458 



MINERALOGY. 



Oryctogno- towards a principal streak, like the disposition of 

 y_-*^!-^_ r , tne P af ts of a feather. We must be careful not 



""" v ~~ to confound it with the plumose external shape. 



It occurs in the folia of Plumose native bis- 

 muth. 



b. Rcticularly. When the streaks either cross each 

 other in a promiscuous manner, or under right 

 angles, forming a kind of flat net-work. It oc- 

 curs on the surface of silver-white cobalt. 

 6. Drusi/. When a crystal is coated with a number of 

 minute crystals of the same kind, so that the new 

 surface acquires a scaly aspect, it is denominated 

 drusy. Example, Common iron-pyrites. 



III. The External Lustre. 



The enter- Here we have to consider the intensity and the sort 

 BIJ lustre, of lustre. 



J. The intensity of the lustre. Of this there are five 

 different degrees. 



A. Splendent. A fossil is said to be splendent, when 

 in full day light (not in the sunshine) its lustre is 

 visible at a great distance. The highest degree of 

 this is termed splecnlar splendent. It generally oc- 

 curs in minerals with a perfect foliated fracture. 

 Example, Galena or lead-glance. 



B. Shining. When a mineral at a distance reflects but 

 a weak light, it is said to be shining. Example, 

 Heavy spar. 



C. Glistening. This degree of lustre is only observ- 

 able when the mineral is near us, and at no greater 

 distance than arm's length. Example, Porcelain 

 jasper. 



D. Glimmering. If the surface of a mineral, when 

 held near to the eye in full and clear day-light, pre- 

 sents a very great number of small faintly shining 

 points, it is said to be glimmering. In strong sun- 

 shine it exhibits a kind of play of colour. As an 

 example of this degree of lustre, we may mention 

 red hematite. It is divided into metallic and non- 

 metallic. 



E. Dull. When a mineral does not reflect any light, 

 or is entirely destitute of lustre, it is said to be dull. 

 Example, chalk. 



2. The sort of lustre. Of the different sorts of lustre 

 we cannot give any definition, but must rest satis- 

 fied with mentioning a few minerals which present 

 these characters in the greatest perfection. 



, a. Metallic lustre, is always combined with opacity. 



It is divided into perfect and imperfect. The per- 

 fect occurs in native metals, the imperfect in tan- 

 talum ore. 



b. Adamantine. Of this lustre there are two varie- 

 ties, viz. metallic adamantine, and common adaman- 

 tine. White lead spar is an example of the first, 

 and diamond of the second. 



e. Pearly. Is divided into common, and metallic -like. 

 Mica is an example of the first, and schiller 

 spar of the second. 



d. Resinous or waxy, as in pitchstone, yellow lead- 

 spar, and tinstone crystals. 



e. Vitreous or glassy, as in rock-crystal and topaz. 



1. In determining the lustre of minerals, we ought 

 to expose them to a strong light, but not to the direct 

 rays of the sun. The specimens should not be hand- 

 led, a practice too often followed, and which very soon 

 alters the lustre, or adds a lustre to such as have none. 



2. When different kinds of lustre occur in the same 

 species, and pass into each other, a lustre suite is form- 



ed. The red silver affords an example of this. In 

 some varieties the lustre is nearly perfect metallic ; in 

 others distinct adamantine ; and other varieties are be- 

 tween metallic and adamantine; so that the whole 

 forms a complete series or suite, where we cannot say 

 where the one begins and the other ends. 



II. THE ASPECT OF THE FRACTURE. 



Here we have to observe the lustre of the fracture, T he aspect 

 the fracture, and the shape of the fragments. of the irac- 



IV. The Lustre of the Fracture. 



The internal lustre, or the lustre of the fracture, pre- The lustre 

 sents the same varieties as the external lustre, and of the frae- 

 therefore requires no particular description. ture - 



V. The Fracture. 



By fracture we understand the shape of those inter- The frac- 

 nal surfaces or planes of a mineral which are produced ture. 

 by breaking or splitting it. These surfaces are either 

 continuous, when the fracture is said to be compact, or 

 are composed of a number of line-like or foliated parts, 

 termed distinct concretions, when the fracture is named 

 split or divided. 

 A. Compact Fracture. There are six different kinds 



of compact fracture, viz. splintery, even, conchoidal, 



uneven, earthy, and hackly. 



a. Splintery. When, on a nearly even surface, small 

 wedge-shaped or scaly parts are to be observed, 

 which adhere by their thicker ends, and allow 

 light to pass through, we say that it is splintery. 

 It sometimes passes into even. Examples, Iron- 

 stone and quartz. 



b. Even, is that kind of fracture-surface which shews 

 the fewest inequalities, and these inequalities are 

 flat and their boundaries never sharply marked, on 

 the contrary, they run into each other impercep- 

 tibly. Minerals possessing this kind of fracture 

 have generally a low degree of lustre and of tran- 

 sparency. It occurs in chrysoprase, calcedony, 

 compact galena or lead-glance, compact red iron- 

 stone, and compact brown ironstone. It passes into 

 large conchoidal and into splintery. Example, 

 Lydian stone. 



c. Conchoidal, is composed of concave and convex 

 roundish elevations and depressions, which are 

 more or less regular : when regular, they are accom- 

 panied with concentric ridges, as in many shells, 

 and hence present a conchoidal appearance. Ex- 

 ample, obsidian. 



d. Uneven. This kind of fracture shews the most 

 considerable elevations and depressions, and the 

 elevations are usually angular and irregular. These 

 elevations are denominated the grain ; and, ac- 

 cording to the size of the grain, the fracture is 

 named coarse-grained, as in copper-pyrites ; small- 

 grained, as in copper nickel; or Jine grained, as in 

 arsenical pyrites. 



This kind of fracture frequently occurs in opaque 

 minerals having some lustre, and is most frequent 

 in metallic minerals. It passes into small and im- 

 perfect conchoidal, and also into earthy. 



e. Earthy. When the fracture surface shews a great 

 number of very small elevations and depressions, 

 which makes it appear rough, it is called earthy. 

 It is always associated with complete opacity and 

 want of lustre, which latter character distinguishes 

 it from the fine grained uneven fracture. It is pe- 

 culiar to earthy minerals. It is distinguished into 



