NO. 3624 METEORITES — MASON" AND MAYNE.S 6 



terms of the conventional subdivision of meteoritic pyroxene, this 

 falls into the compositional range of bronzite. The refractive indices 

 of the turbid twinned clinopyroxene are similar to those of the ortho- 

 pyroxene, and it is evidently clinobronzite of similar composition. 



The acid-insoluble fraction was scanned optically for the presence 

 of diopside, but none was seen. 



Plagioclase: A small amount of this mineral is present. It is fine 

 grained and turbid, and only a mean refractive index, about 1.536, 

 could be measured; this corresponds to a composition of about An™. 



Merrillite: Allegan was one of the meteorites in which merrillite 

 originally was recognized by Shannon and Larsen (1925). Fuchs (1962) 

 published an X-ray powder pattern of merrillite from Allegan which 

 showed that this mineral is structurally identical with the terrestrial 

 mineral whitlockite and a spectrographical analysis which indicated a 

 composition corresponding to the formula (Ca 2 . s Fe . 2 Mgo.iNao.i)(P04) 2 . 

 He did not find any chlorapatite in this meteorite, which is consistent 

 with the very low chlorine content reported by Reed and Allen (1966). 



Chromite: Snetsinger and Keil (1967) recently reported micro- 

 probe analyses of several meteoritic chromites, including that in 

 Allegan. Their results are: Cr 2 3 55.5, A1 2 3 6.3, Ti0 2 2.08, FeO 32.0, 

 MgO 2.31, MnO 1.05, sum 100.24, i.e., the composition corresponds 

 closely to the ideal formula FeCr 2 4 . The presence of over 2 percent 

 Ti0 2 is interesting, representing almost a 20-fold concentration of 

 titanium over the amount reported in the bulk analysis of the 

 meteorite; this is present in solid solution, not as exsolved ilmenite or 

 rutile. The rutile in Allegan is disseminated as discrete grains within 

 the silicate matrix, and Buseck and Keil suggest that it is a primary 

 crystallization. 



Tassin (1908), from chemical tests, recorded the occurrence of 

 oldhamite, CaS, in the Allegan meteorite; however, he was unable to 

 detect it microscopically, and we have found no trace of this mineral. 

 Perusal of his original paper suggests that the evidence for this 

 identification is unconvincing. 



The structure of the Allegan meteorite as seen in thin section 

 under the microscope is highly chondritic, the chondrules ranging 

 from 0.3-2 mm in diameter. Many are perfectly spherical, some are 

 less regular in form, and others appear to have been broken prior to 

 aggregation with the groundmass. A wide variety of chondrule types 

 are present: barred olivine chondrules, the bars consisting alternately 

 of olivine and of dark, turbid, almost opaque material; chondrules 

 consisting of numerous euhedral olivine crystals in a gray turbid 

 matrix, probably devitrified glass; fibrous radiating pyroxene chon- 

 drules; and chondrules containing both olivine and pyroxene. The 



