164 



Proceedings of the Royal Irish Academy. 



that the mineral is the lightest silicate known of, almost. This is very 

 remarkable in a mineral containing such a very large percentage of 

 iron, the peroxide amounting to 20*720 percent. 



Physical characters. — Colour, velvet black. Hardness, about 2, 

 brittle ; lustre, waxy but dull ; streak, olive brown ; Blow-pipe 

 with difficulty fusible at edges to a black glass, which in some speci- 

 mens is magnetic. Very slightly affected by strong HCl or II2SO4 

 when in the mass, but decomposed by the former when boiled in it in 

 powder. Occurs filling amygdaloidal cavities in the basalt of Carnmo- 

 ney Hill, near Belfast; Shane's Castle, Lough JSTeagh^ ; &c. 



Chemical Composition and Formida. 



I shall compare my analysis of this mineral with analysis of deles- 

 site and chlorophaeite. 





Hullitc. 



Delessite.^ 



Chlorophseite.^ 



Silica_ . . . (SiOo), 

 Alumina . . (AI2O3), 

 Peroxide of iron . (FeoOs), 

 Protoxide of iron . (FeO), 

 Protoxide of mana;anese (MnO), 

 Lime . . ' . (CaO), 

 Magnesia . . (MgO), 

 Water . . . (H2O), 

 Carbonic acid . (CO3), 





39-437 



10-350 



20-720 



3-699 



Trace. 



4-484 



7-474 



13-618 



Trace. 



31-07 

 15-47 



17-54 

 4-07 



19-14 



0-46 



11-55 



33-30 



26-70 



40-00 





99-782 



100-00 



100-00 



Formula [CaMgFe"]3[Ar"Fe"']iSi60oi + 7HnO 

 Sp. gr., . . . . . 1-76 



2-89 



FeSiOs + eHoO 

 2-02 



Like other ferrugonous chloritcs (as delessite), to which group 

 this mineral appears in many respects to be allied, it is extremely 

 difficult to express its composition by a chemical formula. In the 

 first place, there is always some degree of alteration, which has changed 

 the characters of the mineral ; and besides, it is difficult to say whether 

 these minerals are true silicates or combinations of silicates with alu- 

 minates. "With regard to the last, it would be very difficult to decide 



^ A specimen of basalt from tMs locality containing very large cavities filled 

 with this mineral is to be seen in the Museum of the College of Science, Dublin, 

 as in the specimen mentioned, p. 161 ; the mineral is called obsidian. 



'^ Dana's System of Mineralogy, 1874, p. 497. 



^ lb., p. 410. Also Western Isles of Scotland, &c. John MaccuUoch, M.D.,. 

 p. 505. 



