Tobacco Documents Online

CS National Bureau of Standards Special Publication 506. Proceedings of the Workshop on Asbestos: Definitions and Measurement Methods held at NBS, Gaithersburg, MD, July 18-20, 1977. (Issued November 1978) THE CRYSTAL STRUCTURES OF AMPHIBOLE AND SERPENTINE MINERALS Jack Zussman Department of Geology University of Manchester England Abstract The crystal structures of the two main asbestos-forming minerals, the amphiboles and serpentines, are surprisingly very different. The amphiboles are "chain silicates" in which Si04 tetrahedra are linked to, form bands four tetrahedra wide and of very great length. These bands run parallel to the asbestos fiber axis and are linked laterally by cations, mainly Ca and Mg in tremolite; Na, Mg and Fe in crocidolite; Mg and Fe in amosite and anthophyllite. The tempting correlation of the chain unit of crystal structure with asbestiform nature is, however, too facile. Many amphiboles are not asbestiform, and as the serpentine minerals show, some asbestiform minerals do not have a chain structure. The serpentine minerals are "layered silicates" in which Si0* tetrahedra are linked to form thin sheets of great lateral extent. The tetrahedra all point in the same direction and their apical oxygens are part of an (O,OH)-Mg-(OH) sheet which is itself formed by Mg-(O,OH) octahedra. Thus the fundamental serpentine layer is polar and has a tetrahedral and octahedral component. The mismatch in dimensions of these two components generally leads to curvature of the layers and in chrysotile asbestos the layers form either scrolls or concentric cylinders with very high length/breadth ratio and with length parallel to the fiber axis. Other forms of serpentine, however, with chemistry very similar to that of chrysotile, do not exhibit asbestiform morphology. For all minerals, the physical and chemical properties are impor- tant both for industrial usage and environmentally in determining the nature of the dusts produced in manufacturing processes and in subsequent abrasion. Factors which may influence properties in addition to the basic chemistry and "average" x-ray structure are the crystal morphology and mode of aggregation, and also the abundance and nature of structural defects. Keywords: Amphibole; asbestos; chemistry; cleavage; defects; dusts; environment; fibers; morphology; serpentine; structure. In this review I would like to describe briefly the crystal structures of the two main asbestos-forming minerals, the amphiboles and serpentines, to consider what they have in common and what are their differences, to identify if possible what are the fundamental criteria that lead to asbestiform habit, and to observe the crystallographic features that may contribute to their physiological behavior. Preceding page blank 35 N ~ 0 ~ w a

,. Amphiboles These minerals are "chain silicates" in which SiO,i tetrahedra are linked so as to form chains with composition 5i,011 as shown in figure 1. The chains are four tetrahedra wide, of very great length,and they lie parallel to the fiber axis in the asbestiform amphiboles. one might say that amphibole asbestos is finely fibrous because of the chain structure, but this is an over simplification. Some amphiboles are not fibrous at all, let alone asbestiform. -0] il. lI L2 ~ ~ E--5~3 A--)-- 0 Oxygen ® OH Cc Figure 1. Plan and end-view of an idealized Si4011 amphibole chain together with additional (OH) ions. No minerals could be formed from Si*011 chains alone and in the amphiboles there are cations linking chains laterally as shown in figure 2. The cations vary from one amphibole to another. In tremolite Mg ions link chains by means of a strip of Mg(O,OH) octahedra. The oxygens of this strip are the apices of the Si-0 tetrahedra and the OH ions occur as in figure 1. Calcium ions link the chains across the bases of the tetrahedra. An alternative view of the structure is one of almost continuous sheets of Mg and Ca polyhedra linked by Si ions. 36

x 4a L-_~ / I A 40 V o Y a      0 0 Figure 2. Schematic end-view of amphibole chains linked by cations in K and Y positions. In some amphiboles the site A is occupied. The amphibole formula can thus be written Ao_1K2Ys(5i,A1)8o22(OH)2. Other important amphiboles are: - anthophyllite in which largely Mg ions play the role of both Ca and Mg in tremolite; the cummingtonite - grunerite series, which contain Mg and Fe in varying proportions; and riebeckite, in which Mg, Fe and Na are the principal cations in addition to Si (see Table 1). The above-mentioned compositions are those most relevant to the consideration of asbestos, since in addition to the less common varieties of asbestos, tremolite and anthophyllite, there are the two more abundant and commercially more important varieties -'amosite,' a form of cummingtonite - grunerite, and "crocidolite" (blue asbestos), a form of riebeckite. 37 O w I

