MINERALOGY. 



as chalk. The furface has a number of minute elevations 

 and depreffions, which makes it appear rough. 



The hackly fradure is peculiar to the malleable metals, 

 and confifts of fhort fharp-pointed protrudmg fibres, which 

 are fometimes only difcoverable by the feel. 



Thefe different kinds of frafture often pafs into each other, 

 and occur together : the moft prevalent one muft be taken as 

 the charafteriftic frafture. In minerals which have a cryf- 

 talline ftrufture, the true or proper frafture is that which is 

 acrofs the direftion of the planes. When cryftalhne minerals 

 are broken, the divifion taking place more readily in the di- 

 rection of the planes, the fragments have generally a tendency 

 to a regular form, as cubic, rhomboidal, &c. according to 

 the ftrufture of the mineral from which they are broken. 



ImperfeAly cryftaUine minerals break into fragments, 

 which are more or lefs regular, and contain the following 

 varieties : the wedge-fhaped, fphntery, fpecular, and tabular. 



Indeterminate fragments, from hard and brittle minerals, 

 which poflefs no cryftalline ftrufture, have fliarp edges and 

 angles. In other minerals, the angles and edges are more or 

 lefs blunt in proportion to their foftnefs and toughnefs. 



External form or fiape of minerals is either indejtnite, 

 definite, or cryflalllne. 



The indefinite or amorphous, called by Mr. Jamefon the 

 common external (hape of a mineral. This charafter is 

 applied when a mineral exhibits no appearance of regular 

 planes or lamins, nor any refemblance to well-known 

 natural or artificial bodies. When the mineral forms a 

 thin coat or cruft on other minerals, it is called fnperjicial 

 or invefting, which is common to friable or pulverulent 

 minerals. Another variety is called plated or membrana- 

 ceous, where the mineral forms thin membranes or flakes 

 not exceeding in thicknefs common paper. When the three 

 dimenfions are not verj' different from each other, if the bulk 

 is not confiderable, the mineral is faid to be in pieces, which 

 may be either angular or rounded. If the bulk of an amor- 

 phous mineral be confiderable, it is called majfive. An enu- 

 meration of Werner's common external forms is given under 

 Oryctognosy ; which fee. 



The definite form, or particular external {hape, appears in 

 many inftances to be derived from cryftaUization modified or 

 difturbed by other caufes. According to Mr. Aikin, many 

 of the definite forms have evidently been occafioned by 

 matter in a femi-fluid ftate having been expofed to the fimul- 

 taneous aftion of cryftaUization, concentric attraftion, and 

 gravitation. To cryftaUization is owing the minute ftruc- 

 ture in ihort prifms or fibres laterally aggregated ; to the 

 concentric attraftion it is owing that each of thefe fibres 

 converges towards a real or imaginary centre, forming a 

 cnrved thick plate of the whole, or feveral plates in fuccef- 

 five coats, like the ftrufture of the onion ; and laftly, it is 

 owing to gravitation that thefe concretions do not form per- 

 feft fpheres, but are more or lefs elongated into the mamil- 

 lary, the reniform, the botryoidal, and the ftalaftitic varieties. 

 Of particular external forms, a great variety are enumerated. 

 (See Oryctognosy.) The definite form that approaches 

 neareft to the regular cryftalline, is the arborefcent or dendritic : 

 it bears a near refemblance to a vegetable fpray ; hence its 

 name. On minute exammation, it will, however, be found 

 to confift of cryftals occafionally very perfetl, implanted 

 one into another, and branching in different direftions. 

 Certain varieties have obtained particular names, as reti- 

 culated or reniform, when the branches interfeft like the 

 mefhes of a net ; and peRinated, when a number of (hort 

 branches rife parallel to each other, at nearly equal dif- 

 tances, on the fame fide or on oppofite fides of a main 

 branch, as in a comb. 



