A Biomolecular Survey of Calcification 111 



(about 50 A in the case of apatite, for instance) and in many mineralized tissues the 

 inorganic particles are at, or below, the threshold of detection; the term "amorphous" 

 applied to such a state is misleading. Finally, a small proportion of crystals of 

 detectable size will give a clear characteristic X-ray diffraction diagram in the 

 presence of relatively large amounts of "amorphous" material of the same, similar 

 or different chemical composition (Pautard, 1964). There is thus no reliable means 

 of estimating the structure of all the inorganic particles in a given tissue and no way 

 of deciding if the crystalline species that is present represents the total inorganic 

 content. The situation is further complicated by comparisons between X-ray diffrac- 

 tion data and observation in the electron microscope. While measurement of the size 

 of crystallites from electron micrographs can confirm the upper, or average, sizes 

 calculated from the width of selected reflexions in the X-ray diffraction patterns, it 

 cannot be assumed that all dense, crystal-like structures are, in fact, crystalline, 

 particularly as the instances of demonstrable single crystals by electron diffraction 

 are very few. 



With these reservations in mind, however, it is still possible to detect a wide 

 range of crystallinity of calcium salts, varying from colloidal suspensions (for 

 example calcium phosphates in cestodes — Brand et al., 1960) to crystals of optical 

 size (oxalates in plants — Pobeguin, 1943) and even larger crystals visible to the 

 unaided eye (calcite in sea urchin spines). An important aspect of the structure of 

 the inorganic phase, in spite of the wide diversity of forms, is the remarkable 

 constancy of the deposits in each calcified tissue. While it is true that there are numer- 

 ous chemical variations in minor cations (Mg, Sr, Na, K, etc.) and minor anions 

 (F, Si03,S04) and mixtures of calcium salts often occur, each mineralized structure 

 seems to be characterized by a careful control of crystallite shape and size. Even in 

 shells hardened by calcium salts at some distance, apparently, from any cells, the 

 crystallite size remains constant in the mature structure, as shown in the valve of 

 Lingula unguis by Kelly et al. (1965). In pathological conditions, a certain sem- 

 blance of constancy is sometimes retained in regions of the inorganic deposit 

 (Gonzales and Sognnaes, 1960, report a laminated, regular structure in dental 

 calculus) but more usually the crystallite size, shape and distribution tends to be 

 random. 



In normal tissues, the crystal characteristics of the three principal calcium salts 

 appear to remain relatively constant also. Calcium carbonate is found most frequently 

 in the form of calcite, occasionally as aragonite; both forms are sometimes found 

 together. Calcium phosphate occurs almost universally as an apatite, or in a closely 

 related crystallographic state. Calcium oxalate appears as several hydrates, the 

 optical characteristics of which have been long established (for example, Frey, 1929). 

 The X-ray crystallography of the oxalate, however, has been confined mostly to 

 comparative powder measurements (in a recent survey, Walter-Levy et Strauss 

 [1962], list the monohydrate, the dihydrate, a 2.25 hydrate, but question the ex- 

 istence of a trihydrate) although there are some limited data for the lattice of the 

 tetragonal polyhydrates (Honegger, 1952). Recently, measurements of the lattice 

 parameters of single crystals of monoclinic monohydrate raphides isolated from 

 Yucca have enabled us (Arnott et al, in press) to establish the dimensions of the 

 unit cell together with some information about the spatial arrangement of the 

 calcium atoms. 



