Rapid Raman mapping of a fulgurite

PAPER IN FOREFRONT

Rapid Raman mapping of a fulgurite

Elizabeth A. Carter & Matthew A. Pasek & Tim Smith &

Terence P. Kee & Peter Hines & Howell G. M. Edwards

Received: 31 December 2009 /Revised: 15 February 2010 /Accepted: 16 February 2010 /Published online: 14 March 2010# Springer-Verlag 2010

Abstract A fulgurite is a naturally occurring glass formedwhen lightning hits sand, rock, or soil. The formation offulgurites is accompanied by mineralogical and sometimescompositional changes, and may record information aboutthe environment in which they were formed. A previousinvestigation using Raman point spectroscopy discoveredthe presence of anatase, a low-temperature polymorph ofTiO2, and polyaromatic hydrocarbons within a fulgurite.These findings indicate that there were regions within the

sample that were not subjected to temperatures of 2,000 Kor more that the matrix is reported to attain when struck bylightning. This paper seeks to expand the previous researchby utilizing the capabilities of a new Raman spectroscopictechnological development that enables rapid mapping. Theentire surface area of a cross-sectioned fulgurite (∼40 mm×23 mm) sample was mapped allowing several regions ofpolyaromatic hydrocarbons and anatase to be located.Furthermore, shocked quartz was found within the boundaryregions of the fulgurite, and is proposed to have resulted fromcontact with vaporized material during the lightning strike.Shocked quartz is typically indicative of extraterrestrialimpact, yet its discovery here suggests that its formation isnot exclusive to the impact process.

Keywords Raman spectroscopy . Rapid mapping .

Fulgurite . Shocked quartz

Introduction

Raman spectroscopy is a non-destructive, non-invasivetechnique that provides information about the molecularcomposition and structure of a sample. The sample isirradiated with monochromatic light, i.e., UV, visible or NIRexcitation, and photons are either inelastically or elasticallyscattered. The inelastically scattered light, known as Ramanscatter, has lost (Stokes) or gained (anti-Stokes) energy duringthis interaction and the emitted photon contains informationabout the molecular structure of the sample.

The application of Raman microspectroscopy in the fieldof geology has led to the investigation of a wide range ofsamples, e.g., minerals (organic and inorganic), fluidinclusions, and natural glasses [1–5]. The technique offersthe benefits of high spatial resolution (∼1 μm), the ability to

E. A. Carter (*)Vibrational Spectroscopy Facility, School of Chemistry,The University of Sydney,Sydney, NSW 2006, Australiae-mail: [email protected]

M. A. PasekDepartment of Geology, University of South Florida,4202 E Fowler Ave,Tampa, FL 33620, USA

T. SmithSpectroscopy Products Division,Renishaw plc, Old Town, Wotton-under-Edge,Gloucestershire GL12 7DL, UK

T. P. KeeSchool of Chemistry, University of Leeds,Leeds LS2 9JT, UK

P. HinesElectron Microscope Unit,Australian Key Centre for Microscopy and Microanalysis,Australian Microscopy & Microanalysis Research Facility(AMMRF), The University of Sydney,Sydney, NSW 2006, Australia

H. G. M. EdwardsCentre for Astrobiology and Extremophiles Research,School of Life Sciences, University of Bradford,Bradford BD7 1DP, UK

Anal Bioanal Chem (2010) 397:2647–2658DOI 10.1007/s00216-010-3593-z

distinguish between polymorphs, in situ analysis and little tono sample preparation is required. The high spatial resolutionprovided by Raman microspectroscopy is very beneficialwhen studying small samples or areas within samples;however, for large samples, only a limited survey is possible.

Mapping

Raman mapping is a technique that can be used toinvestigate the chemical (e.g., polymorph) and mechanical(e.g., stress) properties of a sample with respect to thespatial composition of these components. This is achievedby producing a series of false-color images, which allowvisualization of the type and distribution of the varioussample components. Spectra are collected sequentiallypoint-by-point over a defined region with a user-definedstep size in both the x and y directions. A four-dimensionaldata hypercube is produced that includes the spectrum, aplot of vibrational energy (units of cm−1) vs. intensity(arbitrary units) and the spatial coordinates from where thespectrum was collected (x, y).

Point-by-point mapping

In point spectroscopy, the spectral signal-to-noise ratio(SNR) for opaque samples is dependent upon a number offactors including: excitation wavelength, laser power,collection/exposure time, accumulation number, and theobjective numerical aperture (NA). When mapping, theSNR for each individual spectrum is determined by thesesame variables. However, the objective used is also aprimary consideration because it directly affects the SNR,laser illumination size and collection volume.

