The Elements of Blowpipe Analysis Part 6

BEHAVIOR OF SOME OF THE PRINc.i.p.aL ORES BEFORE THE BLOWPIPE

For the sake of practice, and as a fitting introduction to "Determinative Mineralogy," this chapter is appended. It is not intended to give a detailed account of the minerals, but rather to set before the student the most marked characters, such as hardness, specific gravity, color, l.u.s.tre, etc.

To determine the hardness of a mineral, we try to scratch it with the minerals forming an arbitrary "scale of hardness," proceeding successively from the softest to the hardest. When we say that a certain mineral has hardness = 4, we mean that the mineral is scratched by 4 on the scale, and that 4 on the scale is scratched by the mineral. The scale of hardness chiefly in use is the Mohs-Breithaupt scale, which is as follows:--

1. Talc, common laminated light green variety.

2. Gypsum, crystallized.

3. Calcareous spar, transparent variety.

4. Fluor spar, crystalline.

5. Apat.i.te, transparent.

6. Orthoclase, white cleavable variety.

7. Quartz, transparent.

8. Topaz, transparent.

9. Sapphire, cleavable variety.

10. Diamond.

It seldom happens in determining the hardness of a mineral that its hardness exactly conforms to that of some one member of the scale. In such cases we generally estimate the hardness. For example, suppose a mineral was harder than 4, but softer than 5, and that it was nearer 5 than 4, then we would call its hardness 4-3/4.

In order to preserve the scale some operators use a three-cornered file, first cutting the mineral and then the scale until a number is found, which is abraded to about the same depth as the mineral under examination.

Since a set of minerals forming a scale of hardness is not always at hand, the following scale given by Chapman is appended:--

1. Yields easily to the nail.

2. Yields with difficulty to the nail or just receives an impression from it. Does not scratch a copper coin.

3. Scratches a copper coin but is also scratched by it, being of about the same degree of hardness.

4. Not scratched by a copper coin. Does not scratch gla.s.s.

5. Scratches gla.s.s with difficulty, leaving its powder on it. Yields readily to the knife.

6. Scratches gla.s.s easily. Yields with difficulty to the knife.

7. Does not yield to the knife. Yields to the edge of a file, though with difficulty.

8, 9, 10. Harder than flint.

Specific gravity cannot well be determined without the aid of a balance, and hence its value here is not great.

As in the preceding chapter, alphabetic arrangement will be employed.

ORES OF ANTIMONY

~Stibnite~, Sb_{2}S_{3}, Sb . 71, S . 29.--[A]H = 2, G = 4.52-4.62. Of lead-gray color and metallic l.u.s.tre. Consists of a large number of needle-shaped crystals. Brittle. Fuses in candle flame. In an ignition tube yields a sublimate of sulphur. On Ch. before the Bp. it is volatilized, giving antimony coating and tinges the flame pale blue.

[Footnote A: H = Hardness, G = Specific Gravity.]

ORES OF a.r.s.eNIC

~Native a.r.s.enic, As.~--This contains traces of Sb, Ag, Fe, Co, and Ni.

H = 3.5, G = 5.7-5.8. Dark gray in color. Fracture tin-white, tarnishing rapidly. Volatilizes before the Bp. on Ch. without melting, giving white coating of a.r.s.enious acid and characteristic garlic odor. In ignition tube it sublimes, giving a.r.s.enical ring.

~Realgar~, a.s.s, As . 70, S . 30.--H = 1.5-2, G = 3.56. Bright red to orange-red color and resinous l.u.s.tre. In an ignition tube it fuses and finally sublimes. The sublimate when cool is red and transparent. Fuses readily before the Bp. on Ch. and burns with pale yellowish flame, emitting gray-white fumes having garlic odor.

~Orpiment~, As_{2}S_{3}, As . 61, S . 39.--

H = 1.5-2.0, G = 3.4-3.5.

Lemon-yellow in color and resinous or pearly l.u.s.tre. Sectile. Before the Bp. on Ch. behaves like realgar, but in an ignition tube it gives a dark yellow sublimate which is transparent.

ORES OF BIs.m.u.tH

~Native Bis.m.u.th, Bi.~--This contains traces of As, Te, and S.

H = 2.0-2.5, G = 9.7-9.83. Color, silver-white, slightly tinged with red. Metallic l.u.s.tre. Brittle when cold, but may be laminated when hot.

Before the Bp. on Ch. behaves like pure Bi.

~Bis.m.u.thite~, Bi_{2}O_{3} . 90, CO_{2} . 7, H_{2}O . 3,--

H = 4.0-4.5, G = 6.9-7.8.

Usually of a white or light greenish color and vitreous l.u.s.tre, in acicular crystallizations. In an ignition tube decrepitates, yielding water and turning gray. Before the Bp. on Ch. it fuses easily and is reduced to metallic globule, coating the Ch. with Bi_{2}O_{3}. With Na_{2}CO_{3} it occasionally gives the sulphur reaction.

ORES OF CHROMIUM

~Chromic Iron Ore~, FeO . 32, Cr_{2}O_{3} . 68.--Al_{2}O_{3}, Fe_{2}O_{3}, MnO, and MgO are commonly present. H = 5.5, G = 4.32-4.57. Occurs usually ma.s.sive. Color, iron-black to brownish black. In many varieties strongly magnetic. l.u.s.tre, shining and somewhat metallic. Heated in an ignition tube, remains unchanged. Infusible before the Bp. on Ch.

Before the Bp. on Ch. with Na_{2}CO_{3} and KCN yields metallic iron. In borax bead it slowly dissolves to a clear transparent gla.s.s, which is a beautiful green when cool.

ORES OF COBALT