LESSONS IN MINERALOGY. 



I 



M I XERALOGY. I. 



DEFINITION OF A MINKKAL CHARACTERISTIC* AND PtCU- 

 ulTIKS OF MINERALS COMPARATIVE HABDNK8I 

 8Pt V ITT CRYSTALLISATION. 



ACCOUDINO to Dana, a mineral is " any substance in nature not 

 organised by vitality, whic'u has a homogeneous structure. " 

 That is to say, a minoral is not the product of any process 

 i on by the agent we call lifo ; neither is a minoral a rock, 

 because a rock is an aggregation of particles, it may bo of very 

 :;t compositions, and therefore cannot bo said to have a 

 homogeneous structure. Another definition places a mineral in 

 another light, which describes it to bo " an inorganic substance 

 formed in the earth, possessing a definite geometrical shape, and 

 a definite chemical composition." It is evident from this defi- 

 nition that if the geometrical shape of the mineral and its 

 composition be known, the mineral can bo distinguished, and its 

 name, if it has been before described, determined ; so that the 

 mineralogist must be skilled in two branches of knowledge 

 crystallography and chemical analysis before ho can bo master 

 of his subject. For the processes by which the 

 chemist determines the composition of minerals, 

 works on chemical analysis must bo consulted. In 

 this branch wo only deal with the results, not with 

 the means by which the results have been obtained. 

 Before, however, we enter upon an outline of Crys- 

 tallography, it will be but rational to aid the student to deter- 

 mine some of the most common and conspicuous minerals by 

 means of those physical properties which require no other 

 knowledge than how to observe. 



The physical characteristics of minerals are (1) of structure ; 

 (2) peculiarities depending on 

 light, hardness, gravity, taste, 

 and smell. 



As to Structure: the way in 

 which the particles of a mineral 

 are placed together causes the 

 mineral to have sundry charac- 

 ters. It is said to be 



1. Brittle, when it easily 

 breaks, and when parts of the 

 mineral separate into powder 

 when an attempt is mode to cut 

 it, as many of the ores. 



2. Sectile, when by means of 

 a knife thin slices will separate 

 from the mass, as selenitc. 



3. Malleable, when as native 

 gold will flatten out under the 



.hammer. "'g 



4. Flexible, when the mineral 



will bend and remain in that position, as asbestos, and many of 

 the fibrous minerals. 



5. Elastic, when the mineral returns to its original shape 

 after having been bent, as mica. 



Fracture depends on the state of the aggregation of the 

 particles, and is described as 



1. Conchoidal, when the surface of the fracture is rendered 

 uneven by concave depressions and convexities. This fracture 

 id well exhibited if a lump of flint be split. 



2. Even, when the surface is flat or nearly so. 



3. Uneven, when it is broken. 



4. Hackly, when the surface is very rough with sharp edges ; 

 a broken piece of zinc or of cast-iron illustrates this fracture. 



Some minerals sure fibrous, as asbestos. Some are crystalline, 

 as certain marbles which possoss a glistening surface like loaf 

 sugar. 



Besides these, there are other terms used to express what the 

 words evidently indicate. 



A mineral can only affect tha taste which is soluble in the 

 saliva. 



1. Saline expresses the taste of common salt. 



2. Alkaline, that of soda. 



3. Astringent, that of alum. 



There are but few which are capable of emitting any odour. 

 Serpentine and some other kindred minerals give off an argil- 

 laceous odour when breathed upon ; that is, they smell like clay ; 

 but, generally speaking, minerals are inodorous, except when BO 

 134 N.F. 



heated M to giro off vapour*. Under such treatment many of 

 the artenic ore* produce a smell of garlic ; telenivm give* off 

 the fame* of horseradish, and sulphur the distinctive odour of 

 sulphurous aoid gas. 



Dependent upon light are fire characters of mineral*, viz., 

 colour, lustre, diaphaneity, refraction, and fluoretcence. 



Colour in either metallic or non-metallic. Metallic lustre U 

 that peculiar lustre which distinguishes the metals ; however, 

 it does not belong exclusively to the clous ; for graphite, which 

 is carbon, and scales of iodine, both possess metallic lastre. 

 Minerals whose colour is non-metallic may be found of every 

 hue, from the black onyx to the colourless diamond. 



Iridescence is when the rainbow hues play within the crystal ; 

 this is due to cracks which penetrate it, and the rays of light 

 reflected from the two surfaces " interfere." 



Opalescence is the pearly reflection which is seen with certain 

 opals, and in the mineral called " cat's-eyes." 



Pearly is that silky and often coloured lustre which renders of 

 such value the nacre with which many of the mollusks line their 

 shells. When a grain of sand or some other foreign substance 

 finds its way within the shell, the animal, to allay the 

 irritation, coats the intrusive grain with its beautiful 

 polish. The reason the nacre possesses the pearly 

 lustre arises from the fact that the creature deposits 

 the substance in fine layers ; the light reflected from 

 their edges is in a condition to " interfere," as in 

 the case of iridescence. That the play of colours is entirely 

 due to this, may readily be proved by pressing against the 

 mother-of-pearl a piece of white wax, and it will be found that 

 the wax now exhibits the colours. But pearly is generally 

 applied to minerals having the appearance of nacre without the 



colours; talc and ttilbite are 

 examples. 



In addition to metallic and 

 pearly lustres, there are 



Vitreous; a lump of broken 

 glass possesses this lustre typi- 

 cally. Many minerals share it ; 

 as quartz, calc spar, etc. 



Resinous, as the name implies, 

 is the lustre which distinguishes 

 the yellow resins, and is shared 

 by zinc-blende, opal, etc. 



Silky lustre is the result of a 

 fibrous structure, and is exhibited 

 by fibrous carbonate of lime, 

 fibrous gypsum, and always by 

 minerals which possess a 

 pearly lustre when they become 

 fibrous. 



Adamantine is the lustre 



exhibited by the diamond, and other highly-refractive gems. 

 The degrees of lustre are five : 



1. Splendent, when the reflection is so good as to give the 

 images of objects ; as is the case in Elba iron ore, in some 

 specimens of galena, and pyrites. 



2. Shining, when there is an image, but not one well denned ; 

 as calc spar, celoatino. 



3. Glistening, when there is a general reflection from all 

 points of the surface ; talc, copper pyrites, etc., are examples. 



4. Glimmering, when the reflection from the surface is 

 imperfect, and only takes place from certain points, as is the 

 case in flint and chalcedony. 



5. Dull expresses a total absence of lustre. 



The property which all bodies possess in a more or less degree, 

 of allowing light to pass through them, is termed diaphaneity. 



The most opaque bodies, if in sufficiently thin laminae, will 

 permit light to traverse them, as gold-leaf and silver-leaf. 



There are five degrees of diaphaneity : 



1. Transparent, when objects can be distinctly seen through 

 the mineral, as crystals of quartz, carbonate of lime, and selenite. 



2. Semi-transparent, when the object is seen, but not clearly. 



3. Translucent, when light passes, but not so as to define 

 objects. 



4. Translucent on the edges, when light passes through the 

 edges of the mineral, while the thicker parts are opaque. 



5. Opaque, when no light passes. 



In traversing minerals, light does not always obey the 



