Systematic compositional zoning in the quaftz fibers of agates

Etched sections of agates examined by scanning electron microscopy show three types of compositional zonation'. (1) two-unit oscillatory sequences in which the successive zones are of the same width, on a micron scale, but show a uniform difference in the content of (OH) in structural sites; (2) three-unit oscillatory sequenc€s of various palindromic types in which successive zones are of the same width, on a micron scale, but show a uniform diference at three levels of (OH) content; and (3) disordered sequences in which the zone width is variable, on a micron to millimeter scale, and the (OH) content varies in a highJow but non-uniform way. All three types range in length up to thousands of successive zones, and often occur intermixed. These types of zonation constitute the whole of the fibrous parts of agates in general. The oscillatory zonation is the result of a cyclic interplay between gowth rate and diffusion rates at a crystafsolution interface. In agates the process operates at the interface between the quartz fibers growing on the cavity wall and the inner solution and is imposed on all fibers simultaneously. It is identical in crystallochemical mechanism with the coarse random zonation seen in large individual crystals ofType B quartz. Introduction In briei the fibers are elongated along [1120], are polar The idealized agate represents the infilling of a spheroiand hence are piezoelectric and show unlike growth rates dal cavity by quartz fibers. The fibers tend to grow radially in opposite senses along [1120], are chiral, and show inward. They can be described as forming a crust on the stereospecific twisting along their length. The fibers range cavity wall that thickens up to the point of complete filling. in thickness up to about 0.5 pm and in length up to a few A color zonation caused by pigmenting impurities usually centimeters. They are not cylindrical but tend toward a is imposed during the course of infilling. Agates are usually lath-like shape. described as seen in sawn diametral cross-section. The The study is based on the examination of several color zonation then appears as a succession of concentric hundred sawn agates, from world wide localities, contained bands. The width of the bands varies randomly in the milliin the Harvard mineral collection or obtained from commeter range. The bands usually have been interpreted, inmercial dealers. Ideal study material is provided by com. correctly as will be seen, as marking separate stages or plete diametral thin sections of the nearly colorless longlayers ofdeposition. fiber agates from the Antelope area, Oregon. When polished agate cross-sections are etched and examined by scanning electron microscopy, the visual banding is resolved into a concentric succession of zones on a micron to submicron scale. Study by infrared, X-ray diffraction, optical and thermal techniques has established that the successive zones involve variation in the content of (OH) in structural sites in the quartz itself (Frondel, 1982). The systematic nature of the variation is described in this paper. The compositional zonation is effected at the interface between the inner crystallizing solution and the growing fiber crust on the cavity wall. It is imposed on all fibers simultaneously and can be properly described as a compositional variation along their length. Individual fibers in the converging array may show a succession of hundreds of thousands ofsuch zones or compositional units. The crystallographic characters of the quartz fibers of agates have been described elsewhere (Frondel, 1978,1982\. 0003 004x/85/091H975$02.0O s7s Compositional zonation Three different types of sequencing have been recognized in the compositional zonation that constitutes agates. l. Oscillatory two-unit sequences. These comprise orderly sequences in which successive units of constant length oscillate between two different but uniform levels of (OH) content (Fig. 1). The two units are designated as H (higher content of (OH)) and L (lower content of (OH)). The length of the ... LHLH ... sequences usually ranges from a few hundred to a few thousand units, but examples have been found with up to 15,00 successive units. The length of the individual units is essentially constant in a given agate specimen, with the L units slightly longer in some instances. The length varies in different specimens from submicron dimensions up to about three microns. The difference in (OH) content between successive zones also is constant in a given specimen but differs in different specimens. 976 FRONDEL: COMPOSITIONAL ZONING IN QUARTZ F/BERS Fig. l. Relatively coarse two-unit oscillatory sequence. Deeply etched, with the L zones in relief. The platy or lathlike nature of the quariz fibers can be discerned. Bar is 10 microns. This and the accompanying figures are from a single agate from Antelope, Oregon. Harvard specimen no. 124129. This is shown by the relative depth of etching and, more precisely, by differences in the angular separation of the X-ray reflection angles of the H and L units in different bulk samples of ... LHLH ... sequences. The reflection angles and the etching rate increase with increasing content of (OH), but the relation has not been calibrated. Two-unit sequences as seen in transmitted light also have been illustrated by Reis (192O), Heinz (1930), and Jones (1952). The extended sequences act as optical diffraction gratings in transmitted light and yield the so-called iris agate described by Brewster (1843), Jones (1952), and by Raman and Jayaraman (1953). Many agates were found here to contain oscillatory zonations of appropriate dimension but that were not sufliciently translucent to display the iris effect. An iris effect also has been observed in garnets containing two-unit oscillatory compositional sequences on a submicron scale (Akizuki et al., 1984). 2. Oscillatory three-unit sequences. Numerous types of orderly three-unit oscillatory sequences were observed. The unit length is constant and three different but uniform levels of (OH) content are involved. These are designated as H, L, and M (intermediate content of (OH)). The most common three-unit sequence is . . . LHMHLHMHL ... Other orderly sequences included ... LHMHMHL . '. and similar sequences with longer L to L separation in the identity period (Fig. 2). Sequences of the type LHLHMHMHLHL ... also were observed. The identity periods of the orderly sequences, such as LHMHL in the sequence first cited, in all instances are palindromic. (Palindromes read the same forward and back. They may have either a center or a line of symmetry such as, in their familiar expression as words or sentences, the examples Madam and Hannah.) All observed quartz flber sequences are centrosymmetrical and hence odd-numbered, without any periodic variation in unit length such as represented by the even-numbered sequence LHMMHL. Three-unit sequences are less common than two-unit sequences and on average are shorter in length. Different Fig. 2. Three-unit oscillatory sequence, HLMLMLMLMLH, merging into a two-unit oscillatory sequence on left. Bar is 100