CHAPTER VII. BLOWPIPE-TESTS USEFUL IN THE DETERMINATION OF THE selection of minerals here made includes some that are of far rarer occurrence than others, but which yet are the more common representatives of certain chemical constituents of the earth's crust. It also excludes a number of important minerals, particularly silicates, which must be recognised by physical characters or more complete chemical analysis. The group, for example, of the felspars thus obtains little prominence; but the later section devoted to Szabó's flame-reactions does them, it is hoped, some measure of justice. The worker is presumed to have before him some handy text-book of mineralogy, and questions of colour, hardness, &c., are thus omitted from these notes, except where especially important for distinction. Should the mineral under examination not correspond with any in the following series, reference to the text-book under the head of those that agree with it most closely will generally complete its determination. An alphabetical order has been adopted; but an index at the end of the list serves to connect the ores of the same metal one with another. The abbreviations used are the same as those in the preceding section. The chemical formula is given after each name. la. Alunite (see notes on Websterite). 1. Anglesite. PbSO, O. tube-Fuses, and on prolonged heating forms a slight white sublimate. Ch.-Lead encrustation. Fuses and reduces easily to metal. With soda, sulphur reaction. Add.-Despite its appearance, the specific gravity (= 6·2) suggests the presence of a heavy metal. 2. Anhydrite. CaSO. Flame-Calcium, with HCl. Fus. -About 2.5. Cl. tube-No water. Ch. With soda, sulphur reaction. Add.-Hardness 3 or more; that of gypsum Soluble in H Cl. = = 2. 3. Anorthite. Ca Al, Si, Og. Flame-Calcium, on decomposition with HCl. Fus.-Nearly as high as orthoclase. Micr.Silica. Add.-Best treated by Szabo's method. Decomposed by HCl. Specific gravity about 2.75. = 4. Antimonite. Sb, Sg. Flame-Smoky green. Fus.-1. Cl tube-Red sublimate of antimonous sulphide, darkening to black at base; white oxides, and sometimes sulphur, above. O. tubeSimilar; dense white oxides. Ch. Similar products; in the end completely volatilised. With soda, sulphur reaction. 2 5. Apatite. 3 Ca ̧ P2Og + Ca(Cl2, F2). Flame-With sulphuric acid, green (phosphorus). Fus.-Near 5. Cl. tube-With magnesium, phosphorus reaction. Add.-Soluble in strong H Cl. A drop of sulphuric acid added to the solution precipitates microscopic crystals of gypsum. Treated with nitric acid and ammonium molybdate solution, gives strong yellow precipitate. Small fragments may be thus dealt with on a glass slip. 2 6. Apophyllite. H, Ca Si, O + H, O, with some Fl. FlameWith blue glass, good potassium. Fus.-Easy, with intumescence. Micr.-Silica. Cl. tube-Water. Add.-With H Cl gives gelatinous silica in lumps. 3 7. Aragonite. Ca CO, (Rhombic form). Flame-With H Cl, strong calcium. Fus.-Infusible. Add.-Effervesces freely in cold HCl. The solution, if greatly diluted, gives no precipitate on addition of sulphuric acid, even after long standing. Distinguished thus from strontianite. Distinguished from calcite by its specific gravity (= 2·93) and slightly superior hardness. 8. Argentite. Ag, S. Fus.-Easy. Bor. and Micr.-Silver reactions. Ch.-Silver bead. With soda, sulphur reactions. Add.-Sectile and malleable. Distinguished from stephanite by absence of antimony. 2 9. Atacamite. Cu Cl2+ 3 H2 Cu O2 Flame-Burns luminously, and gives a blue flame near the assay (copper chloride) and copper green beyond. Bor. and Micr.-Copper reactions. Cl. tube-Water. Forms yellow-brown and greenish deposits at lower end of tube. Ch. Similar deposits on the charcoal. Copper bead. io. Augite. (Ca, Mg, Fe) Si Og with some Al, Og and Fe,Og.* Fus.-About 3.5. Micr.-Silica. Add.