0 Table 1. Cation distribution in idealised formulae of the amphibole minerals. Asbestos-forming amphiboles are marked*. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - A X Y Z Cummingtonite-Grunerite* ., - S (Mg,Fe)Z (Mg,Fe)s Sig Anthophyllite ' * Gedrite - (Mg,Fe)2 (Mg,Fe)3A12 Si6A12 Tremolite*-Actinolite - Ca2 (Mg,Fe)s Sig Common Hornblende - Ca2 (Mg,Fe)4A1 Si,A1 Tschermakite - Ca2 (Mg,Fe)3A12 SieA12 Edenite Na Ca2 (Mg,Fe)s Si7A1 Pargasite-Hastingsite Na Ca2 (Mg,Fe)sA1 Si6A12 Richterite Na NaCa (Fe)s Sig Katophorite Na NaCa (Mg,Fe)+A1 5i7A1 Mboziite Na NaCa (Mg,Fe)sA12 Si6A12 Glaucophane-Riebeckite* - Na2 (Mg,Fe)3A12 Sig Eckermannite-Arfvedsonite Na Na2 (Mg,Fe)4A1 Sig CALCIUM AMPHIBOLES ALKALI AMPHIBOLES For the sake of completeness at least, though it may also have some indirect importance, it should be noted here that the amphibole asbestos-forming minerals are monoclinic in sym- metry except for anthophyllite which is orthorhombi c. Cell parameters are given in Table 2. Table 2. Amphiboles. Cell parameters.a aA bA cA p Tremolite [1]1 9.82 18.05 5.28 104°39' Actinolite [2] 9.89 18.20 5.31 104°38' Grunerite [3] 9.56 18.30 5.35 101°50' Crocidolite [4] 9.74 17.95 5.30 103°54' Anthophyllite [5] 18.56 18.01 5.28 900 a These cell parameters relate to particular specimens. Variations in chemical composition, particularly in Fe/Mg ratio, can be expected to yield a range of values but usually within 1 or 2 percent of those given. N 'Figures in brackets indicate the literature references at the end of this paper. ~ w 38 o A W W ~

A well-known physical property of amphiboles is that they generally cleave readily along (110) planes. Assuming that the Si-0 chain is a strong structural unit, and that the strongest inter-chain bonding is across the strips of octahedra joining tetrahedral apices, the probable paths of weakness can be traced as on figure 3, which on a macroscopic scale results in cleavages intersecting at approximately 1200, as observed. / r Figure 3. Schematic view of amphibole structure as seen down z axis, showing likely paths of weakness leading to cleavages intersecting at 57°. Although the good prismatic cleavages explain the readiness of amphibole crystals to splinter into elongated particles, this is not necessarily relevant to the unusual physical nature of asbestos. It would be so if a block of asbestos was a single crystal and the production of hair-like fibers was the process of splitting off cleavage fragments. However, a block of asbestos, even when very small, is not a single crystal but an aggregate of single- crystals all lined up parallel to the fiber axis but with a range of azimuthal orientations. The process of stripping fibrils from asbestos is thus more likely to be one of breaking crystallites away from the aggregate at the grain boundaries across which there is weak cohesion. 39 2063104840

