HARD WICKE'S SCIENCE- G OS SIP. 



i93 



NOTES ON THE COLOURS OF MINERALS. 



N the examination 

 of a collection of 

 minerals we find 

 that many of them 

 are coloured, for 

 instance sulphur 

 is yellow, cinnabar 

 red, sapphire blue, 

 fluor - spar blue, 

 violet, yellow, 

 green, etc. 



Now some of 

 these colours are 

 really due to the 

 substance which is 

 the chief com- 

 ponent of the 

 mineral ; this is 

 the case with sul- 

 phur, which ele- 

 ment is yellow, 

 and with cinnabar (mercuric sulphide) which in one 

 form is of a vermilion colour. But in the case of 

 such as sapphire and fluor-spar the colour is due to 

 the presence of small quantities of other bodies. 



It is to the latter class that I wish to call attention. 

 These minerals have not been systematically investi- 

 gated as to the cause of their colours. 



The following is a list of the minerals which I 

 have up to the present time been able to examine : — 

 Emery. — Composition Al 2 3 . This substance 

 when pure is white, but it generally occurs of various 

 shades of brown, the colour being due to the presence 

 of ferric oxide, which varies in amount from 8 to 33 

 per cent. 



Tinstone.— Composition Sn 2 . When pure is 

 white. 



Black. — The black variety is coloured by ferric 

 oxide. In one specimen Liversedge found 2'3 per 

 cent, of ferric oxide and o*8 per cent, ferrous oxide. 



Brown. — Brown tinstone is also coloured by ferric 

 oxide. 



Quartz. — Composition Si Oo. Often occurs 

 colourless or white. 

 Rose Coloured. — The colour of this variety of the 

 No. 309.— September 1890. 



mineral has been attributed to manganese and also to 

 organic matter. Fuchs found in specimens from 

 Robenstein in Bavaria 1 to 1*5 per cent, of oxide of 

 titanium. On examination of several specimens of 

 this mineral it was found that they did not change 

 colour or become colourless on heating, showing the 

 absence of organic matter ; manganese was also 

 absent in every case. All the specimens contained 

 ferric oxide, and to this body the colour is therefore 

 attributed. The presence of oxide of titanium in 

 the rose quartz examined by Fuchs is not sufficient to 

 explain the colour. 



Violet Quartz. — The colour of this mineral, which 

 is known as the common amethyst, has been 

 attributed to the presence of manganese ; it has also 

 been stated that the colour is due to a ferrate ; and 

 Rose found in one specimen 0*5 per cent, of ferric 

 oxide and 0*25 of potassium oxide, which result led 

 him to believe that a ferrate was present in the 

 mineral. 



Several specimens of this mineral were examined, 

 and it was found that they became colourless on 

 heating, and did not afterwards regain their colour. 

 They all contained iron, but manganese and organic 

 matter were absent. Owing to the small quantity of 

 iron present it was found impossible to determine in 

 what state it exists in the mineral, but it seems 

 probable that the colour is due, as stated by Rose 

 and others, to a ferrate, which becomes decomposed by 

 the action of heat, rendering the mineral colourless. 



Yellow Quartz. — A transparent variety of this 

 mineral on heating did not lose all its colour, but 

 became of a lighter shade. The colour was found to 

 be due to a small quantity of ferric silicate. The 

 colour of an opaque variety was due to ferric hydrate, 

 which latter was easily dissolved by hydrochloric acid, 

 leaving the quartz colourless. 



Stnoky Quartz. — The colour of this mineral has 

 been attributed to allotropic modifications of silica, 

 and also to organic matter. A. Foster found that on 

 distillation this mineral yielded a small quantity of a 

 brownish liquid containing ammonium carbonate, 

 and from this supposed that the colour of the mineral 

 was due to an organic substance containing carbon 

 and nitrogen. 



K 



