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Laser Raman Identification of Silica Phases Comprising Microtextural

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Laser Raman Identification of Silica Phases Comprising Microtextural Mineralogical Magazine, February 2003, Vol. 67(1), pp. 1–13 LaserRaman identification of silica phases comprisingmicrotexturalcomponents of sinters 1 2, K. A. RODGERS AND W. A. HAMPTON * 1 Research Associate, Australian Museum, College Street, Sydney, NSW,Australia 2 Department of Geology, University of Auckland, Private Bag 92019, Auckland, NewZealand ABSTRACT LaserRaman spectroscopy is particularly well suited for routine characterization of the crystalline and thebetter ordered paracrystalline silica phases occurring in silica sinters: opal-C, quartz and moganite. Thenon-crystalline and poorly ordered paracrystalline silica phases (opal-CT and opal-A) are identified withdifficulty, partly as a resultof the high level of fluorescence exhibited by the younger sinters that aredominated by these phases, and partly as a resultof the ill-defined nature of the broad phonon scatteringbands produced by them. Nonetheless, given the limited number of phases present and the highlevel of phonon scattering from the better ordered phases, judicious use of a Ramanmicroprobe enablesthe character of most siliceous microtextural sinter components to be determined readily. Microprobeexamination of a seriesof New Zealandsinters, ranging from Late Quaternary to Pliocene inage shows that a silicaphase inhomogeneity that exists in some outcrops reflects an underlying phaseheterogeneity obtaining in the microstructure of the sinter; it is a consequenceof the phase transformationprocess. In the older sinters, quartz, moganite and opaline cristobalite may be present in asinglefabric element but in quite different proportions to their abundance in adjacent elements. Identificationof opaline lepispheres partially pseudomorphed by quartz+moganite and the association ofquartz+moganite in microcrystals exhibiting common quartz forms, cautions against the use of morphologyas a meansof identifying silica phases at the microstructural level. KEY WORDS: silica,sinter, laser Raman spectroscopy, opal, New Zealand. Introduction thatthrive in the hot-spring environments, and hencethe resulting sinters furnish invaluable SURFACE depositsof silica sinter are a common informationabout mineral-microbe associations expressionof geothermal systems. They are a inhydrothermalsystems (e.g. Walter et al., 1996; signatureofthehydrologicalconditions Farmer,2000). Sinters are a remarkable,persis- prevailingat the time the sinter occulatedas tent,terrestrial, sedimentary deposit. opalinesilica from near-neutral alkali chloride hot When rstdeposi ted,silic ainsinter is springsderived from deep reservoirs hotter than characteristicallynon-crystalline opal-A. With 175ºC (Fournierand Rowe, 1966). Consequently, timethe sinter crystallizes to paracrystalli ne sintersprovide an important exploration guide to opal-CT±opal-C and, subsequently, recrystallizes locatingand interpreting geothermal systems tomicrocrystallinequartz, with moganite forming whosesurface activity has changed, declined or asa transitoryphase (Herdianita, et al., 2000a; evenceased. Further, the texture of the freshly Rodgersand Cressey, 2001). This phase matura- depositedopaline silica is determined, in part, by tionis accompanied by microtextural changes. templetsafforded by the microbial communities Thecomplete transforma tioncan take from 30,000to 50,000 years and is parallelled by changesin thedensity, porosity and water content *E-mail: [email protected] ofthe successive silica phases (Herdianita et al., DOI: 10.1180/0026461036710079 2000a).Thecorrect identi cation of the different # 2003The Mineralogical Society K. A.RODGERSAND W.A. HAMPTON silicaphases is essential to elucidating sinter Herdianita et al. (2000a)andRodgers and history. Cressey(2001)report edsomeprelim inary X-raypowder diffraction is well suited to Ramanscans of opal-A, opal-CT and opal-C, characterizingeach of the different crystalline thesilica phases common among late Pleistocene silicaminerals (Smith, 1997, 1998). Dif culties andRecent sinters, they made no systematic exist,however, in using convention alX-ray study.The present report describes the results of a powdertechniques to identify opal-CT, with its Ramansurvey of all sinter specimens used by limitedlattice order, in the presence of opal-C Herdianita et al. alongwith some additional New whichpossesses an enhanced but still restricted Zealandexamples, particularly from quartz-rich crystalstructure (Smith, 1998; Herdianita et al., Tertiarydeposits. These spectra are presented in 2000b; Campbell et al., 2001).Further, the theirraw, unvarnished form in order to demon- progressionin phase transfo rmationis not stratethe strengths and weaknesses of the laser uniformwithin an outcrop, a