and Laboratory Methods. 1475 



Gareiss, A. Ueber Pseudomorphosen nach Entirely aside from synonyms, some 

 Cordierit & Tschermak's. Min. u. petroe. ^ ^ u u • *. 



Mitth. 20: 1-39, .900. twenty names have been given to 



pseudomorphs after cordierite (iolite) 

 which are simply stages and phases of decomposition, the usual final product of 

 which is muscovite or biotite. The author examines most of these, and con- 

 cludes that the decomposition starts from a network of clefts in the cordierite, 

 sometimes arranged irregularly, at other times parallel, both to base (OUl) and 

 the cleavage (Ul(»). In several instances clefts were also observed parallel to 

 the prism (110). 



These alteration clefts in some instances serve only as central canals for the 

 transportation of material, and enclose a fine grained zone. At other times this 

 zone is surrounded by another, in which extinction takes place in perfect unity 

 with the cordierite, but differs in the color and the lowered double refraction, 

 which may reach isotropy. Frequently this material fills the entire space 

 between the canals, and the entire crystal becomes thus " intermediate " sub- 

 stance composed of undeterminable fibers and little scales. 



A third type of cleft shows little fibers or scales perpendicular to the central 

 canal, and frequently with brilliant interference colors. 



The final products of the decomposition are mica and chlorite, and some 

 quartz. The MgO of the iolite gradually diminishes, and water, alkalies, and iron 

 enter. The mica is usually muscovite, rarely biotite, and in one case paragonite. 

 The tendency to form muscovite is shown by the fact that this exclusively is 

 formed from cordierite in granites and gneisses (which are rich in potassium), 

 whereas in cases such as the cordierite in quartz lenses in mica schist, where 

 little potassium is obtainable, the pseudomorphs consist principally of chlorite. 



Upon the basis of alteration product the author proposes the following 

 nomenclature, which is practically in conformity with the original use of each 

 name : 



Finite. — Preponderating final product mica, and without lamellar parting 

 parallel (001). Here are included the occurrences of Schneeberg, Auvergne, 

 Silberberg, Schonfeld, and Fichtelgebirge. 



Gigantolite. — Preponderating final product mica, and with lamellar parting 

 parallel (001). Here are included the pseudomorphs from Heidelberg and 

 Wasserhauseln. 



Prasiolite (Praseolite, wrong orthog.). — Preponderating final product chlorite, 

 and without lamellar parting parallel (001). Here are included the occurrences 

 from Bamle, Kragerde, and the Alpine pseudomorphs, which show no lamellar 

 parting. 



ChlorophylUte. — The preponderating end product chlorite, and with lamellar 

 parting parallel (001). Here belong the occurrences of Haddam, Unity, and the 

 gigantolite of Tammela, the lamellar fahlunite of the Talkschiefer, and the 

 Alpine pinites with lamellar parting. 



As to the many other names the author points out the essential identity of 

 several, as shown by descriptions. These may be summed up as follows : 



Esmarkite == ChlorophylUte. 



Raumite = Prasiolite. 



Weissite, Triclasite and Huronite ^ Fahlunite, but fahlunite not in every 

 instance a cordierite pseudomorph. 



Iberite = Gigantolite. 



Bonsdorffite = Prasiolite. 



Micarrell Frieskben = Kataspilite Igelstrotn, which is not a cordierite pseudo- 

 morph according to author. a. j. m. 



