MINERALOGY 



MINERALOGY 



a hardness of 5 '5 or less. The 

 thumbnail scratches those with 

 2 '5 or less. 



The specific gravity of minerals is 

 measurable with far more accuracy, 

 but, owing to impurities and inclu- 

 sions, it need not be stated for ordin- 

 ary specimens beyond two places of 

 decimals. Crystals too small for 

 suspension on a balance, and even 

 tiny fragments, the purity of 

 which may be established by ex- 

 amination with the microscope, 

 may be immersed in liquids of 

 known density. A liquid denser 

 than the mineral may be diluted 

 until the particle neither sinks nor 

 swims upon it. This method is 

 especially applicable to gems, and 

 to fragments broken from ordinary 

 rock-forming minerals, where the 

 crystals are, as frequently happens, 

 very small. 



Prof. W. J. Sollas has shown that 

 a solvent or a less dense prepara- 

 tion may be added to a column of 

 a dense liquid in a test-tube, so as 

 to produce a column the density of 

 which increases downwards. This 

 is due to the slowness with which 

 diffusion takes place. The density 

 in the column at any point can be 

 ascertained by dropping in frag- 

 ments of minerals, or glass beads, 

 the density of which has been de- 

 termined. With these as indices, 

 the density of a mineral particle 

 may be found by the level at 

 which it comes to rest in regard to 

 the levels adopted by the indices. 



Practised mineralogists at once 

 notice the relative weight of a 

 mineral specimen when they lift 

 it in the field. There is here a 

 combination of senses ; the eye 

 appreciates the volume and the 

 hand appreciates the weight. In 

 this way such an observer is not 

 likely to mistake a lump of barytes, 

 with a specific gravity of 4 "5, for the 

 similarly white and cleavable, but 

 less valuable, mineral calcite. 



The optical characters of minerals 

 are intimately related to the sym- 

 metry of their crystalline structure. 

 This symmetry is sufficient in the 

 cubic system to render minerals of 

 that system optically isotropic, 

 that is, equally affecting light rays 

 whatever their direction in the 

 crystal. Such minerals possess 

 single refraction, and behave, in- 

 deed, like amorphous substances. 

 The transparency of large masses 

 of calcite long ago called attention 

 to the double refraction of light by 

 minerals of other systems, and this 

 property is now utilised in re- 

 searches on thin slices under the 

 microscope. The translucency of 

 most rock-forming minerals in such 

 circumstances has made optical 

 characters of the first importance 

 to geologists. 



The modes of 

 occurrence of min- 

 erals are often 

 suggestive of their 

 modes of origin. A 

 choice specimen 

 may take its place 

 in the cabinet, but 

 for the correct 

 understanding of 

 it we must learn 

 its place in nature. 

 Many crystals 

 have developed 

 during the cooling 

 of molten masses 

 at the earth's sur- 

 face, or far more 

 slowly in cauldrons 

 underground. In 

 sedimentary rocks 

 the minerals may 

 have been rounded 

 by friction in the 

 beds of ancient 

 rivers or on the 

 shores of long- 

 forgotten seas. 



In the parent 

 rocks we encoun- 

 ter the lodes or 

 veins, which repre- 

 sent the infilling 

 of fissures by 

 emanations from 

 above or from be- 

 low. The main 

 material may 

 be something 

 familiar, such as 

 quartz or calcite, 

 deposited from circulating waters 

 in the crust ; but hi cavities and 

 interstices, or intimately diffused, 

 there may be ores of lead or 

 copper, or even metallic gold. 

 Often nodules or concretions have 

 arisen, looking like flattened peb- 

 bles, but sometimes of gigantic 

 size. The siderite (iron carbonate) 

 of our coalfields has collected in 

 this massive form from a state of 

 diffusion in the surrounding shales. 

 A mineral body is often a true re- 

 placement of the rock in which it 

 occurs, and its concentration has 

 been accompanied by an outward 

 diffusion of the substance origin- 

 ally on the spot. Considerations 

 such as these give mineralogy its 

 rightful place, not only with the 

 physicist, the chemist, and the 

 miner, but in the studies that re- 

 veal the natural history of the earth. 



Another point to be noted in the 

 occurrence of minerals is that 

 certain types of minerals occur 

 together with great frequency. In 

 saline deposits, for example, gyp- 

 sum, rock-salt, carnallite, and epso- 

 mite are commonly found together, 

 while in basic rocks, olivine, augite, 

 and soda lime felspars are group 

 minerals. Such groups of certain 



Mineralogy. 1. Marcasite, showing internal radial struc- 

 ture. 2. Hopper-shaped crystals of salt. 3. Haematite, 

 with nodular exterior and crystalline internal structure. 

 4. Dendritis pyrolusite. 5. Olivine crystal. 6. Pyrite. 

 7. Octahedral crystals of Magnetite in Schist. 8. Crystals 

 of Fluorspar. 9. Quartz crystals 



From specimens in S. Kensington Museum and in the Museum 

 of Practical Geology 



minerals afford a link between the 

 work of the mineralogist and the 

 geologist, for such groups of miner- 

 als have been formed by certain 

 well-defined geological processes. By 

 metamorphic action, for example, 

 garnet, tremolite, vesuvianite, etc., 

 have been formed; by dynamo- 

 metamorphism, i.e. by the effect 

 of pressure on rocks of igneous 

 origin, new minerals have been 

 formed from others, e.g. orthoclase 

 has been transformed into musco- 

 vite, olivine into tremolite, pyroxene 

 into amphibole, etc. 



Though the number of names 

 given to minerals is in the neigh- 

 bourhood of 5,000, due to the 

 erroneous naming of varieties, the 

 number of minerals known is about 

 a thousand. 



Bibliography. Minerals and How 

 to Study Them, E. S. Dana, 2nd ed. 

 1897 ; Mineralogy : An Introduction 

 to the Scientific Study of Minerals, 

 H. A. Miers, 1902 ; System of Miner- 

 alogy, J. D. Dana, with appendix by 

 E. S. Dana, 1909 ; The World's Min- 

 erals, L. J. Spencer, 1911 ; Dana's 

 Manual of Mineralogy, 13th ed.,W. 

 E. Ford, 1912; Mineralogy, F. H. 

 Hatch, 4th ed. 1912; Outlines of 

 Mineralogy, G. A. J. Cole, 1913; 

 Economic Mineralogy, T. Cook, 1921. 