Cot The asbestiform nature of certain amphiboles is thus a consequence of the crystallite morphology which in turn is influenced by the conditions of crystal growth as well as the inherent chemical and physical features of a single crystal. The production of a fiber aggregate as in asbestos must depend upon independent nucleation of each fibril and its preference for growth along z rather than at right angles to it. It is perhaps significant that the group of amphiboles loosely referred to as "hornblendes" occur in roughly equidimensional crystal habits and not as asbestos. Table 1 shows a simplified scheme for the chemical compositions of amphiboles and it is seen that the hornblendes are characterized chemically by having appreciable substitution of Al for Si. Asbestiform amphiboles show little substitution of this kind. The minerals richterite and eckermannite, which also have little Al for Si substitution, are not known to occur naturally as asbestos, but synthetic products have been so described and are at least extremely fibrous [6,7]. I would suggest therefore that the substitution of Al for Si might be responsible for increased potential for growth of prism faces relative to growth in the z direction. Even if true, the above suggestion cannot be the only criterion that governs asbestos formation since tremolite itself can occur in asbestiform or non-asbestiform habit, each variety having the same major element chemical composition. In such circumstances other parameters such as the pressure and temperature conditions, rates of cooling or heating, or minor or trace element concentrations may be critical factors. The mechanical properties of asbestos and related minerals are of importance both for the desirable physical attributes of articles made from asbestos, and environmentally in determining the nature of the dusts produced during the processes of manufacture or during subsequent abrasion. Factors which can give different mechanical properties are the nature of the fundamental particles and their state of aggregation (bundles of fibers versus single crystals). For single fibrils or crystals in the {110} cleavages, and the resistance to breakage across other planes (roughly perpendicular to fiber length), will help to determine the morphology of the dust particles produced. Structural defects may also have an influence on physical properties. Structural defects It should be emphasized here that the published crystal structures of amphiboles (and serpentines), determined by x-ray diffraction, are the content of the "average" unit cell, the volume of specimen investigated consisting of something like 1015 unit cells. In real crystals the unit cells do not repeat perfectly and several kinds of defects may occur. These departures from the perfect structure are no doubt important in questions concerning crystal growth and they may well influence physical properties and physiological effects. The two principal kinds of imperfection in amphibole structures are stacking defects and Wadsley defects. Stacking defects are illustrated schematically in figure 4. In the normal monoclinic amphibole, slabs of structure parallel to (100) are stacked alongside one another with regular displacements. In a faulted structure occasional errors in the direction of this displacement occur and the frequency of such faults varies from one specimen to another. When the faults are relatively infrequent the result can sometimes be described as a twinned crystal. Figure 5 shows a high resolution electron micrograph displaying twin components. Such defects are also seen in lower magnification electron micrographs and they have important effects on diffraction patterns. When the faults are frequent and regularly repeating, they are no longer really faults but are the regular displacement of a structure with a super-cell and perhaps different symmetry. The latter describes approximately the relationship between the orthorhombic and monoclinic amphiboles. 40

CS monoclinic monocUnic twin WO orthorhombic monoclinic with stacking fauLt Figure 4. Illustration of the stacking of blocks of amphibole structure to forni a) regular monoclinic, b) monoclinic twinned, c) monoclinic faulted, and d) orthorhombic structures. The fault plane is (100). Figure 5. High-resolution electron micrograph of amosite showing faulted and twinned struc- tures. (Electron beam parallel to Y) Figure from J. L. Hutchinson et a1. [8]. 41 2063104842

Figure 6 illustrates the Wadsley defect by showing how parts of an amphibole crystal might contain occasional triple or single Si-0 chains distributed among the normal double chains. In low magnification electron micrographs such defects are seen as linear features parallel to (010) (fig. 7). PYROXENE- AMPHIBOLE /-'a7Ma A/LJ\A ~ single chains double chains I triple chain double chain double chain double chain double chain single chain Figure 6. Schematic illustration of a) pyroxene structure, b) amphibole structure, c) amphibole with triple chain Wadsley defect, and d) amphibole with single chain defect. Figure 7. Electron micrograph of amphibole with beam perpendicular to y showing Wadsley defects on (010). Figure from J. E. Chisholm [9]. 42

