CRYSTALLOGRAPHY. 



477 



represented in Fig. 10. Its symbol is B. 2. The ?n- 



X 



s 



verse, represented m Fig. 11. Its symbol is E' *E. 



/ 



3. The metastatic, represented in Fig. 12. Its symbol 



2 



is D. 4. The contrasting, represented in Fig. 13. Its 



r 3 ... 



symbol is c. 5. The regular six sided prism, repre- 

 ss , 

 sented in Fig. 1 4. Its symbol is e A. The equiaxe, 



i 



CO 



with a very small prism interposed, is called dog tooth 

 spar. 



2. Bitter spar, or magnesian ca?-bonale of lime. It 

 was supposed formerly to have the same primitive form 

 as calcareous spar. But Dr Wollaston has discovered, 

 that the angle at the summit is 106° 15'. 



3. Carbonate of iron, or fiosferri. It was supposed 

 to have the same primitive form with calcareous spar ; 

 but Dr Wollaston has discovered, that the angle at 

 the summit is 107°. 



4. Quartz. This is a very abundant mineral. The 



primitive form is a rhomboid, with angles of 94° 4' and 



85° 56', and therefore differing but little from a cube. 



This primitive form is uncommon ; but Mr Philips of 



London has specimens of quartz crystallized in this 



form. They are small ; but very distinct. By far 



the most common variety of its crystals is that repre- 



i 



sented in Fig. 15. The symbol for which is ePr. 



rP: 

 The terminating pyramids are usually less regular in 

 the size of the faces than is represented in the Figure. 



5. Tourmaline or schorl. The angle at the summit is 

 113° 34' 31". It usually occurs in nine sided prisms. 

 A common variety is represented in Fig. 16. Its sym- 

 bol is DE e^P'E 1 *>•* e I -°. 



si P o 



6. Dioptase, or copper emerald. This mineral comes 

 from Siberia, and is still scarce. But few crystalline 

 forms of it have been observed. 



7. Chabasic, sometimes called cubic zeolite. It oc- 

 curs in trap rocks. The angle at the summit is Q3° 30', 

 so that it does not differ much from a cube. 



8. Red siher ore, antimoniated sulphuret of silver. 

 Hauy at first conceived, that the primitive form of this 

 ore was the rhomboidal dodecahedron. But he after- 

 wards ascertained, that it was a rhomboid. The crys- 

 talline forms are pretty numerous. But as the one is 

 easily distinguished by its colour and other properties, 

 we need not give figures here. 



X. Rhomboid ivith an acute Summit. 



To this primitive form belong three species of miner- 

 als; namely, 



1. Corundum. This is the name given by mineralo- 

 gists to the sapphyr, oriental ruby, corundum, adamant- 

 ine spar, and emery, which have been shewn by Bour- 

 non to belong all to the same species, and to have the 

 same primitive form. 



The best description of its crystals was published by 

 Count Bournon in the Philosophical Transactions. It 

 occurs in six sided prisms, and in dodecahedrons form- 

 ed by two six sided pyramids appl : ed base to base. 



2. Oligiste iron ore, or glance iron ore. The primi- 

 tive form does not differ much from a cube, the angles 

 of the rhombs being 87° and 93°. One of the most 



Sulphate of 

 iron. 



common varieties of its crystals is represented in Fig. Crystalline 

 17. Its symbol is P E* 3E A. J^w 



P n S Plate 



3. Sulphate of iron. The angles of the rhomboidal CCXXV1. 

 faces are 79° 50' and 100° 10'. It occurs as common- *j£j& 

 ly in its primitive form as in any other. 



II. Crystals whose Primitive form is the Tetra- Tetjube- 



HEDRON. DnoN. 



Only two species of minerals belong to this class, and 

 both occur among the copper ores ; namely, 



1. Copper pyrites ; 2. Grey copper ore, or fahlore and Copper py- 

 graugiltigcrz. These two have the same crystalline rites; 

 forms, namely, modifications of the tetrahedron, and Gie 7 cf> P' 

 probably ought to constitute but one species. The ptr 

 first consists of copper, iron, and sulphur ; the second, 

 of copper, iron, arsenic, and sulphur ; while the grau- 

 giltigerz is composed of copper, iron, antimony, and 

 sulphur. If the arsenic and antimony were mechani- 

 cal mixtures, then the constituents of all would be the 

 same. 



III. Crystals whose Primitive form is a Hexago- 

 nal Prism. 



This crystalline form belongs to eight species of mi- 

 nerals ; namely, 



1. Apatite or phosphate of lime. It occurs often crys- 

 tallized in the primitive form which is represented in 

 Fig. 1 S. Most of the varieties bear so close a resem- 

 blance or connection with the primitive form, that they 

 may be easily traced to it with the ej T e. 



2. Carbonate qfstronlian. It seldom occurs crystal- 

 lized. The only crystallized specimens of it that we 

 have seen had the primitive form. 



3. Emerald. This mineral occurs frequently in the 

 primitive form. Indeed all the crystals of it that we 

 have seen are slight modifications of that form. We 

 suspect some of the varieties figured by Hauy as be- 

 longing to the euclase, are emeralds. 



4. Nepheli?ie or sommite. It occurs often in the pri- 

 mitive form. A common variety is given in Fig. 19. 



Hexago- 

 nal Pkism. 



Apatite. 

 Fig. 18. 



Carbonate 

 of stron- 

 tian. 



Emerald. 



Nepheline. 

 Fig." 19. 



The symbol for which is M B P. 

 M r P 



5. Pinite or micarell. Has considei'able resemblance Finite; 

 to mica in appearance. We have only seen it crystal- 

 lized in the primitive form. 



6. Dipyre. Two different varieties of crystals belong- Dipyre. 

 ing to this species have been announced by Hauy. 



We have not seen them, and cannot therefore describe 

 them. 



7. Sulphurate of copper or glance copper ore. As far Sulphurate 

 as we have seen, the crystals of this species are always of C0 PP er - 

 primitive. 



8. Cinnabar, or sulphuret of mercury. It occurs also Cinnabar. 

 usually in the primitive form. Only one other variety 



is described by Hauy. 



The reader will observe, that those species which 

 have the six sided prism for a primitive form, occur 

 usually crystallized in that form. The same observa-- 

 tion applies to the cube, the tetrahedron, the rhomboidal 

 dodecahedron, and the regular octahedron. But it does 

 not apply to the other- primitive forms. 



IV. Crystals whose Primitive form is the Rhom- 

 boidal Dodecahedron. 



This primitive form belongs only to two species of 

 minerals. The inclination of the faces to each other is 

 obviously 120°. 



Rhomboi- 

 dal DODC' 



cahlbron. 