The illumination size influences the mapping spatialresolution and sample coverage. When the step size is equalto the diameter of the laser spot, ‘optimal sampling’ isachieved. If the step size increases beyond this diameter,‘under sampling’ occurs, where the spatial resolution isreduced but sample coverage also drops dramatically. Such‘under sampling’ is appropriate if the sample domain sizesare significantly larger than the step size and enables verylarge sample areas to be analyzed. ‘Over sampling’ occurswhen using a step size smaller than the laser spot diameter,is usually only performed when using a high NA ×100objective. This allows the generation of Raman images thatare more detailed than those obtained using the ‘optimallysampled’ methodology.

Line-focus mapping

The development of line-focus techniques decreased thetime required to map a sample by a factor of approximately

20 c.f. point-by-point mapping. The speed increase resultedfrom the ability to directly image the laser line onto theCCD detector. This allowed multiple spectra at spatiallocations on the sample to be collected simultaneously. Thestep size is controlled in the x direction in the same way asstandard point-by-point mapping, with the line widthequivalent to the laser spot diameter. The y step size iscontrolled by binning neighboring line spectra to produce asingle spectrum with reduced spatial resolution. A line-focuslens is used to expand the laser beam from a point into a lineand reduces the power density at the sample [6]. Thisreduces the likelihood of sample damage, photochemicalreactions, and thermal band broadening. However, theillumination of the beam is uneven and leads to variationsin spectral intensity along the laser line. This limits thetechnique where semi-quantitative information is desired andrequires normalization methods to produce acceptableimages. This hybrid imaging/mapping technique does,however, show the ability of Raman instruments to combinethe speed of direct imaging of the laser line with the richspectral information of mapping.

New technology—StreamLine rapid mapping

The StreamLine mapping method (Renishaw plc., Wotton-under-Edge, UK) is an evolution of line mapping, wherethe limitations of uneven illumination and variation inspectral intensity are addressed and the benefit of reducedpower density is maintained. The direct laser line imagesynchronously moves with the mapping stage such thateach sample point experiences each laser power point alongthe line. Any given sample point therefore experiences anaverage of the laser line resulting in ultra smooth andseamless images. Samples sensitive to laser power benefitfrom the line-focus method and can result in data collectiontime order of magnitudes faster than equivalent pointmapping methods.

Data analysis

Mapping can produce large datasets and if the band(s) ofinterest are intense, unique to a particular sample componentand are not overlapped by other spectral features, then dataanalyses are straightforward and individual wavelength-specific maps can be constructed. The inherent complexityof samples and consequently their spectra inevitabilityrequires a more rigorous approach to data analyses. This isoften facilitated by the application of multivariate statisticalanalyses, e.g., principal component analysis (PCA), clusteranalysis, hierarchical cluster analysis (HCA).

Maps collected using the StreamLine technology can begenerated using a variety of data analysis techniques, whichare categorized into (1) direct, and those requiring (2)

2648 E.A. Carter et al.

curve-fitting or (3) multivariate analysis, see Fig. 1. Theunivariate maps can be either generated during (livemapping) or after data collection. These methods allow theuser to create a false-color map based on discrete spectralchanges, which usually represent a specific Raman band orcomponent within the sample. The benefit of live mapping isthe instant feedback about data quality and domain sizewithin the sample. This allows convenient modification ofthe experimental set-up if any changes are necessary.

Image generation after data collection can allowpre-processing methods to be applied to the entire dataset. This can allow the removal of cosmic ray features ina quick and efficient manner. This ability becomes moreimportant when considering the use of multivariate

methods such as principal component analysis. Suchmethods allow the user to understand complex spectralvariations without prior knowledge.

Application of rapid mapping technology:fulgurites—a case study

Fulgurites, obsidian, and impactites are a group of naturallyoccurring glasses formed by a number of differentterrestrial phenomena such as lightning strikes, volcaniceruptions, and meteorite impacts, respectively (a man-madeanalogue is trinitite, formed by the Los Alamos nuclearexplosion). Fulgurites form when a lightning bolt hits thesurface of sand, soil, or rock and an exchange of a thermalenergy occurs, estimated to be as large as 1 GJ [7]. This isaccompanied by temperatures momentarily reachingbetween 18,000 and 39,000 K in the air and a minimum of2,000 K in the superheated matrix [8, 9]. The silicaceouscomponents of the sand, soil, or rock fuse to form a fragilehollow tube with a diameter of a few centimeters and alength that can vary from centimeters to meters [10, 11].

Recently, it has been suggested that fulgurites are mostlyclosely related to impactites in terms of composition andconditions experienced [12]. Given the huge role of impactsin the history of planetary surfaces [13], investigatedchanges that occur during fulgurite formation would helpto elucidate processes that occur during large impacts or inother planetary environments. Furthermore, fulgurites havebeen recovered from a number of different materials, whichmay permit significant study of these changes; in contrast,impactites are limited to a few sources [12].

