American Mineralogist, Volume 83, pages 512-545, 1998 HRTEM investigationof microstructuresin length-slowchalcedony HurnnNc Xurt,* PnrBn R. BusBcrr2 ANDGunnNc Luo, rDepartmentof Earlh and PlanetarySciences, University of New Mexico, Albuquerque,New Mexico 87131-1116,U.S A. '?Departmentof Geology, Arizona State University, Tempe, Arizona 85287-1404,U.S.A. 3Departmentof Earth Sciences,Nanjing University,Nanjing, Jiangsu 210093, China Ansrrucr High-resolution transmissionelectron microscopy reveals denseBrazil-twin boundaries on the unit-cell scalein length-slow chalcedony.The twins can be consideredas the result of stacking of left- and right-handedquartz with a (101) twin composition plane. Although most twin sequenceson the unit-cell scale are nonperiodic, moganite-typedomains result where they are periodic. It is proposed that the twins formed during rapid crystallization rather than as transformation products of a precursor phase such as moganite. The twin boundariesare energetically less stable than twin-free areasand may indicate non-equilib- rium crystallization at a high supersaturationof aqueousSiO, speciesin the parent fluid. INrnooucrroN Smrpr.n AND ExpERTMENTALMETHoDS Microcrystalline quartz, which includes length-fastand The sample is from a Tertiary fine-grained conglom- length-slow chalcedony, cherl, and flint, is common in eratefrom Fang Hill, near Liihe, JiangsuProvince, China. sedimentaryand volcanic rocks (FrondeT1978; Fltjrke et A thin section shows zones of length-slow chalcedony al. 1991). Chalcedony is a variety of fibrous qtrafiz. separatedby narrow, dark, length-fast chalcedony (Fig. Length-slow chalcedonyis consideredto be a precipitate 1). The thicknessesof the length-slow zoneschange along from brine water and therefore an indicator of vanished the growth directions of the fibers. evaporates(Folk and Pittman 1971; Hattori 1989). Dif- Chalcedony with optically positive elongation was se- ferencesbetween chalcedony and quartz in composition, lected for TEM study from a petrographic thin section optical properties, and microstructures at the unit-cell and mounted on Cu grids. Thin specimenswere prepared scalewere detailedby Flcirkeet al. (1982,1991),Midgley by Ar-ion bombardmentand then coatedon one side with (1951),Frondel (1978), Miehe and Graetsch(1992),Hea- a thin carbon layer. SAED and HRTEM images were ob- ney and Post (1992),and reviewedby Heaney(1993). X- tained by using a JEOL-4000EX high-resolution TEM ray powder diffraction of microcrystalline quartz shows operatedat 400 keV. The main obstaclefor the TEM in- that it contains variable amounts of moganite-typestruc- vestigation was extremely fast irradiation damage under high-energy ture, a material that consistsof periodic unit-cell twins of the electron beam. A charge-coupleddevice (CCD) camera,which can record images left- and right-handed quartz (Miehe and Graetsch 1992; digitally at low electron-beamdoses, was used to achieve good defocus Miehe et al. 1984; Heaney and Post 1992). Selected-area electron diffraction (SAED) and conventional transmis- sion electron microscopy (TEM) studiesshow that length- fast chalcedony contains moganite domains and dense Brazil twin boundaries (Graetschet al. 1987; Heaney et al. 1994). Such planar defects also occur in authigenic quartz (Cady et al. 1993). Most planar defects in micro- crystalline qtrmtz, including intergrown length-slow and length-fast chalcedony, are apparently caused by Brazll twinning. There are no reported TEM results of microstructures in length-slow chalcedony,nor any high-resolution TEM results for either length-slow or length-fast chalcedony. In this paper we present high resolution (HR) TEM im- ages of length-slow chalcedony and discuss the twin boundariesin it and other microcrystalline qaartz. Frcunn l. Thin section of chalcedony that displays zoning of length-slow and intervening zonesof narrow, dark, length-fast * E-mail: [email protected] chalcedony(cross-polarized light with gypsum plate inserted). 0003-004x/98/0506-0542s05.00 542 XU ET AL.: MICROSTRUCTURESIN CHALCEDONY 543 of right-handed qttartz,i.e., moganite, occur in areaswith densely distributed composition planes (Figs. 3A and 3B). The periodicity of lattice spacingis 6.7 A perpen- dicular to {101} (Figs.3A and 3B). Calculatedimages show the doubling of the unit cell along (101)* of quartz under certain imaging conditions (Fig. a). The HRTEM image of periodic twin (moganite)domains showing pe- riodicity doubling(i.e.,6.7 A periodicity)along (l0l)* correspondsto the focus condition of -90 nm (Fig. 4). Some areasof length-slow chalcedony fibers contain rel- atively few compositionplanes (Fig. 3C). A SAED pat- tern from the area of Figure 3C containing few twin boundaries shows weak streaking. Heaney (1993) proposed