Section IV., 1908. [ 16B ] Trans. R. S. C. 



IX. — Dawsonite: A Carbonate of Soda and Alumina. 

 By EiCHARD p. D. Graham, M.Sc, 



Lecturer in Mineralogy, McGill University. 

 (Presented by Dr. F. D. Adams and read May 26, 1908.) 



The mineral described below was first collected as a probably new 

 species by Sir William Dawson from a trachyte dyke which cuts through 

 the Trenton limestone, near the western end of McGill College, Montreal, 

 and an examination of the chemical composition by Dr. B. J. Har- 

 rington having proved this surmise to be correct, the latter named the 

 mineral Dawsonite in honour of Sir William.^ 



The dyke in question is shattered by numerous joints, and along 

 the narrow fissures so formed there has been a subsequent deposition of 

 minerals, notably of calcite, dolomite and pyrite, with some galena and, 

 less often, a black manganiferous substance. The surfaces are also 

 occasionally found to be coated with a thin layer of whita or colourless 

 bladed crystals, which have a fibrous appearance and a bright glassy or 

 somewhat silky lustre, and it is this latter mineral to which the name 

 dawsonite has been given. The crystals often closely resemble the white 

 variety of tremolite, and, indeed. Dr. Harrington found several speci- 

 mens labelled as such in the mineral collection of Dr. Holmes, which the 

 University had acquired in 1856; these, too, had been collected near 

 McGill College. It is 'also interesting that tliis mineral had been 

 observed by Dr. Sterr}'^ Hunt as early as 1863, when, in a description 

 of the same dyke, he reported the occurrence of " thin bladed crystals of 

 an aluminous mineral, apparently a zeolite" coating the surfaces of 

 the joints; but he did not collect sufficient material for a complete 

 analysis. 



Harrington made several analyses, of which he published two in 

 the paper referred to .above, and these are reproduced on page 174 

 (columns I and II). Although these analyses agree fairly closely with 

 one another, they do not lead to any simple formula, and Harrington 

 could only conclude thai the mineral is a carbonate of lime, soda, and 

 alumina. It was, however, especially interesting from the fact that 

 normal aluminium carbonate is unlmown as a chemical substance, and 

 although a naturally occurring carbonate of alumina and lime had been 

 described by J. H. and G. Gladstonje^ under the name hovite, this latter 



1 Can. Nat. New Series, VII. 6, p. 30.Ï, 1874. 



' Phil. Mag. XXm. 461. 1862. The hovite occurs in association with 

 collyrite, (SiOo.2A1.2O.i.l0H2O.) The analyses all show the presence of silica, 

 but the authors argued that it is really an Al-Ca-caibonate intimately mixed 

 with collyrite. Both are soft white amorphous substances. 



