450 



MINERALOGY. 



Oryttogno- 



y- 



The genu- 

 ineness of 

 cryMali. 



A. The Genuineness of Crystals. 



This refers to the division of crystals into True and 

 Supposititious. The true are the forma which the same 

 substance always assumes, and which are peculiar to 

 it ; the supposititious are those regular figures whose 

 shape does not depend on the substance of which they 

 are composed, but is owing to pre-existing crystals, or 

 crystal-moulds. 



Supposititious crystals are formed in two ways: 



1. When an imbedded crystal falls out and leaves an 

 empty mould, which is afterwards filled up with fos- 

 sil matter, a figure or crystal corresponding in shape 

 to the mould is formed. The supposititious crystals 

 formed in this manner are smoother, and have sharp- 

 er edges and angles than the succeeding kind, and 

 their interior is often hollow and drusy. 



2. When a mineral is deposited over a pre-existing 

 crystal, and assumes its figure, the second kind of 

 supposititious crystal is formed. The pre-existing 

 crystal either remains, forming the nucleus, or it dis- 

 appears when the supposititious crystal is hollow. 

 It differs from the first kind in having generally a 

 rough and drusy surface, blunter edges and angles, 

 and the inner surfaces smooth. 



The first kind of supposititious crystal is a cast or 

 filling of the space formerly occupied by true crystals ; 

 the second is merely an incrustation of true crystals *. 

 True and supposititious crystals are distinguished 

 from each other by the following characters : 

 a. True crystals. 



a. Are transparent and semi transparent. 

 ft. Their planes are smooth and shining or splen- 

 dent, or they are regularly streaked, 

 y. Their angles and edges are sharp. 

 3. Are seldom drusy upon the surface ; but when 

 this is the case, it is remarked that all the axes 

 of the small crystals forming the drusy surface 

 are in the same position. 

 i. They form particular characteristic suites. 

 i. Supposititious crystals. 



*. The planes are never smooth and shining, or 

 regularly streaked j on the contrary, are gene- 

 rally rough and dull. 



ft. The angles and edges are not so sharp as in 

 true crystals, but are generally somewhat round- 

 ed. 



y. They are usually hollow, and their internal sur- 

 face is drusy. 



i. They are almost always drusy, internally, in 

 those formed by moulding ; externally, in those 

 formed by incrustation ; and it is remarked, 

 that the small crystals forming the drusy sur- 

 faces are disposed irregularly. 

 f. They are not, like true crystals, connected by 

 transitions with other crystals of the same 

 species : Thus the octahedral supposititious 

 crystals, of quartz, which originate from fluor- 

 spar, do not belong to the suite of quartz. 

 . Even in their internal structure they are differ- 

 ent from true crystals ; for they seldom present 

 a fracture inclining to foliated. 

 11, Single crystals are never all around crystallized. 



The following are well known instances of suppositi- Oryctojno. 

 tious crystals. y- 



1. Octahedral crystals of quartz, originating from s- ^<^**' 

 fluor-spar. 



2. Cubic crystals of quartz, from fluor-spar. 



3. Flint in double three-sided pyramids, from calca- 

 reous-spar. 



4. Quartz in oblique four-sided tables, from heavy- 

 spar. 



B. The Shape of Crystals. 



The shape of crystals is determined by the number The hpe 

 and form of the planes or faces, and the edges and of crystals, 

 angles which form the contour or outline f. Amidst 

 the great variety of crystals that occur in the mineral 

 kingdom, there are some simple ones, which are com- 

 posed of but few planes, that do not vary much in 

 shape ; and of others, in which the planes are not only 

 numerous, but present great differences in form. 

 These simple forms are nearly allied to the more com- 

 plex ones, and gradually pass into them by a change in 

 the shape of their planes. On this circumstance Wer- 

 ner has founded a crystallographic system, remarkable 

 for its simplicity, and the ease with which it enables us 

 to acquire distinct conceptions of the most complicated 

 crystallizations. He considers these simple forms as 

 the basis of the others, and names them Fundamental 

 Jigure*. We can distinguish in them one, or at the 

 utmrst, two sets of planes, which run in two directions, 

 and enclose the crystal on all sides. The cube is an 

 example of a fundamental figure with one set of planes ; 

 the prism, pyramid and table, are examples of funda- 

 mental figures with two sets of planes, which are named 

 lateral and terminal planes. All those crystals in which 

 we observe many different sorts of planes, he considers 

 as changed or altered fundamental figures ; and names 

 the other planes which are generally smaller, and differ 

 from the planes of the fundamental figure in direction, 

 and in being further removed from the centre of the 

 crystal, Alterating planes. We have thus, according to 

 this method, to consider, first, The Fundamental Fi- 

 gures, and then their Alterations or Modifications. 



I. THE FUNDAMENTAL FIGURES. 



The fundamental figures, as already mentioned, are The fund- 

 composed of one or two sorts of planes. In order to mental fi- 

 discover these planes in the altered fundamental 8 ures - 

 figures, we have only to conceive the planes that lie 

 nearest the centre of the crystal, and which are gene- 

 rally the largest, extended on all sides until they join. 



In the fundamental figure are observed and attended 

 to, I. Its Paris. II. Its Varieties or Kinds. III. The 

 Differences of each Fundamental Figure in Particular. 



I. Parts of (he Fundamental Figures. 

 The fundamental figure is composed of lateral and Parts of 



terminal planes; of lateral and terminal edges ; and of' he fund - 

 ... f mental B- 



solid angles. 



1. Lateral planes are the greatest planes that bound the 

 smallest extent. Terminal planes are the smallest 

 planes that bound the greatest extent. In the prum 

 they form the bases, but in the table they are the 

 smaller planes that surround the two largest planes. 



A third kinil of crystal has been particularly pointed out by Breithaupt. He names it the metamorphosed or changed cryrtal; the 

 form is not altered, but the substance is changed by certain processes hitherto but imperfectly known. Thus, cubes of iron pyrites are 

 changed into cubes of brown iron-stone, and crystals of augite, without any change of form, are converted into chlorite, or green earth, 

 _rV iile Uebcr die Acclilhnt der Krystalle, von A. Breithaupt, Freiberg, 1815. 



y When the faces are very small, they are named facett. 



