's comb " barytes occurs as rounded aggregations of thin lamellar crystals . Barytes is of common occurrence in metalliferous veins, especially those which yield ores of lead and silver; some of the largest and most perfect crystals of colourless barytes were obtained from the lead mines near Duf ton in Westmorland . It is found also in beds of iron ore, and the haematite mines of the Cleator Moor district in west Cumberland have yielded many extremely fine crystals, specimens of which may be seen in all mineral collections . In the neighbourhood of Nottingham, and other places in the Midlands, barytes forms a cementing material in the Triassic sandstones; amber-coloured crystals of the same mineral are found in the fuller's earth at Nutfield in Surrey; and the septarian nodules in London Clay contain crystals of barytes as well as of calcite . Crystals are found as a rarity in the amygdaloidal cavities of igneous rocks . Artificially prepared crystals of barytes may be obtained by allowing a solution of a soluble barium salt to diffuse slowly into a solution of a soluble sulphate . Barium chloride is present in some natural waters, and when this is the case the interaction of sulphates results in a deposition of barytes, as has occurred in the pipes and water-boxes of the Newcastle-on-Tyne coal mines . Commercially, barytes is used in the preparation of barium compounds, as a body for certain kinds of paper and cloth, and as a white pigment (" permanent white ") . The finely powdered and washed mineral is too crystalline and consequently of insufficient opacity to be used alone as a paint, and is therefore mixed with " white lead," of which material it is also used as an adulterant .

(L . J .