II 



MINERALOGY 



MINFJULOOY. 



Hi 



slender like a needle. 



Stalaetitio : baring the form of a cylinder or cone hanging from 

 the root* of carities or caves. The term stalactite it usually restricted 

 to the cylinders of carbonate of lime hanging from the roofs of 

 carenu ; bat other mioeraU are laid to hare a stalnct itie fonn when 

 resembling these in their general ahape and origin. Chalcedony and 

 brown iron-ore are often stalactitic. 



Reticulated: net-like. 



Drniy : a surface U said to be dnuy when oorcred with minute 

 crystal*. 



Amorphotu shapeless : baring no regular structure or form, either 

 crystalline or imitative. 



Crystals are also called Pseudomorphous. A pseudomorphous crystal 

 is one that has a form which is foreign to the species to which the 

 substance belongs. 



Crystals sometimes undergo a change of composition from aqueous 

 or some other agency, without losing their form ; for example, octa- 

 hedrons of spinel change to steatiU), still retaining the octahedral 

 form. Cubes of pyrites are changed to red or brown iron-ore. 



Again, crystals are sometimes removed entirely, and at the same 

 time and with equal progress, another mineral U substituted ; for 

 example, when cubes of fluor-spar are transformed to quartz. The 

 petrifaction of wood is of the same kind. 



Again, cavities left empty by a decomposed crystal are refilled by 

 another species by infiltration, and the new mineral takes on the 

 external form of the original mineral, as a fused metal the form of 

 the mould into which it is cast. 



Again, crystals are. sometimes tncrusted over by other minerals, as 

 cubes of fluor by quartz ; and when the fluor is afterwards dissolved 

 away, as sometimes happens, hollow cubes of quartz are left. 



The first kind of Pseudomorphs are Paeudomorphs by Alteration ; 

 the second, Pseudomorphs by Replacement ; the third, Pseudomorphs 

 by Infiltration ; the fourth, Pseudomorphs by Incrustation. 



Pseudomorphous crystals are distinguished by having n different 

 structure and cleavage from that of the mineral imitated in form, and 

 a different hardness, and usually little lustre. 



A large number of minerals have been met with as pseudomorphs. 

 The causes of such changes have operated very widely and produced 

 important geological results. 



The characters of minerals depending on light are also arranged. 

 They are of five kinds, and arise from the power of minerals to reflect, 

 transmit, or emit light They are as follows : 1, Lustre ; 2, Colour; 

 S, Diaphaneity ; 4, Refraction ; 5, Phosphorescence. 



The lustre of minerals depends on the nature of their surfaces, 

 which causes more or less light to be reflected. There are different 

 degrees of intensity of lustre, and also different kinds of lustre. 



The kinds of lustre are six, and are named from some familiar 

 object or class of object* : 



Metallic : the usual lustre of metals. Imperfect metallic lustre is 

 expressed by the term Sub-Metallic. 



Vitreous : the lustre of broken glass. An imperfect vitreous lustre 

 is termed Sob- Vitreous. Both the vitreous and sub-vitreous lustres 

 are common. Quartz possesses the former in an eminent degree; 

 calcareous spar often the latter. This lustre may be exhibited by 

 minerals of any colour. 



Resinous : lustre of the yellow resins. Opal and zinc-blende are 

 examples. 



Pearly : like pearl. Talc, native magnesia, stilbite, &c., are examples. 

 When united with sub-metallic lustre, the term Metallic-Pearly is 

 applied. 



Silky : like silk ; it is the result of a fibrous structure. Fibrous 

 carbonate of lime, fibrous gypsum, and many fibrous minerals, more 

 especially those which in other forms have a pearly lustre, are 

 examples. 



Adamantine : the lustre of the diamond. When sub-metallic, it is 

 termed Metallic-Adamantine. Varieties of white lead-ore are examples. 



The degrees of intensity are denominated as follows : 



Splendent : when the surface reflects lights with great brilliancy, 

 and gives well denned images. Elba iron-ore, tin-ore, some specimens 

 of quarts and pyrites are examples. 



Shining : when an image is produced, but not a well-defined image. 

 Calcareous spar and celestine are examples. 



Glistening : when there is a general reflection from the surface, but 

 no image. Tale and copper-pyrites are examples. 



Glimmering : when the reflection is very imperfect, and apparently 

 from points scattered over the surface. Flint and chalcedony nro 

 examples. 



A mineral is said to be Dull when there is a total absence of lustre, 

 as chalk. 



In distinguishing minerals, both the external colour and the colour 

 of a surface that has been rubbed or scratched, are ulnervud. The 

 latter is called the Streak, and the powder abraded, the Streak-Powder. 



The colours are either metallic or non-metallic. 



The Metallic are named after some familiar metal, as copper-red, 

 bronze-yellow, brass-yellow, gold-yellow, steel-gray, lead-gray, iron- 



The Non-Metallic colours used in characterising minerals, are various 

 shades of white, gray, black, blue, green, yellow, red, and brown. 



