MINERALOGY. 



fummits of the ridges. I'lie hardnels Ihould, therefore, 

 be tried by a fcratch parallel to the diretlioii of the fibres, 

 or, ftill better, on the furface of the tranfverfe frafture. 

 7\nother precaution is always to feleft a fecond undecom- 

 pofed fpecimen to make a trial of the hardnefs, this cha- 

 rafter being affefted fooner than any other by the fpon- 

 taneous alteration of a mineral. In examining the relative 

 degree of hardnefs of two minerals, by trying which will 

 Icratch the other, it is neceffary to be aware that the 

 folid angles and edges of the primitive forms are very fen- 

 fibly harder than thofe of the derivative forms, or than 

 the angles or edges produced by cafual frafture, either of 

 cryftals or maflive varieties of the fame fpecies. This 

 fact has been long known to diamond-cutters, who always 

 diftinguifli between the hard and foft points of the gem, 

 that is, between the folid angles belonging to the primitive 

 octahedron, and thofe belonging to any of its modifications, 

 tlie latter being eafily worn down by cutting or rubbing 

 them with the former. 



The whole range of hardnefs obtained by the ufe of the 

 knife may be thus clafled. When a mineral does not yield 

 to the point of a knife, it may be called very hard, as quartz 

 and flint. When it yields with great difficulty, it may be 

 called hard, as felfpar. When a mineral yields more readily 

 than the former, it may be called femi-harii, as hornblende 

 and fluor fpar. When it is eafily fcratched with a knife, it 

 is called yi//, as calcareous fpar and barytes. And when it 

 yields to the nail, very foft, as gypfum and chalk. 



Tenacity. — By this property is undcrilood the relative 

 mobility of the particles of minerals, and the different degrees 

 of coherence. In fome metallic minerals, particularly native 

 gold and filver, the particles, though they cohere with great 

 force, are capable of a confiderable degree of motion, and 

 may be cut with a knife or extended with a hammer. Such 

 minerals are called malleable. When a mineral may be cut 

 into fragile (havings, orcoarfe grains, adhering to the knife, 

 it is c'AXiiA. fed'ile, as in plumbago and foap-ftone. When on 

 cutting a mineral with a knife, the particles dart off with a 

 grating noife, it is faid to be brittle. All hard minerals, and 

 the greater number of femi-hard minerals, are brittle, as 

 quartz and fluor fpar. 



Frangibilily — By this property is underftood the refiftance 

 which minerals oppofe to the ftroke of a hammer before they 

 are broken into fragments. The degrees of frangibility 

 depend partly on the cohefion of the particles, and partly on 

 the Itruftureof the mineral. Frangibility muft not be con- 

 founded with hardnefs ; many foft minerals are more infran- 

 gible than hard ones. Quartz is much harder than horn- 

 blende, but may be broken with greater facility. The brit- 

 tle minerals are the molt frangible, whilft thofe which yield 

 to the knife and are feftile are generally very tough ; and 

 the malleable minerals, fuch as native gold, can fcarcely be 

 faid to be frangible. 



A mineral is more eafily frangible by a fharp blow from 

 a fmall hammer, than by a heavier blow from a large ham- 

 mer ; hence this property appears to depend much on elaf- 

 ticity. Some earthy minerals, as beryl, flint, and topaz, are 

 more frangible when firft obtained from their native beds, 

 than when they have been expofed for fome time to the 

 atmofphere, owing to their containing a portion of moiilure 

 which IS afterwards evaporated. The degrees of frangibility, 

 from very difficultly frangible to very eafily frangible, are 

 enumerated under Okyctognosy. 



Some earthy minerals, and all malleable minerals, bend 

 without breaking, or are flexible ; and fome minerals are both 

 flexible and elaftic, as mica. 