fabricor textural Ramanmicroprobe as aroutinedeterminative tool type,or evena singlehand specimen. In orderto inmicrotextural sinter studies. The results prove fullyascertain the stage in maturationthat a sinter sufcient to demonstrate that a silicaphase hasattained and, importantly, to gain an under- inhomogeneityofte nobservedin a sinter standingofthe process esinvolve dinthat outcropalso pertains at the microscopic level. transformation,it is desirable to determine the phasecomposition of individual morphological componentsof different sinters. This is not Samples possiblewith conventional X-ray powder techni- Herdianita et al. (2000a)reportedon 29 samples ques.The standard 20 61061mm aluminium ofsilica sinter from ten different active and diffractionsample holders require 200 –300 mg extinctgeothermal systems, speci cally collected ofpowder .Thisis some 5 –10 orders of toprovide a rangeof ages and occurrences of magnitudegreater than the mass of many of the typicalthin-bedded moderate- to low-temperature sinterfabric elements .Clearly,each X-ray sinters.Less common textural types and silica powderscan serves to identify only the dominant residuewere excluded. Mineral phases other than phasespresent in several million microtextural silicawere rare. Twenty one were from active and components.Phases present at low levels tend to extinctQuaternary geothermal eldsof theTaupo beoverlooked as has proved to be the case with VolcanicZone (TVZ). Fivecame from the long- moganite(Smith, 1998; Rodgers and Cressey, livedbut still active Ngawha eldof northern 2001).Heaney and Post (1992) found it necessary New Zealand,and three from Tertiary occur- toundertake Rietveld re nement of their data rencesin North America (Hausbr ouckand afterhaving counted for 2 –10 s/0.03º2y from 2 to Lincoln,Montana). All had been deposited from 80º2y,i.e.for 1.4 –7.2h persample, in order to near-neutralalkali chloride waters. None had conrm the presence of moganitein microcrystal- sufferedsubsequent heating or steamalteration as linequartz. Such requirements are not conducive faras couldbe ascertained.All were examined in forroutine analyses of silica-rich samples and an thepresent study. Specimen details are given in alternativemethod to X-raypowder techniques is Herdianita et al. (2000a),althoughthe ages of requiredif the mineralogy of individual sinter someof theirsamples have recently been revised. microtexturesis to be determined and the inter- Inaddition, the present study includes samples relationshipsbetween the different silica phases fromthe quartz -richmid- to late-Pl iocene comprehendedat a microscopiclevel. Whenuaroasi nterat Plumduf fandMount Theroutine identi cation of theprincipal silica Mitchell,Northland, two of three southernmost specieswith Raman microspectroscopy has been sintermasses of the fossil Puhipuhi geothermal demonstratedby Kingma and Hemley (1994) in eld,aswellasfromlatePleistocene, thecourse of using a Ramanmicroprobe to >40,000BP, sintersof Umukuri (Campbell et characterizemoganite in microcrystalline quartz al.,2001)and Atiamuri, from tephra-rich sinter samples.Subseque ntly,Go ¨tze et al. (1998) remnantscapping hillocks in the Otamakokore employeda Ramanmicroprobe to explore the andWhirinaki stream valleys, and from the mid- distributionof moganite in agate while Allen et TertiaryWaitaia Sinter of Coromandel (Skinner, al.(1999)used Raman techniques successfully to 1976).Conventional X-ray powder diffraction characterizethe aggulinated silica (and titania) showedall these additional specimens to contain particlesof marine foraminifera tests. Although highproportions of opaline cristobalite (opal-CT 2 MICROTEXTURALCOMPONENTSOF SINTERS and/oropal-C) and/ orquartz. Rodgers and non-crystalline,paracrystall ineand crystalline Cressey(2001) had demonstrated that moganite microtexturalcomponents. The crystalline silica wasintimately associated with microcrystalline phases,quartz, a-cristobalite, a-tridymiteand quartzin both the lower horizons of Umukuri, moganiteall possess distinctive, intense scattering quartz-richareas of the Otamakokore erosional bandsin the region 350 –550 cm – 1 as do the residuals,and some older Atiamuri sinters. betterordere dofthe paracr ystallineopals Referencesamples included an unlocate d, (Fig. 1). colourless,transparent quartz macrocrystal from Initialexperiments focused on collecting the Switzerlandand tridymite from Vechec Quarry, Ramanspectra of different textural portions of EasternSlovakia (AM D52115).A suitable eachsample from 150 to ~724 cm – 1,i.e.across cristobalitespecimen was unavailable and the an ~575 cm –1 widecounting window, using a Ramanspectral data given by Bates (1972) were DilorXY spectrometerof 500 mm focallength used.Sample numbers pre xed AU arethose of akinto that used by Kingma and Hemley (1994) theUniversit yofAuckland, Departmen tof andemployingsimilar
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