460 



MINERALOGY. 



Oryctogno- o f transparency, which are generally crystallised, and 



v 8 - Y ' usually afford regular fragments. It is 'a more fre- 



""Y""" quent fracture than either the radiated or fibrous. 



In the foliated fracture we have to attend to the 

 size of the folia the degree of perfection of the folia- 

 ted fracture the direction of the folia the position of 

 the folia the aspect of the surface of the folia and 

 the passage of the folia or cleavage. 



a. The size of the folia. The size of the folia is de- 

 termined by that of the distinct concretions ; so 

 that a mineral which is composed of large granu- 

 lar concretions, must have a large foliated fracture, 

 or, of small granular concretions, a small foliated 

 fracture. When a mineral with a foliated fracture 

 is not composed of distinct concretions, but is one 

 uniform undivided mass, the folia pass uninter- 

 ruptedly through the whole extent of the mass, 

 and afford the largest variety of foliated frac- 

 ture. 



b. The degree of perfection of the foliated fracture. 

 This depends on the facility with which the folia 

 are separated from each other by splitting on the 

 lustre, and the smoothness of the fracture surface. 



Thus it is 



*. Highly perfect or specular splendent : when the 

 folia are perfectly smooth, and specular splen- 

 dent, as in galena or lead-glance, yellow blende, 

 transparent calcareous-spar, and selenite. 



. Perfect foliated, in which the folia are pretty 

 smooth and shining, and sometimes splendent, 

 as in mica and felspar. 



y. Imperfect foliated, when the folia are slightly 

 uneven or even rough, and the lustre lower than 

 in the perfect foliated, as in fluor spar, and be- 

 ryl. 



3. Concealed foliated, when the folia are separated 

 from each other with difficulty, and the foliated 

 fracture appears only in a few places of the frac- 

 ture surface, as in rock-crystal. 



c. The direction of the folia. 



,. Straight foliated, as in selenite and calcareous- 

 spar. 

 ft. Curved foliated, which is either 



i. Spherical curved foliated, when the folia are 

 so bent, that they resemble either whole 

 spheres, or segments of spheres, as in brown- 

 spar and mica. 



ii. Undulating curved foliated, when the folia are 

 so laid over each other, that a transverse sec- 

 tion gives a serpentine line, but the longitu- 

 dinal one a straight line, as in mica, 

 iii. Floriform foliated, when the folia are va- 

 riously curved, and the curvatures are ar- 

 ranged in a scopiform manner, as in galena or 

 lead-glance. 



iv. Indeterminate curved foliated, when the folia 

 are irregularly or indeterminately curved, as 

 in iron-mica or micaceous iron-ore, and mica, 

 d. The position of the folia. 



a. Common foliated, when the folia extend through- 

 out the whole mass, and cover each other com- 

 pletely, as in calcareous-spar, and most other 

 minerals with a foliated fracture. 

 ft. Scaly foliated, when the folia cover each other 

 only partially in their arrangement, somewhat 

 resembling the scales on a fish. It is divided 

 into large, small, and^ne scaly foliated, and oc- 

 curs in mica. 



e. The aspect of the turf ace of the folia. The foliated Oryetogno. 



fracture is either 

 a. Smooth, as in calcareous spar, and felspar ; or > 

 . Streaked, which is either 

 i. Simply streaked, and in the direction of the 



length, as in common hornblende, 

 ii. Variously streaked, as in iron-mica, 

 iii. P/umosely streaked, as in mica, 

 f. The passage of the folia or cleavage. 



The cleavage is the number of determinate di- 

 rections in which a mineral exhibits a foliated 

 fracture, and according to which it can be split. 

 It is distinguished 



it. According to the number of the cleavages. 



i. Single, when it splits only in one direction, as 



in mica. 



ii. Twofold or double, when it splits in two di- 

 rections, as in felspar, hornblende, and tremo- 

 lite. 



iii. Threefold or triple, when it splits in three 

 directions, as in calcareous-spar, rock-salt, 

 and galena or lead-glance, 

 iv. Fourfold or quadruple, when it splits in four 

 directions, as in fluor-spar, specular iron, or 

 iron-glance, and beryl. 



v. Sixfold, when it splits in six different direc- 

 tions, as in blende and rock-crystal. 

 /3. According to the angle under which the cleavages 

 intersect each other ; and these exhibit the fol- 

 lowing varieties. 



i. In the twofold cleavage, the two folia or clea- 

 vages intersect each other rectangularly, as in 

 felspar and hyacinth; or oblique angularly, 

 as in hornblende. 



ii. In the threefold cleavage, the folia intersect 

 each other rectangularly, as in galena or lead- 

 glance; oblique, yet equiangular ly, as in cal- 

 careous-spar and sparry ironstone; oblique but 

 unequiangu/arly, as in heavy-spar ; and partly 

 rectangularly, partly oblique-angularly, as in 

 selenite. 



iii. In the fourfold cleavage, all the cleavages are 

 equiangular and oblique-angular, as in fluor- 

 spar, iron-glance, and diamond ; or three 

 cleavages are equiangular and oblique-angu- 

 lar, in a common axis, and are intersected by 

 a fourth, which is horizontal and rectangular, 

 as in beryl. 



iv. In the sixfold cleavage, all the cleavages meet 

 under equal oblique angles, as in rock-crystal ; 

 or three of the cleavages are equiangular and 

 oblique-angular, in a common axis, which are 

 obliquely intersected by three others, which 

 also intersect the axis iu an oblique direction. 

 Example, Blende. 



These angles of the various cleavages may 

 also be more particularly measured by means 

 of the goniometer. 



D. Slaty fracture. This fracture, like the foliated, con- 

 sists of plane-like portions, in which the length and 

 breadth are nearly alike, but in which the thickness 

 begins to be discernible. The fracture-surface is 

 generally rough, with but little lustre. It is nearly 

 allied to the foliated fracture, but is less perfect, and 

 never occurs in regularly crystallised minerals, but 

 always in those which are found in large masses, or 

 in beds. Minerals with this fracture are generally 

 opaque. This fracture is further distinguished ac- 



e.y. 



