Wright — Three Contact Minerals. 545 



Art. LI. — On Three Contact Minerals from Velar dena, 

 Durango, Mexico. (Gehlenite, Spurriie and Hillebrand- 

 ite) / by Feed Eugene Weight. 



Dueing the summer of 1907, a geologic examination of the 

 Yelardena mining district in Mexico was made by Mr. J. E. 

 Spurr, assisted by Mr. G. H. Garrey. Several of the thin 

 sections of the material there gathered were sent to the writer 

 for examination, and in one of these a mineral with peculiar 

 optic properties was observed. At the suggestion of the writer 

 an adequate collection of the rock from which the thin section 

 had been cut, was then made by Mr. Garrey, and in this col- 

 lection the three minerals to be described below were found. 

 Two of these minerals proved to be new mineral species, sili- 

 cates of interesting composition, while the third, gehlenite, is 

 apparently novel for this continent. All three are contact 

 minerals, formed near the junction of altered limestone and 

 intrusive basic diorite and their relations to the contact and 

 conditions of formation have been carefully studied by Messrs. 

 Spurr and Garrey. As the results of their extended investi- 

 gation will soon be ready for publication it has not been 

 deemed necessary to consider in detail in this present paper 

 conditions of occurrence and formation of these minerals 

 and their relations to the ore deposits in general. 



The chemical analyses of the three minerals and their specific 

 gravity determinations were made by Dr. E. T. Allen of the 

 Geophysical Laboratory, and to him the writer is deeply 

 indebted for the courtesy. 



Gehlenite* 



This mineral occurs in massive granular aggregates, usually 

 dark gray or gray -black in color, from minute inclusions of 

 magnetite and other particles. Rarely small pieces of gehlen- 

 ite of amber-yellow color and free from magnetite inclusions 

 were observed. The grains are rounded in outline and not 

 suitable for crystallographic measurement. The physical and 

 optical properties, however, are similar to those recorded for 

 gehlenite from other localities, and the chemical composition 

 also agrees as well with the prescribed formula as the analyses 

 of the type material. 



Crystal system, probably tetragonal, judging from the cleav- 

 age, which is imperfect after 001 and much less well marked 

 after a prism. Fracture, uneven and irregular, conchoidal to 

 splintery. Hardness, between 5 and 6, about 5*5. Luster, 



* Type specimen from contact aureole of the Terneras intrusion, Velardena, 

 Durango, Mexico. 



