116 RE-EXAMINATION OF AMERICAN MINERALS. 



excess of sal ammoniac to the solution before the addition of the 

 ammonia, we redissolved the precipitate, added sal ammoniac, 

 and reprecipitated the oxide of iron ; this was done even a 

 third time, before the last traces of magnesia were got rid of, 

 or that we were sure that the amount of iron would not be in- 

 creased by containing magnesia — a circumstance in which 

 sufficient precaution is not always used. "What is here said 

 of oxide of iron is equally true of alumina. 



8. Lancasterite, a mechanical mixture of Brucite and 

 Hydro-magnesite. 



While on a mineralogical excursion to the localities near 

 Texas, Pa., a few months since, in company with Mr. W. W. 

 Jeffries, we observed at Wood's Mine a peculiar magnesian 

 mineral, somewhat resembling lancasterite ; a chemical exam- 

 ination showed it to be hydro-magnesite. The composition of it, 

 as well as its strong resemblance to some specimens of lancas- 

 terite, led to a re-examination of the latter species. 



Lancasterite is described as occurring "foliated like brucite," 

 but sometimes in crystals "resembling somewhat stilbite or 

 gypsum." As we desired to see whether these forms were 

 identical in chemical composition, a portion of the foliated 

 mineral was carefully selected and the amount of carbonic acid 

 determined. It was but a trace; the magnesia and water being 

 estimated gave the same amount as is found in brucite; there 

 was also a trace of manganese and iron. 



Some of the small crystals " resembling stilbite or gypsum " 

 were then examined ; analysis showed them to have the same 

 composition as the hydro-magnesite of Kobell. 



These results go to prove that lancasterite is not a dis- 

 tinct species, but a mechanical mixture of brucite and hydro- 

 magnesite. In Dr. Erni's analyses of this mineral (made in 

 the Yale Laboratory) we are aware he found great difficulty 

 in obtaining a constant composition, and it was only after a 

 series of analyses that he obtained any concordant results. 

 The specimens he examined were both crystallized and foliated, 

 the folia in some cases overlying the crystalline portion. 

 With this explanation the composition he obtained is easily 

 understood. 