There are thus snow-white, reddish-white, greenish-white, in ilk- 

 white, yellowish-white ; bluish-gray, smoke-gray, greenish-gray, pearl- 

 gray, ash-gray ; velvet-black, greenish-black, bluish-black ; azure-blue, 

 violet-blue, sky-blue, indigo-blue; emerald-green, olive-green, oil- 

 green, (Trass-green, apple-green, blackish-green, pistachio-green 

 (yellowish) ; sulphur-yellow, straw-yellow, wax-yellow, ochre-yellow, 

 honey-yellow, orange-yellow; scarlet-red, blood-red, flesh-red, brick- 

 red, hyacinth-red, rose-rod, cherry-red ; hair-brown, reddish-brown, 

 chestnut-brown, yellowinh-brown, pinchbeck-brown, wood-brown. 



The expression a Play of Colours is used when several prismatic 

 colours appear in rapid succession on turning the mineral. The 

 diamond is a striking example ; also precious opal. 



Change of Colours : when the colours change slowly on turning in 

 different positions, as in labradorito. 



Opalescence : when there is a milky or pearly reflection from the 

 interior of a specimen, as in some opals, and in cat's eye. 



Iridescence : when prismatic colours are seen within a crystal ; it is 

 the effect of fracture, and is common in quartz. 



Tarnish: when the surface-colours differ from the interior; it 

 is the result of exposure. The tarnish is described as Irisoil, when it 

 has the hues of the rainbow. 



Polychroism : the property, belonging to some prismatic crystals, of 

 presenting a different colour in different directions. The term Dichro- 

 ism has been generally used, and implies different colours in two 

 directions, ai in the mineral iolite, which has been named dichroite 

 because of the different colours presented by the bases and sides of 

 the prism. Mica is another example of the same. The more general 

 term has been introduced, because a different shade of colour has been 

 observed in more than two directions. 



These different colours are observed only in crystals with unequal 

 axes. The colours are the same in the direction of equal axes, and 

 often unlike in the direction of unequal axes. This is the general 

 principle at the basis of polychroism. 



Diaphaneity : the property which many objects possess of trans- 

 mitting light ; or in other words, of permitting more or less light to 

 pass through them. This property is often called transparency, but it 

 is properly one of the degrees of diaphaneity. The following terms 

 are used to express the different degrees of this property : 



Transparent : a mineral is said to be transparent when the outlines 

 of objects, viewed through it, are distinct Glass and crystals of 

 quartz are examples. 



Sub-Transparent, or Semi-Transparent : when objects are seen, but 

 their outlines are indistinct 



Translucent: when light is transmitted, but objects are not seen. 

 Loaf-sugar is a good example ; also Carrara marble. 



Sub-Translucent: when merely the edges transmit light faintly 

 When no light is transmitted, the mineral is described as opaque. 



Those minerals whose faces emit light exhibit two sets of pheno- 

 mena, Refraction and Polarisation. [HEFBACTIOX, and POLAHISATION, in 

 ARTS AND Sc. Div.] 



The index of refraction has been obtained for many minerals, of 

 which the following are a few : 



Air 



Tabashccr . 

 Ice . 

 Cryolite . 

 Water . 

 Fluor-Spar 

 Hock-Salt . 

 Quartz 



Many crystals possess the property of refracting light in two 

 directions instead of one, and object* seen through them conse- 

 quently appear double. This U called Double Refraction. It is most 

 conveniently exhibited with a crystal of calc-spar, and wns first noticed 

 in a pellucid variety of this ininer.il from Iceland, called from the 

 locality Iceland-Spar. On drawing a line on paper and placing the 

 crystal over it, two lines are seen instead of one one by ordinary 

 refraction, the other by an extraordinary refraction. If the crystal, 

 as it lies over the line, be turned around, when it is in one position 

 the two lines will come together. Instead of a line make a dot on 

 the paper, and place the crystal over the dot : the two dots seen will 

 not come together on revolving the crystal, but will seem t<> revolve 

 one around the other. The dot will in fact appear double through the 

 crystal in every direction except that of the vertical axis, and thin 

 direction is called the Axis of Double Refraction. To view it in thin 

 direction the ends must be ground and polished. Tho divergence 

 increases on passing from a view in the direction of the axis to one 

 at right angles with it, where it is greatest. In some substances the 

 refraction of the extraordinary ray is greater in the latter direction 

 than that of the ordinary ray, and in others it is less. I 

 it is lew, it diminishing from 1-654 to 1-483. In quartz it is greater, 

 it increasing from 1-64S4 to 1-6582. The former in said to have a 

 Negative Axis, the latter a Positive. 



This property of double refraction belongs to such of the funda- 

 mental forms as have unequal axes ; that is, to all except those of the 

 monomctric system. Those forms in which the lateral axes are equal 

 (the diinetric and hexagonal systems) hare one axis of double rofrac- 