Strudure of Minerals — This is the internal arrangement of 



the particles of a mineral. The three great divifions of ftrilc- 

 ture are, the perfcdly cryflalline, imperfeSly cryJlalUne, and the 

 promifcuous ffrudiire. The perfeftly cryflalline ftrufture is 

 defcribed under the article Crystal ; and the Wernerian de- 

 fcription of cryllallne forms will be treated of in the follow- 

 ing feftion. For the imperfectly cryflalline and promifcuous 

 flrudture, fee Structure of Minerals, where thefe impor- 

 tant characters are defcribed. The flrudture of minerals is 

 afcertained by the number of joints, or determinate direc- 

 tions in which a mineral can be fplit, or exhibits diflindt 

 laminae. This is called the cleavage by the German 

 mineralogifls. 



When a mineral fplits in one direction, it is faid to have 

 a fingle cleavage, as in mica. The cleavage may be double, 

 as in felfpar ; triple, as in calcareous fpar ; quadruple, as in 

 fluor fpar ; or fix-fold, as in blende and rock-cryftal. 



The Wernerian fyilem takes no meafure of the angles 

 under which the planes or laminae of a mineral meet, except 

 as being reitangular, equiangular, or oblique. But the 

 angular meafurement of the inclination of the planes forms 

 the bafis of Haiiy's fyfliem of cryftallography. ( See Crystal 

 and Goniometer.) According to Werner, the two-fold 

 cleavage is defcribed either as reftangular, (examples, felfpar 

 and hyacinth,) or oblique, as in hornblende. 



In the triple cleavage, the laminae may interfeCt each other 

 rectangularly, as in lead-glance or galena ; or the cleavage 

 may be oblique, but equiangular, as in calcareous fpar ; or 

 oblique and at unequal angles, as in heavy fpar ; or may be 

 partly reCtangular and partly oblique, as in felenite. 



The four-fold cleavage may either be equiangular and 

 oblique, as in fluor fpar and the diamond, or three cleavages 

 may be equiangular and oblique in the common axis of the 

 cryftal, and interfeCted by a fourth, which is at right angles 

 with the axis, as in beryl. 



In the fix-fold cleavage, all the laminae may meet under 

 equal oblique angles, as in rock-cryftal, or three of the 

 cleavages may form equal and oblique angles in a commoa 

 axis, and be obliquely interfeCted by three others, which alfo 

 interfeCt the axis in an oblique direCtion. 



Fraflure. — This property is carefully diftinguifhed front 

 the ftruClure by Haiiy. The fraCture is the cafual divifion 

 of the whole into fragments, and depends much on the kind 

 of ftroke by which it is produced, whereas the ftruCture 

 exifts in the mineral before it is broken. FraCture is either 

 conchoidal, which is compofed of convex or concave eleva- 

 tions or depreffions more or lefs regular. When regular 

 they have fmooth concentric ridges, as in many fhells ; hence 

 the name is derived. The conchoidal fraCture is diftin- 

 guifhed according to the magnitude of the elevations and 

 depreffions, into large conchoidal, as in obfidian or flint, and 

 fmall conchoidal, as in pitch-ftone. It is further diftin- 

 guifhed into deep or fiat conchoidal, and into perfeCt con- 

 choidal and imperfect conchoidal. The conchoidal fraCture 

 is charaCteriftic of brittle minerals, which have fome degree 

 of luftre and tranfparency. The uneven fraHure prefents 

 elevations which are commonly irregular and angular. This 

 fraCture is moft frequent in metalHc minerals, and in opaque 

 minerals which have fome luftre ; it paffes into fmall and 

 imperfeCt conchoidal, and alfo into earthy. 



The even fradure is that kind of furface whicli fliews the 

 feweft inequalities, and thefe inequalities are flat and never 

 fliarply defined. It paffes into large conchoidal and 

 fplintery. 



Thfifplinteryfraaure, improperly fo called, denotes a nearly 

 flat furface, on which are numerous fmall wedge-fliaped 

 fcales, adhering by their thick end. 



The earthy frahure is peculiar to opaque earthy minerals. 



