COMPOSITION AND STRUCTURE OF METEORITES 



11 



Crystals are brittle and insoluble in acids, even resisting the fluxes 

 potassium and sodium carbonates. Its composition is uncertain, 

 but it is commonly regarded as a titanium or zii'conium oxycliloride.^^ 



Pyroxenes. — -Pyroxene is common in meteorites in both ortho- 

 rhombic and monoclinic forms. 



1. Orthorhombic pyroxenes: enstatite, bronzite, and hypersthene. 

 These minerals, next to the olivines, are the most common of the 

 meteoric silicate minerals. The composition is somewhat variable, 

 owing to the varying proportions of ii'on and magnesia, as in the 

 olivines. A typical enstatite corresponds to the formula MgSiOa, 

 but through the assumption of iron this passes over into the bronzite 

 variety (AlgFe) SiOa. So far as known, the highly pleocliroic hyper- 

 sthene rarely occurs in meteorites, though in at least one instance — ■ 

 that of Shalka, India — -the percentage of iron is fully as high as in the 

 strongly pleochroic terrestrial mineral. The name clino-enstatite 

 has been proposed by Wahl -^ for a monoclinic variety with a smaller 

 extinction angle on clinopinacoidal sections than normal monoclinic 

 pyroxenes, and which is characterized further by a marked tendency 

 toward polysnj^thetic twinning. The varying composition of ensta- 

 tite and bronzite from some of the best known meteorites is given 

 below : 



' Smith, J. L., Amer. Journ. Sci., vol. 38, 1864, p. 225. 



« Maskelyne, Philos. Trans. Roy. See. London, vol. 160, 1870, p. 206. 



' Tschermak, Sitz. Akad. Wiss. Wien, vol. 61, 1870, p. 467. 



« Maskelyne, Philos. Trans. Roy. Soc. London, vol. 161, 1871, p. 359. 



' Rammelsburg, Monatsber. Akad. Berlin, 1870, p. 314. 



' Borgstrom, Bull. Comm. Geol. Finlande, No. 14, 1903. 



' Teclu, Rammelsburg's Mineralcbemie, 1875, p. 382. 



8 Meunier, Ann. Chem. Phys., vol. 17, 18P9, p. 12. 



« Rammelsburg, Monatsber. Akad. Berlin, 1870, p. 319. 



10 Winkler, Cohen's Meteoritenkunde, Heft 1, 1894, p. 281. 



11 Shannon, Amer. Mufeiim Novitates, Nov. 30, 1925. 

 " Also 0.69 Cr203 and 0.56 MnO. 



As with olivine the mineral rarely occurs in good crystal form 

 excepting in the porphyritic chondrules. A more common form, as 

 noted later, is in that of radiating and cryptocrystalline "kugels." 

 (See pi. 22.) 



" The fact seems not generally recognized that the oldhamite and osbornite occur only ia that portion 

 of the Busti stone which is plainly an inclusion, and further that osbornite has as yet not been identified 

 in any other stone, although oldhamite is comparatively common. 



26 Tschermak's Min. u. Pet. Mitteilungon, vol. 26, 1907. 



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