qUA ( 269 )
qUA
found native. The known varieties which itexhibits are the following.
1. The pyramidal dodecahedron, compofedof two fix fided pyramids joined bafe to bafe,the angle formed by the plane of one pyramidand the correfponding plane of the other is=1030 20' and the angle formed by any twoof the adjacent planes of the fame pyramidis—133 0 4°’.
2. The preceding, with a ftrait hexahedralprifm interpofcd between the two pyramids, theangle formed by any plane of the pyramids andthe adjoining plane of the prifm is =141° 40'.
3. The fame as var. 2, with the alternatefolid angles at the bafe of each pyramid replacedby fmall rhombic planes.
4. The fame as var. 2, with all the folid
angles at the bafe of each pyramid replaced bytrapezoidal planes.
5. The fame as var. 2, with the horizontaledges of the prifm replaced by trapezoidal,almoft parallelogramatic planes.
The fize of the cryftals varies from fmall tovery large; fome of the largeft prifms are morethan two feet in length, and weigh above100 lbs. The planes of the prifms are tranf-verfely ftriated, but thofe of the terminatingpyramids are fmooth. The cryftals are com-monly brightly fhining, both externally andinternally, with a vitreous luftre. Its fraftureis conchoidal, paffing occafionally into obfcure-ly foliated. The fragments are indeterminatelyangular, and very {harp-edged. The colourlefsor llightly coloured varieties are ufually tranf-parent, but thofe that are much loaded withiron ochre are often only tranflucent: whentranfparent it exhibits a double refradbion.
Its hardnefs is fuperior to that of glafs, andit gives vivid fparks when ftruck againft a fteel.It is brittle and eafily frangible. Sp. gr. 2.65when pure, but when deeply coloured bymetallic oxyds it is confiderably more.
When two cryftals are rubbed together theyafford a phofphorefcent light, and exhale apeculiar odour. By expofure to the blowpipethis mineral undergoes no change, except thatcertain of its varieties lofe their colour. Evena blowpipe charged with oxygen gas is whollyincapable, according to Lavoifier, of inducingtne ieaft fign of fufion. According to Ehrmannhowever, it melts into an enamel, but in thisca e ’ne quartz was probably heated on charcoal. om w nch it might acquire ootafh enough toierve as a flux. *
The purple variety, called amethyft, is com-pofed according to Role, of
97.5 Silex0.25 Alumine
0.5 Iron and a trace of manganefe
98.25
Several of the varieties of cryftallized quartzare ufed as feal-ftones and ornaments of variouskinds, on account of their hardnefs, the exqui-fite polifh that they are capable of receiving, andthe pleafing tone of their colours. The moilperfectly tranfparent and colourlefs is called bythe lapidaries rock cryfal, and befides beingapplied to various purpofes of ornament, it iscut into fpethicle glaffes, which from theirHardnefs are not fo liable to be- fcratched asthofe which are made of flint glafs. Thereddiih purple or violet coloured variety iscalled Amethyft, and is occafionally ranked a-mong the gems: it nnift not however be con-founded with the oriental amethyft, which is atrue gem, being a variety of corundum. Thepearl grey or pale blue variety is called falfe, or1 voter fappbire. The yellow and fmoke colouredvarieties are called falfe topaz. The greenvarieties are not unfrequently miftaken forchryfolite.
The largeft and moft efteemed cryftals ofquartz come from Madagafcar, alfo from theAlps, from Norway, and Scotland , where theyoccur lining cavities in granite. Amethyft iscommonly found in geodes of agate, and ischiefly procured from Catalonia in Spain ; fromBohemia, Saxony, Silefia and the Palatinate inGermany; from Hungary; from Auvergne inFrance, and the Uralian mountains near Cathe-rinenburg in Ruffia. Quartz cryftals alfo occurin moft metallic veins both of ancient andrecent formation in every kind of rock j
There is fcarcely any cryftallized mineral foliable as quartz to contain foreign fubftances;fometimes drops of water and bubbles of airmay be obferved in it, alfo fibres of amethyft,capillary cryftals of fchorl and ruthil, feales ofchlorite, and dendritical iron ore, &c.
2 Subfp. Fibrous quartz.
Its colour is yellow;(h or greyifti white, andfometimes pale amethyft. It occurs in mafs: ithas a faintly fhining vitreous luftre: its fra£Iureis thick and ftiort fibrous, either parallel, di-verging, or radiated: the radiated is compofedof globular diftinfb concretions, two inches ormore in diameter. It is tranflucent, fometimesfemi-tranfparent. In its other charadfters itagrees with the proceeding iubfpecies.