MINERALOGY. 



TKe chara(3:ers of minerals, arranged according to the 

 Wernerian method, are enumerated under tlie article 

 Oryctognosy; but a feleftion of the moft important 

 ones, with a further definition of fome of them, appears 

 neceffar)- to render more complete this department of our 

 work, and to enable the general reader to underi^and the 

 defcriptive language ufed by mineralogifts. The charafters 

 of minerals are either phyjical or chemical. Phyfical cha- 

 rafters are thofe properties which can be difcovered without 

 decompofing or changing the nature of minerals : under 

 this term we comprife the external characters of Werner, 

 and alfo fome of tliofe properties which are elicited by the 

 action of other bodies on the mineral examined, fuch as 

 magnetifm and eleftricity. 



Chemical characters in mineralogy are thofe properties 

 which are moft eafily difcovered by the aftion of heat, of 

 acids, and of various re-agents. It is in this limited fenfe 

 that thefe charadters are underftood by the mineralogiit. 

 A complete analyfis of minerals belongs to chemiftry. 



In our enumeration of phyfical charafters, we (hall com- 

 mence with thofe which depend on the adtion of light ; 

 namely, colour, tranfparence, luftre, and refraction. 



Colour is regarded by the German mineralogifts as an im- 

 portant character, and it is that which firft arrefts the attention 

 of the obferver ; butin earthy minerals, the colours of the fame 

 fpecies are often fo various, that this charafter lofes much 

 of its value as applied to them. The colours of minerals, 

 as far as they have been chemically examined, are princi- 

 pally owing to metaUic oxyds and inflammable matter : 

 the earths, the acids, and the alkalies, in a ftate of purity, 

 are white or colourlefs. The colour of earthy minerals 

 may, therefore, be regarded for the moft part as arifing 

 from accidental admixtures \vith unimportant ingredients. 

 But in the inflammable minerals and metaUic ores, and in a 

 few of the earthy minerals, the colouring matter is as im- 

 portant as the other parts, and generally more fo ; hence 

 in the latter, the colour varies but little in each fpecies, and 

 is a character of importance precifely in proportion to its 

 fimplicity. For an enumeration of the different varieties 

 of each colour, and the minerals in which they are moft 

 charaAeriftic, and for change of colour, fee Oryctog- 

 nosy. 



Tranfparence, in its different degrees, from perfect tranf- 

 parence, femi-tranfparence, tranducence, and opacity, are 

 terms perfectly intelhgible to every reader. When the 

 cloudinefs in femi-tranfparent minerals increafes, fo that the 

 outline of objects can fcarcely be feen, tranflucency com- 

 mences, as in common chalcedony. 



Double Refraalon, or Duplicating, is the property which 

 fome tranfparent minerals poffefs of prefcnting a double 

 image of an object when feen through them in particular 

 direftions, of which calcareous fpar, an Iceland cryilal, offers 

 a remarkable example. See Crystal of Icelaxd. 



Opalefcence, a term not unfrequently applied to fome 

 minerals, is thus defined by Mr. Jamefon : " Some minerals, 

 when held in a particular direction, refleft from fome fingle 

 fpots in their interior a coloured ffiining luftre ; this is 

 what is underftood bv opalefcence : it is diftinguiftied into 

 fimple iXiA. Jlellular ; in the latter, the luftre diverges in fix 

 rays in the form of a ftar, as in the Jiar-fapphire." 



Lujlre. — The luftre of a mineral produced by the reflec- 

 tion of light is of different kinds, and is called metallic, 

 femi-metallic, adamantine, pearly, refinous, and vitreous. 

