PLAIN DIRECTIONS FOR TESTING MINERALS.—Continued

Waikato Times, Volume I, Issue 28, 4 July 1872, Page 2

PLAIN DIRECTIONS FOR TESTING MlNEßALS.—Continued

t>. —To ascertain the Metal by the Color it imparts to Borax. Mauy metals impart- a color to fused borax, by which their presence can be recognised. To try this experiment a boad of fused borax must be first obtained on the platinum wire. Tho end of the wire is bent into a loop or ring about the twelfth part of an inch in diameter. The wire is then heated in the blowpipe flame and dipped whilst hot into the borax; the portion of borax that adheres is thon fused on to the wire in tho blowpipe flame, and tho hot wire is again dipped ; this is repeated until the loop contains a glass-like boad of fused borax. If the bead has become cloudy, the soot causing this may be burned off in tho oxidising point of the flame. Having thus obtained a clear, colourless, transparent bead, the next step is to add to it a minute portion of the mineral which is to bo tested. By touchiug a little of the finely pulverised mineral with the borax bead, whilo sof.enod by heat, enough will adhere to the boad for a first trial. The bead is then lcopt at a white heat in the oxidising point of tho fiame for a few seconds, and on removal its color is noted, both whilst hot; and when cold. If no color is imparted a frosh trial may bo made with a larger quantity of the powder ; but if the bead is opaque owing to the depth of colour, as is often the case, a fresh experiment must bo made, using a still smaller quantity of powder. The color can only be fairly judged in a perfectly transparent bead. If no color can be obtained in tho oxidising point, further experiment with tho borax bead is needless ; but if a color is obtained, it is then advisable to try the effect of tho reducing flame upon the same bead. The following observations and inferences may result from this test: — If the color of bead in oxidising is green (hot), blue (cold), and red in reducing, it shows tho presence of Comm. If tho colour of bead in oxidising is blue (both hot and cold), and bluo in reducing, it shows the presence of . . . Cojialt. If the colour of the boad in oxidising, is amethyst, find colorless in reducing, it shows the presence of . . . . Ma.ngane.sk. If the colour of the bead is oxidising is green, and green in reducing, it shown the presence of . . . . . Chromium. If the colour of the bead in oxidising is red or yi-llow (liot); yellow or colourless (cold), and bottle greea in reducing, it shows the presence of .... Inox. If the colour of the bead in oxidising is violet (hot); red-brown (cold), and grey and turbid, difficult to obtain, in reducing, it shows the presence of .... Nickel. This mode of testing may often be used to test the presence of the above-mentioned metals. It requires some practice before reliable results can be obtained in reducing j the reduced bead, if brought out of tho flame at a white heat into the air, may at once oxidise ; but this may be prevented by placing it inside the dark inner cone of a common candle flame, and allowing it to cool partially there. 6.—Tes'iwj with Carbonate of Soda on CharcoaL

The mode of testing with carbonate of soda on charcoal isperformed as follows : —A sound piece of charcoal half an inch square is chosen, and a neat cavity is scooped out of its surface, into which is placed a mixture containing the pulverised mineral to be tested, with three or four parts of carbonate of soda, the whole not exceeding the bulk of a pea; after lightly preesing the mixture into the cavity, the blowpipe flame may be cautiously applied to it; and afterwards when the mixture no longer shows a tendency to fly off, the charcoal may be advanced nearer to the blowpipe, and finally be kept at as high a temperature as possible in the reducing part of the flame.

This treatment is designed to extract metals from minerals ; it favors in the highest degree the removal of oxygen. Bat like tho borax test it is limited in its application ; it can only be used to detect certain metals. Tho failure of the test, in any case, must not be looked upon as a conclusive proof of the absence of the particular metal sought for. For instance, copper can be easily abstracted from carbonate of soda by this test, but not from copper pyrites. Still the test is a most valuable and indispensable one to the mineralogist The test is complete when the metal is obtained as a globule in the cavity of the charcoal ; in many cases the globule will be found surrounded with the oxide of the metal forming an incrustation on the charcoal ; and the color of such incrustation should be carefully noted, both at the moment of removal fiom the flame and after cooling. By pressing the globule between smooth and hard surfaces, it can be determined whether the metal is capable of being flattened out or crushed to pieces (brittle). The following observations and inferences result from this test:— If the globule is yellow and malleable, with no incrustation, it indicates the presence of Gold. •If the globule is white and malleable, with no incrustation, it indicates the presence of . • • • # • Silvek. If the globule is red and malleable, with no incrustation, it indicates the presence of ..... Copfeb. If the globule is white and malleable, with white incrustation, it indicates the presence of ... i Tin. If the globule is white and malleable, with red (hot) and yellow (cold) incrustation, it indicates the presence of Lead. If the. globule is white and brittle, with red (hot) and white (cold), incrustation, it indicates the presence of » Bismuth*.

If there i« no globule, but nn incrustation of yellow (hot) and whito (cold) it. iuflieufos the prcHonftc of , . Zinc, If tin; globule in white and brittle, KiviriK off fumes when removed from tin; Hiinie, with white incrustation, it indicates the presence of . . Antimony. in testing- it is necessary to bear in mind ill at a pieco of ore may consist of a single mineral only, cither pure or mixed with a readily distinguished gangne ; or it may bo composed of finely desseminafed particles of two or more metal liferoui minerals. But by nearcli in localities where such mixtures occur, pure and isolated specimens of each kind may often he found. Where this is not the case, the operator must proceed to extract a portion of the ore from tho gangue by vanning, as previously described. Moreover, after a little practice the complication of charactcrs which mixtures present, will not interfere with tho recognition of their components, Tt'Hiinu Mund'tc or Pyrilen. Perhaps no mineral is bo frequently mistaken for gold, by the uninitiated, as mundic, whether of iron pyrites or copper. Sulphur and arsenic enter largely into tho composition of pyrites, and hence its character may be readily discovered by heating a fragment of tho mineral, supported on a piece of charcoal, or held in a forceps, in the oxidising point of Iho flame, and comparing the odour which is given off; a smell of burning sulphur indicates that tho mineral contains that substance, and whito lurries having a garlic odour indicate the presence of arsenic. Mercury, antimony, and other substances may escape as fumes when heated in this manner.

iYJundic is of two descriptions, viz : — 1. Ikon Pyiutks.—The chemical composition of iron pyrites is : Iron, 4(57 ; sulphur, f>oo. It occurs in cubes and allied forms; sides often marked by fine parallel lines; also massivo. JSrass yellow ; lustre metallic ; fracture irregular ; specific gravity, 4.3 to fi.l ; hardness, 0 to G. 5 ; cannot be scratched with a knife, though quartz will scratch it; scratches glass with great facility; strikes lire with steel (lienco the origin of the term "pyrites.") IJefore the blow-pipe it burns with a blue flame, giving off an odour of sulphur, and ultimately fuses into a black magnetic globule. It is abundant in many localities, and is used as a source of sulphur ; also in tho manufacture of alum. Occasionally auriferous. It is easily distinguished from copper pyrites by its hardness, copper pyrites being easily cut with a knife. Distinguishable from gold by its hardness and in not being malleable, and in giving off sulphurous odours in tho blowpipe flame. 2. Ausknicai, Pyrites.—This mineral has a white metallic lustre. Heated before the blowpipe it gives off white arsenical fumes of a garlic odour, and ultimately fuses into a black globule. Abundant, in mining districts; sometimes auriforous. This ore and iron pyrites form tho " roundic" of miners.