The crystal structure of the chondrodite series

The chondrodite group of minerals comprises prolectite (H 2 Mg 3 SiO 6 ), chondrodite (H 2 Mg 5 Si 2 O 10 ), humite (H 2 Mg 7 Si 3 O 14 ) and clinohumite (H 2 Mg 9 Si 4 O 18 ). We have so far been unable to obtain a specimen of the first—there seems, indeed, to be some doubt as to its existence—and, in consequence, the present paper is concerned with the last three members only. Our conclusions are based on X-ray data obtained by the rotating crystal method and by the ionisation spectrometer. None of the crystal specimens was suitable for absolute measurements of the intensity of reflexion, and the investigation has therefore been wholly qualitative in character. This, of course, increases considerably the difficulty of the problem, especially in so far as an accurate determination of parameters is concerned; for in attacking crystal structures of even moderate complexity, the advisability of a quantitative study is being more and more realised. In the present case, we are dealing with crystals possessing a very large number of parameters ( e. g ., 24 in chondrodite, 36 in humite and 45 in clinohumite), and even a detailed quantitative examination might be unsuccessful without the use of some guiding principle or principles. These principles will be apparent later.