-Can be just scratched with a knife. Prism-angle of 87° distinguishes the pyroxenes from the amphiboles. 11. Azurite. 2 Cu CO2+ H2 Cu Og Flame With HCl, copper colours. Fus.-2. Bor. and Micr.-Copper reactions. Cl. tube-Blackens; water given off. Ch. Copper bead. Add.-Effervesces freely in hot H Cl. Distinguished from malachite by blue colour; from chalcanthite by absence of sulphur and insolubility in water. 12. Barytes. Ba SO Flame-Barium green. Fus.-About 3; commonly decrepitates. Ch.-With soda, sulphur reaction. * See Hintze, Mineralogie, Bd. ii., p. 958. Add.-Specific gravity = 4.5, a character noticeable even in small specimens. 13. Bismuth. Bi. Fus.-Very easy. O. tube-Fuses and forms white to pale yellow sublimate; if potassium iodide and sulphur are fused with it, a vermilion sublimate results. Ch.Bismuth encrustation, which becomes a strong red when potassium iodide and sulphur are added to the assay. These materials should be powdered up with the bismuth. Fuses to a brittle but slightly malleable bead, which does not mark paper. Bismuthine (Bi, S3) gives similar reactions, but with evidences of sulphur. 13a. Blende (see Zinc Blende). 14. Bornite. (Cu2, Cu, Fe) S. Flame-Copper. Fus.-Easy. Bor. and Micr.-Copper reactions. Ch. Fuses to a magnetic globule. With soda, copper beads in a black magnetic residue (iron). Sulphur reaction. Add.-Distinguished by purple-red tarnish from copper pyrites; yellowish when freshly fractured. 15. Brucite. H, Mg O2 Fus.-Infusible; becomes opaque. Cl. tube-Water. Ch. With cobalt nitrate gives a somewhat poor magnesium reaction. Compare with talc. 16. Calamine (Smithsonite of Beudant). Zn COg. Ch.-Zinc encrustation, which, with the residue, becomes a fine green with cobalt nitrate. Add.-Effervesces in H Cl. Distinguished from hemimorphite (calamine of Brongniart) by not yielding gelatinous silica. 17. Calcite. Ca CO, (rhombohedral form). Like Aragonite. Add.-Specific gravity= 2.72. Perfect rhombohedral cleavage very marked, even when fragments are examined with a lens. 18. Cassiterite. Sn O. Micr.-Often some silica. Ch.-Tin encrustation, blue-green with cobalt nitrate; near the assay, and very characteristic if examined when cold. Powder with one part of charcoal and two parts of soda, to obtain good metallic beads. The metallic bead, treated alone on charcoal in O. F., oxidises and turns white, unlike lead or silver. Add.-Specific gravity about 6.8, an important character when examining the dull brown pebbles of Stream Tin. = 19. Celestine. Sr SO4. Flame-Strontium. Ch. With soda, sulphur reaction. Add.-Distinguished from gypsum, should the flame be doubtful, by hardness = 33.5 and specific gravity = 3·95; also absence of water, and insolubility in HCl. The latter character distinguishes it from anhydrite. 20. Cerussite. Pb CO. Flame-Lead. Cl. tube-Becomes yellow on cooling (lead oxide). Ch.-Lead encrustation; rapidly reduced to metal. Add.-Effervesces in Fus.-Very easy. hot H Ol. Specific gravity=64 when crystallised, suggesting, in spite of its appearance, the presence of a heavy metal. 20a. Chalcopyrite (see Copper Pyrites). 21. Chalybite. Fe C Og. Fus.-Near 5; blackens. Bor. and Micr.-Iron reactions. Cl. tube.-Blackens; magnetic residue. Ch.-Ditto. Add.-Effervesces in hot H Cl. 22. Chloanthite. (Ni, Co, Fe) As, Fus.-Easy. Bor.