that chalcedony can precipi- tate from slightly saturatedaqueous solutions at relatively low temperatures(<100 'C) by a spiral growth process, and the solubility of chalcedony is intermediatebetween the solubilities of opal-A and quartz. Later, Heaney and Davis (1994, 1995) proposedthat oscillation of defect Frcunr 2. [01] SAEDpatterns of length-slowchalcedony from (A) an areawith denselyperiodic and nonperiodictwins and(B) an areawith only denselynonperiodic twins. The addi- tionalweak, sharp reflections in A arecaused by intersectionof the Ewaldsphere with streakedreflections not in the zero-layer reciprocalplane (Heaney 1993). conditions for the images. Relatively low magnifications and electron beam doseswere used to minimize damage. Simulated HRTEM images were calculated using the MACTEMPAS computer program. The input structure model was based on the structure refinement of Miehe and Graetsch(1992\. Rrsur,rs AND DISCUSSToN SAED patterns from the length-slow chalcedonyshow strong quartz reflections and streaking along (101)* (Fig. 2). SAED patternsfrom some a"reasshow additional weak reflections that indicate doubling of the unit cell of quartz characterizethe moganite-type structure (Fig. 2A) based on XRD and TEM results of Miehe and Graersch(1992) and Heaney et al. (1994). The streaking is produced by the disordered intergrowth of Brazil twins with compo- Frcunn 3. HRTEM images of length-slow chalcedonyfrom sition planes parallel to HRTEM images from {101}. (A) an area with densely nonperiodic twin boundariesand peri- most areaswithin length-slow bands in our sample show odic unit-cell twins. (B) an area dominatedby periodic unit-cell non-periodic arrangementsof the composition {101} twins with a 6Jl A, periodicityalong the nor-ul to {101}, and planes (Fig. 3A). TEM observations from coarser do- (C) an area with relatively few twin boundaries. Some twin mains show features similar to quartz. Periodic iurange- boundariesare marked by white vertical bars. Best viewed at a ments of one unit layer of left-handed and one unit layer low angleparallel to the fringes. 544 XU ET AL.: MICROSTRUCTURES IN CHALCEDONY take {101} as compositionplanes becauseof the rela- ft tively low strain energy induced by structural distortion between the neighboring twin lamellae (Mclaren and Phakey 1966; Mclaren and Pitkethly 1982; Wenk et al. 1988). Quartz and amorphous silica can result from precipi- tation of aqueousSiO, from supersaturatedsolutions. The -120nm -1L0nm -100nm crystallization rate is related to the saturation index by k(O- ly (Rimstidt and Barnes 1980), where k = rate con- stant and /, = adjustableexponent. The activities of aque- ous silica in equilibrium with quartz and amorphoussilica at different temperaturesand pressuresare illustrated in -90nm -80nm -70 nm Figure 5. If the fluid is in an equilibrium or near-equilib- rium state (i.e., O is slightly higher than 1), only ther- Frcunr4. Calculated zoneaxis HRTEM image of mo- [111] quartz ganite(see text for detail). modynamically stable crystallizes. If the fluid is over saturatedwith respectto amorphoussilica (i.e., high f,)), amorphoussilica or opal forms. If the solution is un- concentrationsor zones of quartz and chalcedony result der saturatedwith respect to amorphoussilica but super- from fluctuating degreesof silica polymerization. In con- saturatedwith respect to qvarlz, chalcedony with dense trast, Wang and Merino (1990), based on their self-or- Brazil twins forms. A possible region for the crystalli- garuzation diffusion model, proposed rhat chalcedony zation of chalcedony is also labeled in Figure 5. crystallizes from silica gel. A fluid that is highly supersaturatedwith respect to We think the occurrence of multiple, fine-grained, quartz but under saturatedwith respect to amorphoussil- closely spaced fibers indicates high densities of nuclei ica only occurs at relatively low temperature (Fig. 5), during crystallization. Such high densities are related to which may be the reason for the restricted occurrenceof the nucleation rates, which, in turn, are roughly propor- chalcedony to low-temperature rocks. Rapid crystalliza- tional to the saturationindex (O) of the fluid (Zhang and tion of quartz resultsin growth mistakesof left- and right- Nancollas 1990). A supersaturatedsolution of aqueousSiO, handed qvartz. Similar phenomena have also been re- is necessaryfor producing high concentrationsof quartz nu- ported for a fibrous cristobalite ("lussatite") with dense clei for crystal.li2aliel of chalcedony and other microcrys- stacking faults (Cady and Wenk 1992). talline quartz varieties during rapid crystallization. The relatively higher solubilities of chalcedony with An interesting phenomenon was observed by Lund (1960), who described two neighboring