2 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 124 



Even allowing for its rapid recovery, the Allegan meteorite has a 

 remarkably fresh appearance. Many chondrites, even fresh falls, soon 

 develop rust stains around the nickel-iron particles. A plausible 

 explanation of this rusty staining is the decomposition of minute 

 amounts of lawrencite originally on the surface of the metal grains. 

 The Allegan specimens have remained perfectly fresh and free from 

 rusting after more than 60 years of exposure to the atmosphere. 



In hand specimens the meteorite shows an even granular structure 

 of gray color, with numerous chondrules that average one to two 

 millimeters in diameter, rarely more. Nickel-iron is visible in the form 

 of numerous brilliant metallic particles. The stone is extremely friable, 

 crumbling easily with handling. 



The Allegan meteorite was described by Merrill and Stokes (1900), 

 whose chemical analysis, although rated by Urey and Craig (1953) as 

 superior, shows some anomalous features, such as unusually high 

 metal content (23.06 percent nickel-iron), low NazO, and high K 2 0. 

 Since this meteorite is ideal for research, being very fresh as well as 

 available in comparatively large amounts, we have decided to re- 

 analyze it. 



Mineralogical composition and structure. — The principal 

 minerals in the Allegan meteorite are olivine and pyroxene. Nickel- 

 iron (kamacite and taenite), troilite, and plagioclase are present in 

 minor amounts. Accessor}^ minerals include chromite and merrillite. 

 Buseck and Keil (1966) recorded trace amounts of rutile, and Prof. 

 P. Ramdohr (pers. comm.) recognized chalcopyrrhotite (cubanite), 

 native copper, and mackinawite in polished surfaces. Notes on some 

 of these minerals follow: 



Olivine: The refractive indices are a=1.668, 7 = 1.706, corre- 

 sponding to a composition of 18 mole percent of the Fe 2 Si0 4 component, 

 according to the determinative curve of Poldervaart (1950). Keil and 

 Fredriksson (1964) reported an olivine composition of Fan.5 from 

 microprobe analyses of this meteorite. By the X-ray method of Yoder 

 and Sahama (1957) the composition was determined as Fai 9 . The 

 olivine peaks on the diffractometer chart are sharp, symmetrical, and 

 well defined, indicating olivine of uniform composition. 



Pyroxene: In an acid-insoluble fraction of this meteorite the 

 pyroxene is a mixture of transparent orthopyroxene and turbid poly- 

 synthetically twinned clinopyroxene. The refractive indices of the 

 orthopyroxene are a=1.672, 7=1.682, indicating a content of 16 mole 

 percent of the FeSi0 3 (Fs) component, according to the determinative 

 curve of Kuno (1954). Keil and Fredriksson (1964), by microprobe 

 analysis, determined the composition of the orthopyroxene to be 

 Fsi 6 . ; they reported that the pyroxene contains 0.37 percent Ca. In 



