HRTEM Investigation of Microstructures in Length-Slow

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-equilibrium 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 microcrystalline 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 images 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 periodicity 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 relatively 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 precipitate 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 precipitation 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 aqueous 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-equilibrium 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 chambers filled _1.0 with drusy quartz and chalcedony,respectively, in a fossil coral. The temperatureand pressurefor the crystallization -1.5 of silica were presumablythe samein both chambers.The only difference is that the chamberwith drusy quartz was _2.0 A*"1yq closed, while the other one was open to the outside en- vironment by a channel.The fluid was presumablyslight- _2.5 ly over-saturatedwith respect to quartz in the drusy ;-""t" chamber becauseof the slow diffusion rate of the fluid 6 s -11, into the closed chamber.However, the fluid was probably Oo supersaturatedin the chamber with chalcedony because a the fluid could pass though the channel and maintain its -J.f supersaturatedstate containing respectto the crystallizing _4.0 qvartz. The drusy quartz is apparently a product of near- equilibrium crystallization from a less supersaturatedso- _4.5 lution than the solution that resulted in the precipitation 0 100 200 300 400 of chalcedony.Therefore, the activity of aqueousSiO, in the fluid during crystallization was lower in the chamber Temperature("C) with drusy quartz than that with chalcedony. Bonev (1990) suggestedthat some natural fibrous min- Frcunn 5, Calculated diagram showing the boundaries of aqueoussilica activities for quartz and amorphoussilica forma- erals crystallize under highly non-equilibrium conditions. tion as a function of temperatureat a pressureof 500 bars. The We infer that the chalcedony and other microcrystalline diagram was calculated using SUPCRT92 software and the da- quartz with dense Brazil twin boundariesand moganite- tabaseof Johnson et al. (1992). The region for chalcedonyfor- type domains in this study are also products of non-equi- mation (betweenthe saturationcurves for quartz and amorphous librium crystallization. The Brazil twins result from silica) is constrainedbased on synthesizedchalcedony that oc- growth mistakesduring rapid crystallization. These twins curs only below 300'C. XU ET AL.: MICROSTRUCTURES IN CHALCEDONY 545 respectto quartz reportedby Fournier (1977) and Walther Hardie, L A and Euster,H.P (1970) The evolution of closed-basinbrines and Helgeson (1977) indicate that the saturationindex of Geological Society of America Special Publication, 3, 273-290 Harvie, C E. and Weare,J H ( I 980) The prediction of mineral solubilities the mother fluids for chalcedony formation is higher than in natural waters: the Na-K-Mg-Ca-CI-SO"-H,O system from zero to that for qtrartz.The densetwin boundariesin chalcedony high concentrationat 25 "C. Geochimica et Cosmochimica Acta, 44, are presumablyresponsible for the high solubility of chal- 981-997 cedony. We observedthat the boundariesamorphize fast- Hattori, I. (1989) Length-slow chalcedony in sedimentaryrocks of the its use for tectonic er than defect-freeareas, which indicates Mesozoic allochthonousterrain in central Japanand the twin bound- synthesis.In J.R. Hein and J Obradovic,Eds., Siliceousdeposits ofthe aries are less stablethan twin-free areas.A suddenchange Tethys and Pacific regions, p.2Ol-215 Springer,Berlin. in fluid temperature,pressure, or pH could induce super- Heaney,PJ. (1993) A proposedmechanism for the growth of chalcedony sahrrationof aqueousSiOr. Changesin concentrationof Contribution to Mineralogy and Petrology, ll5,66-74. NaCl in the fluid can also affect the growth rate of quartz Heaney, PJ and Davis, A.M. (1994) Geochemical self-organizationin scale.Geological Society of America Abstracts (Huston agatesat the sub-micro and Walter 1995). The effect of the dissolved with Programs,26, A-111. NaCl is similar to supersaturationof the fluid with respect - (1995) Observationand origin of self-organizedtextures in agates to quartzgrowth rates.Brines and brine-like fluids in geo- Science.269. 1562-1565. logical systems (Garrels and MacKenzie 1967; Harvie Heaney,PJ. and Post, lE (1992) The widespreaddistribution of a novel polymorphs quartz varieties. Science, 255, and Weare 1980; Hardie and Eugster 1970) silica in microcrystalline may be a 441-443 factor contributing to the rapid crystallization of quartz Heaney,P.J., Veblen, DR, and Post, JE. (1994) Structuraldisparities and the formation of chalcedonywith denseBrazil twins. betweenchalcedony and macrocrystallinequartz. 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