 Perfectly opaque minerals, as the metals, and moft of the 

 metallic fulphurets, reflect the light wholly from the furface 

 without undergoing any refraClion, and exhibit the metallic 

 luftre of various degrees of intenfity ; and the litftre is 



Vol. XXXIX. 



mcreafed, and the colour is uiiclianged, when a fcratch is 

 made in them with a knife or file. Minerals having a femi- 

 metalhc luftre yield a lighter colour, or have their luftre 

 deftroyed when fcratched. The adamantine luftre is 

 exhibited by minerals which are tranflucent, and poffefs 

 great refraftive power; the luftre is refleded from the 

 interior of the mafs with great vivacity, and is produced 

 both by reflefted and refrafted light. Examples, the dia- 

 mond, fulphur, and the native falts of lead. In thefe 

 minerals, though the luftre is increafed by pohftiing, yet its 

 particular charader is lefs diftinct, owing to the increafed 

 reflection of unrefraCted light from the furface. 



The pearly or nacry luftre is well exhibited in fomr 

 kinds of zeolite, and in kyanite. When it proceeds from 

 fibrous minerals, as in fatin fpar and fibrous gypfum, it is 

 fometimes called a filky or fatiny luftre. 



The refinous luftre is well reprefented by that of pitch : 

 it exifts in pitch-ftone and refinous flints. 



The vitreous luftre is perfeftly reprefented in rock- 

 cryftal. 



Each of thefe kinds of luftre may vary in degree from 

 the moft fplendent, which can be feen at a great diftance, 

 to ftiining, gliftening,.-or ghmm.ering. When entirely def- 

 titute of luftre, a mineral is called dull. 



Thtjlreak implies the colour or luftre which a mineral 

 exhibits when fcratched with a knife or file : the colour is 

 the fame as that of the mineral when pulverized. 



Soilmg is a character that occurs in fome foft minerals, 

 which leave a mark when drawn on the furface of other 

 bodies, or on the fingers, as plumbago, chalk, and reddle. 



The above are the principal characters depending on the 

 aftion of light. 



Phofphorefcence Certain minerals give out light when 



rubbed againft each other, as quartz ; or when fcratched 

 with a knife, as dolomite. Other minerals give out light 

 when thrown on hot coals, or heated iron, as fluor fpar ; and 

 certain minerals emit light when expofed to the action of 

 the blow-pipe. 



Hardnefs and Solidity. — Solids are the only bodies to 

 which the terms hard or foft can properly be applied. In 

 common language, hardnefs and frangibUity are often con- 

 founded. A ftone that endures many heavy blows before 

 it breaks, is confidered as harder than another which requires 

 fewer blows for its frafture ; but the property which dif- 

 ferent minerals have of refifting the point of a knife or file 

 of hardened fteel, or the effeCk produced when a mineral is 

 rubbed on other minerals, or fcratched by them, is the moft 

 unexceptionable teft of their hardnefs. Thus fome minerals 

 fcratch cryftallized quartz, a ftone eafily recognized, and 

 whofe hardnefs in that ftate is always the fame ; other 

 minera's fcratch fteel, glafs, fluor fpar, &c. This method 

 is precife, and gives the real hardnefs of the parts ; whereas 

 ftriking fl -"'th fteel, which is often mentioned as a cha- 

 rafter, is a .ague teft, fubjeCt to variation from the form of 

 the mineral, the fi:a!-nnefs of its edges, &c. ; and foft mine- 

 rals not unfrequently contain minute grains of harder ones, 

 which will give fparks with fteel. 



As a knife is the moft convenient and portable inftrument 

 for determining the hardnefs of moft minerals, except gems, 

 the following judicious obfervations on the ufe of it, by 

 Mr. Aikiii, are deferving the attention of the ftudent. In 

 fibrous n. :!="rals, a fcratch direfted acrofs the fibres will 

 always inc :. > a lower degree of hardnefs than the true one ; 

 for the fibroub ruCture prefenting an alternation of ridges and 

 furrows, the knife glances acrofs the intervals, thus interrupt- 

 ing the uniformity of the ftroke, and producing a fucceffion 

 of finall blows, which rather break down than divide the 

 3 Z fuQunitg 