The outer surface of a fulgurite is often described as veryrough and this is attributed to fragments of minerals androcks, which did not fully melt, that are incorporated intoits structure [14]. The interior surface is smooth and glassyand may also contain spherical inclusions, some of whichhave been found to contain air and gases, such as carbondioxide and monoxide from the surrounding area [15]. Thechemical composition of a fulgurite is dependent upon thenature of the surrounding environment in which it wasformed but literature suggests that they are typically rich inSiO2 (72–95%), which is not surprising as silicates are themost abundant crustal minerals [11, 14, 16].

Astrophysicists and astrobiologists have postulated thatviolent electric storms would have been a major feature ofthe evolutionary development of planets and their satellitesand may have been observed, for example, on Titan [17].These storms are a potential candidate mechanism for theformation of chondrules, small (1–2 mm) spheres of silicatesthat are believed to represent a major portion of the rock-forming solids in the Solar System [18]. Lightning hashistorically held an important role in the development of life

Fig. 1 Schematic diagram illustrating the steps taken in datacollection and the multiple options available for data analysis

Rapid Raman mapping of a fulgurite 2649

on the Earth, as it has been invoked in the generation ofamino acids [19], provides one of the few means ofabiotically fixing nitrogen [20], and affects the phosphoruscycle by reducing the oxidation state of phosphates [21].While lightning is less frequently involved in prebioticsyntheses of organic compounds in modern research,supplanted in part due to research on exogenous sources oforganic compounds [22, 23], the historical relevance oflightning to origins of life still influences the focus of currentresearch [19, 24]. Further, the preservation of organic materialinside glasses synthesized in developing evolutionary systemsfrom electric storms is a novel example of an extremeenvironmental scenario that needs to be considered in anastrobiological context, as well as a terrestrial one.

Previously, the authors have analyzed a number ofregions within an individual fulgurite sample using Ramanspectroscopy [25]. Spectra revealed several forms ofcrystalline quartz, fused silica, and the presence of poly-aromatic hydrocarbons (PAH) found in an interfacial zoneof a glass bubble. Together with the presence of anatase, alow-temperature polymorph of TiO2, this suggested thatsome regions within the fulgurite specimen were notsubjected to temperatures greater than 2,000 K, which arerequired to fuse silica. In addition, some data was found tobe similar to that of shocked crystalline quartz and shifts inthe position of numerous bands indicated that these regionswere subjected to pressures ranging from between 22 to30 GPa [26].

In the previous study, only a small sample area(∼1,000 μm2), of a large fulgurite sample ∼40×23 mm wasanalyzed using point spectroscopy. This paper seeks toexpand this area of research by utilizing a new technologicaldevelopment that enables rapid mapping. The aim of thiscurrent study is to better understand the nature and spatialrelationship of the various inorganic and organic componentsfound within the fulgurite using both analytical andmodeling techniques. The rapid mapping capabilities allowfor large-scale survey maps to be collected with smallerhigher spatial resolution maps then obtained once a region ofinterest (ROI) has been defined.

Experimental

Fulgurite samples

The fulgurite is comprised of a dark heterogeneous glassymaterial containing several white and grayish-brown areasand is classified as a Type II [21], see Fig. 2. This fulguritewas found in Greensboro, North Carolina, USA in ared-brown clayey soil, classified as an ultisol by the USDAsoil taxonomy (USDA 1999). Organic material (e.g., rootsand leaf debris) was present throughout the topsoil. The total

mass of the fulgurite was estimated to be greater than 20 kg,with two to three large pieces greater than 1 kg in mass, andover 50 individual pieces recovered.

Scanning electron microscopy/energy dispersive X-rayspectroscopy (EDS)

Specimens were imaged in an FEI Quanta 200 3D SEM atan accelerating voltage of 20 kV. The instrument wasoperated in controlled pressure mode at a pressure of0.5 mbar. Images were captured with a four-quadrant diodeback-scattered electron detector in composition mode.Energy dispersive X-ray spectroscopy (EDS) maps andspectra were collected using an EDAX 10 mm2 SiLi detectorwith a 132-eV resolution running under EDAX Genesis 3.52software. Mapping was performed using the qualitative ROImethod as a means of locating carbon-rich features.

Raman spectroscopy

Spectra were collected using a Renishaw Raman inViaReflex Microscope (Renishaw plc., Wotton-under-Edge,UK), equipped with two air-cooled charge-coupled device(CCD) cameras—one used for standard point spectroscopy

Fig. 2 Optical image of a fulgurite from Greensboro, North Carolina,USA. The sample is characterized by a large white semi-circular bandof crystals (marked by a red dotted line) surrounded by a glass matrix


and point mapping and the second used for StreamLinerapid mapping. The spectrometer is fitted with holographicnotch filters and two gratings (1,200 mm/line (visible)2,400 mm/line (NIR)). The attached microscope is a LeicaDMLM and is equipped with three objectives (×50/0.75 NA, ×20/0.40 NA, ×5/0.12 NA) and a trinocularviewer that accommodates a video camera allowing directviewing of the sample.