-Cobalt reaction. If much iron is present, the bead will be green when bot. To obtain evidence of the nickel, prepare a large wellcoloured borax bead and transfer it to charcoal; oxidise for some time; separate the blue glass from the metallic residue by breaking the bead, wrapped in paper, on the anvil; fuse the residue again with more borax on the charcoal, and repeat the operation until the bead becomes brown (nickel) or colourless (no nickel present). To confirm this result, treat the residue now with microcosmic salt, which will show the characteristic yellow due to nickel in O. F. If copper is also present, the microcosmic salt bead will be green, and will become red in R. F. on addition of tin. Where the quantity of nickel is small, a gold button weighing about 75 milligrammes should be fused with the borax bead on charcoal. This withdraws the nickel and any copper, and, after fusion with fresh borax to remove all cobalt, gives with microcosmic salt the nickel or nickel and copper reaction. The colours of beads thus treated on charcoal can always be examined by picking up some of the hot material on platinum wire. Cl. tube-Arsenic sublimates. Ch.-Abundant arsenic encrustation. 23. Chromite. (Fe, Cr) (Fe, Cr2) O4, often with MgO and Al, Og. Fus.-Practically infusible; becomes magnetic in R. F. Bor. and Micr.-Fine chromium reactions; mingled with those of iron when hot. Ch.-In R. F. somewhat feebly magnetic residue. 24. Chrysocolla. Probably H,CuSiO+H,O. Flame-Copper. Fus.-Infusible. Bor. and Micr.-Copper reactions. In latter, cloudy silica. Cl. tube-Water; becomes black. Ch.-With soda, metallic copper. Add.-With H Cl, separation of powdery silica. Commonly some carbonate present. 25. Cinnabar. Hg S. Cl. tube-Black sublimate, which has a red streak. With soda added, yields globules of mercury. Ch.Volatilises. With soda, sulphur reaction. With potassium iodide and sulphur, faint yellow encrustation. Add.-Red streak. 26. Cobaltine. Co As2 + Co S2 (= Co As S). Bor. and Micr.Cobalt reactions. Cl. tube-Fuses. Arsenous sulphide, sulphur, white oxide, and sometimes some metallic arsenic. O. tubeWhite oxide and some sulphide. Ch.-Arsenic encrustation. 27. Copper Pyrites (Chalcopyrite). Cu Fe S2. Flame-Copper colours with HCl. Fus.-Easy. Bor. and Micr.-Copper reactions; green in O. F. when hot, owing to presence of iron. Cl. tube-Some sulphur. Ch.-Fuses to a magnetic globule. Roast in O. F., and then reduce; a copper bead separates in the mass. Soda only obscures the reaction. Add.-Hardness = 3.5; easily distinguished thus from iron pyrites, which cannot be scratched by the knife. 27a. Copper Glance (see Redruthite). = The 28. Corundum. Al, Og. Fus.-Infusible. Micr.-Very slowly soluble; often some silica. Ch.-With cobalt nitrate the residue becomes pale blue (alumina). Add.-Hardness = 9; specific gravity 4. These characters are both important when dealing with rough forms, such as the pebbles found in streams. parting-planes are generally traceable even in these. 29. Cryolite. 6 Na F+ Al2 F6 ( Nag Al2 F12). Flame Sodium. Fus.-Very easy. Cl. tube-Fluorine reaction with sulphuric acid. Ch. After thorough heating, the residue gives alumina reaction with cobalt nitrate. Add.-Fused with microcosmic salt on a glass slip, leaves a dulled and etched area when the slip has been washed and dried. Distinguished from fluorspar by its lower hardness (= 2·5) and its easy fusibility. = 30. Cuprite. Cu, O. Flame-Copper colours with H Cl. Fus. -Easy. Bor. and Micr.-Copper reactions. O. tube-Blackens (Cu O). Ch.-Copper bead in R. F. easily obtained. Add.Soluble in H Cl. Streak red. 31. Dolomite. (Ca, Mg) CO,. Orystallised dolomite should be Ca O Og + Mg CO,. Flame-With H Cl, calcium. Fus.-Infusible. Add.--Effervesces in hot HCl, calcite effervescing freely in cold. Intermediate forms give intermediate re actions. To the H Cl solution add slight excess of ammonia, and then solution of hydric disodic phosphate (1 part of the salt to 10 of water). Allow to stand for some time. If magnesia is present, a minutely crystalline precipitate of ammonic magnesic phosphate will be formed. The specific gravity of dolomite is about 2.85, calcite being 2.72; the hardness is between 3·5 and 4. 32. Epidote. H, Ca, (Al, Fe), Sig O26 fusible than actinolite. Intumesces somewhat. Add.-Hardness 6.5, that of the amphiboles being 5.5. = Fus. Slightly more |