MINERALOGY 



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MINERALOGY 



MINERALOGY: SCIENCE OF MINERALS 



Orenvtlle A. J. Cole. P.R.8.. ProfeMor ot 

 Geology, Royal College ot Science. Dublin 



This Encyclopedia has articles on all the important minerals, e.g. 

 <>ind ; Gypsum ; Iron, etc. See also Crystallography ; 

 Geology; Rocks 



portance. Crystalline material Is 

 requisite, and, if possible, the work 

 should be done on one or more 

 actual crystals showing traces of 

 external form. These can be 

 scrutinised for possible inclusions, 

 and here a polished surface of a 

 thin section examined under the 

 microscope is of service. 



The results of analysis have in 

 many cases been checked by 

 synthesis, i.e. the production of the 

 mineral artificially from known 

 quantities of pure chemicals. In 

 this way certain anomalies re- 

 vealed by analysis have become 

 ascribed to the solid solution of 

 some substance in the others that 

 determine the fundamental char- 

 acters of the mineral. The excess 

 of silica hi natural nepheline when 

 compared with the corresponding 

 artificial sodium -aluminium or 

 potassium-aluminium silicate, has 

 been thus explained. The anoma- 

 lous sulphur in pyrrhotine, min- 

 eral ferrous sulphide, provides an- 

 other example. 



Grouping of Molecules 



The same chemical molecules, 

 however, may group themselves 

 variously under various conditions, 

 and may produce a series of crystals 

 which are not related to one an- 

 other under the fundamental law 

 of crystallography. Commonly this 

 difference of structure is accom- 

 panied by a difference in specific 

 gravity and in hardness. Two or 

 even more distinct mineral species 

 may thus possess the same chemi- 

 cal composition. 



In cases of polymorphism, where 

 the same substance appears in two 

 forms (dimorphous), three forms 

 (trimorphous), or even more, one 

 form is usually far more commonly 

 produced in nature. The pre- 

 dominance of calcite over aragon- 

 ite, which is a good example, is 

 further emphasised by the change 

 of the latter mineral into the for- 

 mer in geological time. 



It is clear that an amorphous 

 mineral cannot be regarded as 

 an independent mineral species. 

 Separate names, however, must be 

 assigned to amorphous materials, 

 since we cannot speak, for ex- 

 ample, of amorphous quartz, but 

 only of amorphous silica. 



One of the most beautiful results 

 of the measurement of the angles 

 of crystals (goniometry) is the dis- 

 covery that differences of chemical 

 constitution involve differences of 

 crystalline form ; while even the 



is the science of 



initu nls, and a mineral may be 

 strictly il-timd as an inorganic 

 Milt;mcc with a constant chemical 

 i->.ni|i.i-iii.iii. or a composition that 

 i inly by the partial substitu- 

 tion of one element for another 

 under recognized chemical laws. 

 I'mlrr favourable conditions, a 

 mineral ussumes a characteristic 

 Him- form. 



This phenomenon of crystallisa- 

 tion at once distinguishes a mineral 

 from a rock, and excludes from 

 scientific classification as minerals 

 smli substances as mineral oils, 

 coal, and slate. Natural alloys 

 are regarded as minerals, as also 

 are the constituents of a mineral 

 occurring in their proper propor- 

 tions, but in an uncrystalline or 

 amorphous state. Natural glass 

 (obsidian) is not a mineral, since 

 it contains the constituents of 

 several species, which would have 

 separated out from one another 

 under suitable conditions. 



The classification of minerals by 

 cl(l(r writers was hampered by 

 the slow development of chemical 

 science and methods of analysis, 

 and the proper appreciation of 

 minerals dates only from the last 

 half of the 18th century. Miner- 

 alogy, however, is not merely a 

 classificatory science, but one of 

 immense philosophic interest. It 

 is the natural history branch of 

 chemistry. Since we receive only 

 meteoritic substances from beyond 

 the earth, even our organic com- 

 pounds have those found in minerals 

 as a basis. Radioactivity, more- 

 over, with all the questions raised 

 by it as to the succession of ele- 

 ments in time, is fundamentally 

 concerned with minerals. 

 Chemical Examination 



The most important character of 

 minerals is undoubtedly chemical 

 composition, and other characters 

 depend very largely upon this. 

 Since these characters are often 

 more easily determinate than the 

 composition, many minerals can 

 now be identified without analysis. 

 A qualitative chemical examina- 

 tion can, however, be made with 

 ease in the case of common 

 minerals, and especially of metallic 

 ores, which often serves for identi- 

 fication, and the mineralogist or 

 prospector who is an adept in the 

 use of the blowpipe has here a 

 notable advantage. 



For quantitative work, the purity 

 of a specimen is of the first im- 



substitution of one element for 

 another to an extent that doe* not 

 warrant the erection of a separate 

 species, such as the substitution of 

 calcium for sodium, in many fel- 

 spars, produces slight change* in 

 the angles of forms that remain 

 very similar in appearance. A 

 number of minerals have been 

 styled isomorphous on account of 

 their close similarity in crystalline 

 structure, which can commonly be 

 traced in such cases to similarity in 

 molecular structure. 



Planes of Cleavage 



The fact that many crystalline 

 minerals possess regular planes 

 along which they fracture, a pro- 

 perty styled cleavage, is of great 

 utility in determinative work. 

 Quartz has no Cleavage, breaking 

 with a curving fracture like glass ; 

 other transparent and colourless 

 minerals which show cleavage 

 when struck cannot, then, be mis- 

 taken for this very common sub- 

 stance. Planes of cleavage, more- 

 over, are related to the crystalline 

 structure of the mineral Where 

 two or more series of such planes 

 occur, the angles between them 

 are angles which might be pro- 

 duced by the same substance in its 

 external form. Hence, many frac- 

 tured and imperfect specimens 

 serve just as well as good crystals 

 for purposes of measurement. 



A broken mass of calcite, in the 

 hands of the pioneer mineralogist 

 Haiiy, led to the recognition of the 

 fundamental law of crystallisation 

 in mineral species. The remark- 

 able cleavage of mica, whereby it 

 can be split into thin elastic sheets, 

 ia well known through its com- 

 mercial applications. 



Crystallographic studies are, 

 however, for the laboratory. The 

 hardness of a mineral, and, with the 

 aid of very simple instruments, its 

 specific gravity, can be readily de- 

 termined in the camp. Hardness is 

 roughly stated by comparison with 

 that of certain well-known minerals. 

 This is the method of Mohs, who 

 selected the following, arranged in 

 ascending order : 1, talc ; 2, rock- 

 salt or gypsum ; 3, calcite ; 4, 

 fluor-spar ; 5, apatite ; 6, ortho- 

 clase felspar ; 7, quartz ; 8, 

 topaz ; 9, corundum ; 10, dia- 

 mond. The intervals between the 

 hardnesses hi this scale of minerals 

 are known to be very unequal; 

 nevertheless, it serves well enough 

 for reference. A mineral which 

 scratches orthoclase and is also 

 scratched by it has a nominal 

 hardness of 6 ; one that scratches 

 apatite and is scratched by ortho- 

 clase has a hardness stated as 5'5. 

 A steel knife scratches, with vary- 

 ing degrees of readiness that are 

 soon appreciated, all minerals with 