The cryflalline form of minerals is called by Mr. Jamefon 

 the regular external thape. When a mineral occurs cryftal- 

 lized in a fimple form which has received a name in geome- 

 try, as the cube, the rhomboid, the oftahedron, &c. it is eafy 

 to give an idea of it by referring it to that form ; but when 

 a cryftal prefents a great number of unequal planes, or is 

 very complicated, the defcription becomes difficult v.ithout 

 a drawing or model. Mr.Werner has, however, confider- 

 ably facilitated the mode of defcribing cryftals by confider- 

 ing them as modifications of certain fimple forms ; and this 

 mode, though not ftriftly fcientific, is found moft con- 

 venient in praftice. 



The fmiple forms, or what he calls the fundamental 

 forms, are, the cube {fig.l.); the rhomboid {fig. 2.); the 

 prifm, which may have three, four, or a greater number of 

 fides {figs. 3, 4, 5, &c.) ; the pyramid, which may have 

 three, four, fix, or eight triangular planes (j%.6.) ; the 

 table, which has two equal and parallel planes, which are 

 very large compared with the thicknefs of the table, and 

 is bounded by an indeterminate number of fides {fgs. 7 and 8. ) 

 The three following forms are very rare. The icofahedron, 

 having twenty equilateral planes {fig.<).) ; the dodecahedron, 

 having twelve pentagonal faces {fig- 10.); and the lens, 

 which has two curved faces {fig. 1 1.) 



Mr. Aikin is of opinion, that the number of fimple forms, 

 or models, to which almoft all cryftals can be referred, may 

 be reduced to four. The prifm, the rhomboidal dodecahe- 

 dron {fig. 12.), the regular tetrahedron ( fig. 13.), and the 

 double pyramid formed by two equal and fimilar pyramids 

 joined together by a common bafe. The pyramid, hke the 

 prifm, may have a greater or fmaller number of fides, and 

 the edges of the bafe of each pyramid may be in the fame 

 plane, as infg. 14, or fet on obliquely, as mfg. 31. 



Thefe forms, or models, it muft be carefully noticed, have 

 no connexion with the true primitive forms of cryftals (fee 

 (Crystallography', ylddenda), but are merely adopted as 

 convenient types for the defcription of crj'ftals. The changes 

 which thefe forms are fuppofed to undergo by truncation and 

 bevelment may take place either on the edges or folid angles 

 of the cryftal. As the prifm and the cube are the moft com- 

 mon forms of cryftals, we fhall proceed to defcribe them 

 modified by thefe changes. The prifm, as we have before 

 obferved, may have feveral fides, and may be triangular, or 

 reftangular, as in fig. 3 ; oblique, as injig. 4 ; or polygonal 

 and equiangular, as infg. 5. The fides are technically called 

 the lateral planes, they are parallel to and furrounding an 

 imaginary axis. The bafes at each extremity of the prifm 

 are called the terminal planes. The lateral edges are formed 

 by the junAion of two contiguous fides or planes, and the 

 terminal edges are formed by the junftion of the lateral 

 planes with the bafe or terminal planes, and the folid 

 angles are formed at the point of junftion of the ter- 

 minal planes with the lateral planes. The cube may alfo 

 be defcribed as a (hort reftangular four-fided prifm. When 

 a folid angle is removed and one plane is formed in its place, 

 as in Plate Yll.fig. 16, the cryftal is faid to be truncated on 

 the angles. When planes are formed on the edges of a cryftal, 

 as in ^^. 17, it is defcribed as truncated on the edges: 

 and when two planes are formed on an edge of the cryf- 

 tal, as in ^^. 18, it is faid to be bevelled on the edges. 

 Fig. 19. is a three-fided prifm bevelled on its lateral edges. 

 When the fohd angles of a cryftal, or the termjnal planes, 

 appear cut off by three or more planes converging to a 

 point, the cryftal is faid to be acuminated : in fig. 20. each 

 angle of the cube is acuminated by three fmall planes fet 

 on the lateral planes ; and in fg. 2 1 . each angle is acumi- 

 nated by planes fet on the lateral edges. Fig. 22, according 

 3 Z 2 to 



