CRYSTALLOGRAPHY. 



441 



Theory. 



Paralltlo- 

 iped. 



from each other in the proportional length and breadth 

 of their faces, and in the angles which these faces 

 make with one another. About 40 different parallelo- 

 pipeds have been hitherto observed in the mineral 

 kingdom. It may be useful to give a kind of general 

 arrangement of them here. They may be divided into 

 nine distinct kinds. The following description will 

 give some notion of each of these. 



1 . The first is the cube. It is a well known rectangu- 

 lar figure bounded by six square faces, all equal to ejich 

 other. Now it deserves attention, that this and all the 

 other regidar mathematical figures which occur in the 

 mineral kingdom, constitute the primitive forms of a 

 variety of species, while the other figures, which do 

 not possess this mathematical symmetry, are confined to 

 one species. The cube, for example, constitutes the 

 primitive form of no less than 1 1 species of minerals ; 

 as examples may be mentioned, common salt, pyrites, 

 galena, native gold, silver, and copper. 



2. The second is a right quadrangular prism with 

 square bases. It is a cube somewhat longer in one di- 

 rection than the other. Of course the only way it can 

 vary is in the relative lengths of two contiguous faces, 

 the base, and one cf the faces of the prism. For all the 

 faces of the prism are, of necessity, equal and similar, 

 being all rectangles. There are seven species which 

 have this kind of primitive form. In four of these the 

 prism is shorter than the square base, while in three it 

 is longer. Meionite, wernerite, sulphate of magnesia 

 and mesotype, have the prism shorter than the base ; 

 while in Vesuvian, chromate of lead, and titanite, it is 

 longer. 



3. The third is likewise a rectangular prism, but the 

 base, instead of a square, is a rectangle; of course it ad- 

 mits of greater variation than the preceding, as both 

 the length of the base and of the prism may vary. Yet, 

 as far as we recollect at present, only six species of mi- 

 nerals have been hitherto observed, whose primitive 

 forms belong to this variety of parallelopiped. These 

 are apophylkte, euclase, chrysoberyl, tungsten, chryso- 

 lite, and foliated zeolite. 



4. The fourth is likewise a rectangular prism, but the 

 base is a rhomb. Hence it is nearly in the same cir- 

 cumstances with the second kind; the square in it 

 being a rhomb in this kind. The primitive forms of 

 sulphate of barytes, sulphate of strontian, arsenical 

 pyrites, mica and granatite. There may be others that 

 belong to this kind of form, though we do not at pre- 

 sent recollect them. 



5. The fifth kind is likewise a rectangular prism ; but 

 the base is an oblique angled parallelogram. It bears 

 the same relation to the third kind that the fourth does 

 to the second. We recollect at present only three species 

 of minerals that have this primitive form ; namely, 

 pistazite, axinite, gypsum, and borax. They dif- 

 fer from each other of course in the relative propor- 

 tions of the faces, and in the angles of the base. 



6. The sixth kind is an oblique angled prism, the 

 base of which is a rhomb. Hornblende, actinolite, 

 augite and grammatite, are species, which have this 

 kind of primitive form. 



7. The seventh kind is an oblique angled prism,"the base 

 of which is an oblique angled parallelogram. Felspar 

 and sulphate of copper belong to it. If there be any 

 other minerals that have this primitive form, we do not 

 recollect them at present. 



8. The eighth kind is a rhomboid with an obtuse sum- 

 mit. The distinction between this and the succeeding 



VOL. VII. PART U. 



kind is rather artificial than real. But it deserves atten- Theory, 

 tion, because it is useful in the theory, and facilitates the V """"Y"™ 

 description of crystals. By a rhomboid, in crystallogra- 

 phy, is meant a figure bounded by six equal and simi- 

 lar rhombs. So that it is nothing more than an oblique 

 angled cube, or a cube twisted a little awry ; and in 

 some of the species, the deviation of the rhomboid from 

 the cube is not veiy evident to the eye. This is, in some 

 measure, the case with the primitive figure of quartz, 

 and still more with that of chabasie, to which the name 

 of cubic zeolite, on that account, has been often given. 

 Now, if you examine the eight solid angles of any of 

 the rhomboids belonging to this kind, you will find 

 that two of them, which are opposite to each other, dif- 

 fer from the other six. They are composed each of 

 three obtuse plain angles meeting together in a point ; 

 whereas the six others are formed of two acute and 

 one obtuse angle. The line joining these obtuse solid 

 angles is called the axis of the crystal, and the angles 

 themselves constitute the summits of the crystal. The 

 axis is the shortest line joining any two opposite angles 

 in the respective rhomboids. The following species 

 belong to this kind of parallelopiped. Calcareous 

 spar, red silver ore, quartz, chabasie, dioptase or cop- 

 per, emerald, and tourmaline. 



9. The ninth kind is a rhomboid with an acute summit. 

 If you examine the eight solid angles of these rhom- 

 boids, you will find two of them to differ from the 

 other six. They are formed by the inclination of three 

 acute plain angles to each other, whereas the other six. 

 are composed of two obtuse and one acute plain angle. 

 The line joining these two acute and opposite solid 

 angles, is called the axis of the crystal, and the angles 

 themselves are called the summits. This axis is the 

 longest line joining any two opposite angles of the crys- 

 tal. The species of minerals belonging to this kind of 

 rhomboid, are sulphate of iron, corundum, and oligiste 

 iron ore. The primitive form of the two last differ but 

 little in appearance from a cube. 



II. The second primitive form is the octahedron. Octahe ? 

 The simplest way of forming a notion of this figure, is clron» 

 to conceive two four-sided pyramids applied base to 

 base ; thus united they form the octahedron. There 

 are about SO species of minerals which have the octa- 

 hedron for their primitive form. They may be divi- 

 ded into four kinds. 



1. The regular octahedron. In it the triangular 

 faces are equilateral and equiangular, and, of course, 

 the base of the two pyramids is a square. A consider- 

 able number of minerals have the regular octahedron 

 for their primitive form. We recollect at present 1 1 

 different species. The following are their names : 

 alum, salammoniac, fluor spar, diamond, spinel, mag- 

 netic iron ore, native antimony, native bismuth, red 

 copper ore, native amalgam, muriate of copper. 



2. Octahedron composed of two pyramids, having 

 rectangular bases applied base to base. Each triangu- 

 lar face, of course, is isosceles, the two angles at the 

 base being equal, and the angle at the summit different. 

 It maybe either acute, rectangular, or obtuse, according 

 to the length of the rectangular base of the pyramids. 

 The faces are equal and similar, four and four. The> 

 following species have this primitive form : carbonate 

 of lead, sulphate of lead, calamine, topaz, chiastohtp, 

 nitrate of potash, and perhaps also arragonite. 



3. Octahedron composed of two pyramids having a 

 square base. The pyramids, in general, are very low, 

 when compared with the size of the base, though this 



3k 