C3 For environmental health considerations we do not yet have a causative understanding of the harmful effects of asbestos and do not know which properties of asbestos are involved. It is conceivable therefore that more subtle structural factors than those described above might be important. Although the structure described is broadly correct for all amphiboles, minor differences in atomic coordinates occur from one amphibole to another. Structure determinations have been performed for non-asbestiform tremolite, actinolite, anthophyllite and grunerite, but not for asbestiform specimens because of technical difficulties. For crocidolite, a fiber approaching a single crystal was used rather than a hair-like strand of asbestos. As part of the details of structure, variations can occur in the way in which Fe and Mg atoms are distributed amoQg similar but not strictly equivalent octahedral sites. In some amphiboles the role of Fe3 may be significant in oxidation-reduction processes, and there is the possibility of Na (or Ca) having a degree of cation exchange capacity. Serpentine Chrysotile, another important variety of asbestos is not an amphibole but is a member of the serpentine group of miner;1s. Because of its asbestiform character, and repeat distance in the unit cell of about 5.3 A parallel to the fiber axis (similar to that in amphiboles), it was once thought to have a chain-like crystal structure. Later work, however, showed it to be a layered silicate with structure analogous to that of the clay mineral kaolinite, but with Mg instead of Al in its composition. The paradox of how a layered mineral could have asbestiform habit was solved largely by Whittaker [10,11,12] who deduced from x-ray diffraction patterns that layers are rolled to form concentric cylinders or scrolls with their long axes parallel to the fiber. This indirect evidence was supported by electron microscopy of transverse sections of chrysotile, culminating in the spectacular high-resolution photographs published by Yada (fig. 8). Figure 8. High resolution electron micrograph of transverse section of chrysotile asbestos. Figure from K. Yada [13]. 43

0 The reason for the curving of the fundamental layers in chrysotile can be seen by examination of their chemical composition and structure. Each layer has two components, one a sheet of linked Si-0 tetrahedra, and the other (,joined to the first by sharing apical oxygens), a sheet of (Mg-O,OH) octahedra. A plan and elevation view of the composite layer, Mg3SizOs(0H)*, is shown iP figure 9. In order to form a flat-layered composite the dimensions of each component would need to match fairly closely. Reasonable estimates of the repeat distance of each show that the tetrahedral Si sheet has smaller dimensions than the octahedral Mg sheet, and this mis-match can be overcome by curvature, with the Mg sheet outermost, or by some other means of relieving structural strain. This leads to a number of strange structural configurations in serpentines, one of which is the tube-like character of chrysotile. W. O -{JT! I 1 . I b~ ~I I I ~ ~ I , I I 1 I I I j I I I I oti o e I ~ b O O O o 0 Me x x x x x x x x x x O.OH O O 0 O O 0 Si 0 e ~-b~o9~2A ~ Ol4 O O O O O O O O O O O cd•2A Figure 9. Plan and elevation views of idealized serpentine structure. Electron micrograph studies of chrysotile asbestos show that diameters of natural fibrils are of the order of 100 to 500 A, and the length/breadth ratios are of the order of 100 to 1 or greater. The limitation on growth in the radial direction is more easily understood for chrysotile than for amphiboles in that as successive layers are added during the growth process, the radius of curvature increases, eventually deviating too far from its ideal strain-free value to be energetically favorable. Thus chrysotile asbestos probably forms by multiple nucleation, usually on the walls of veins in massive fine-grained serpentinite rock, with relatively rapid growth in the fiber direction and limited growth at right angles to it. It is pertinent in the context of possible environmental problems to consider the structure and morphology of other serpentine minerals which have very similar composition but are not asbestiform. One such mineral is antigorite. It too has a curved sheet structure, but the layers are corrugated rather than rolled (fig. 10). The corrugations have a rather regular wavelength so that quite well-formed crystals result. Sometimes they are equidimensional but they have a tendency to be thin, and lath-like parallel to y. Antigorite 44