Sample excitation was achieved using an argon ion laser(Modu-Laser, Utah, USA) emitting at 514.5 nm. Calibrationof the wavenumber axis is achieved by recording theRaman spectrum of silicon (one accumulation, 10 s) forboth static and extended modes. If necessary, an offsetcorrection is performed to ensure that the position of thesilicon band is at 520.50±0.10 cm−1. Spectra were notcorrected for instrument response. The spectrometer wascontrolled by PC with instrument control software(Renishaw WiRE™, Version 3.2).

Mapping parameters

Figure 1 presents a schematic diagram of the various stepsundertaken to collect a StreamLine map. First, an opticalimage of the entire sample is obtained by stitching a seriesof images together using the montage capabilities of theWiRE™ software, see Fig. 3a. The sample is placed on thestage and focused using the ×5 microscope objective whichhas a field of view (FOV) of ∼1,200 μm. The sample ispositioned using the motorized stage such that the bottomright-hand corner of the sample is located and the x and ycoordinates are set to zero. The number of images to becollected in the x and y directions are inputted into thesoftware (x=13 and y=21 in Fig. 3a). The motorized stagemoves the sample and the software collects each newimage adding all the frames to the final montaged image.In addition, there is an option to apply an algorithm toimprove the montage image by modeling the variation inbackground intensity across all images.

Spectra were collected in StreamLine mode over thespectral range of 1,460 to 100 cm−1 with a laser power of13 mW (point mode). A ×20 microscope objective(0.40 NA) was used for data collection which produced a50 μm long laser line. Acquisition times ranged from 0.3 to2.55 s per spectrum, however, the overall time required forsample mapping was also dependent upon the followingfactors: (1) the size of the defined area for mapping, (2) thestep size, and (3) binning. Table 1 lists the data collectionparameters for the maps presented in Figs. 3, 4, and 6.

Data analysis

Cosmic rays were removed from the spectral data using anearest neighbor cosmic ray removal method (Renishaw

WiRE 3 software tool). The false-color maps shown inFigs. 3, 4, and 6 were generated from the data using either(1) the direct analysis approach by calculating the signal-to-baseline, or (2) a combination of a direct classical leastsquares (DCLS, Renishaw WiRE 3 component analysismethod) and alternating least squares multivariate curveresolution algorithm (ALS-MCR, Renishaw's EmptyModeling™ method). The Empty Modeling™ producedhigh-quality reference spectra of the pure componentsfrom the mixed spectral data. Individual maps weregenerated using the Empty Model spectra as referencespectra using supervised DCLS as well as using ALS-MCR.These maps were combined using a layering method andfalse-color representation to reveal the relative componentlocations.

Results and discussion

Figure 2 presents the optical image of a cross-sectionedfulgurite specimen. The sample is clearly heterogeneousand is characterized by the presence of a semi-circular bandof white crystals including several large crystals (∼5 mmlong×∼2–3 mm wide) surrounded by a vitreous glassmatrix. The glass matrix contains a range of sphericalhemispheres that were presumably inclusions containingbubbles of trapped gases [14, 16] and vary in size frommicrons to millimeters. A previous study using Ramanspectroscopy investigated a small region within the sample(∼1000 μm2) and established that the fulgurite wascomposed of amorphous glass, crystalline quartz, shockedcrystalline quartz, anatase, and polyaromatic hydrocarbons[25].

Figure 3 presents an optical image of the fulguritetogether with a series of red boxes, both solid and dashed,which indicate the areas from where the various Ramanmaps were collected.

Survey map

Figure 3b is a Raman map that illustrates the distribution ofcrystalline quartz (blue). It is evident from this figure thatcrystalline quartz, of various sizes, is found throughout theentire sample. Notably, the Raman quartz map is coincidentwith the white semi-circular band of crystalline materialoutlined in the optical images shown in Figs. 2 and 3a. Thismap identifies the white band of material as quartz. Theother area of interest is very small and is located at thequartz/glass matrix interface (marked by the arrow) and isattributed to a pocket of polyaromatic hydrocarbons (red).These survey maps were generated using a direct analysisapproach and involved calculating the signal-to-baseline ofthe A1 mode of quartz (474–455 cm−1) centered at


∼464 cm−1 together with a polyaromatic hydrocarbonsν(C–C) mode (1,237–1,188 cm−1) centered at ∼1,145 cm−1.

Polyaromatic hydrocarbons

The location of the polyaromatic hydrocarbons was asurprise as previous research had found a different pocketof material located within the centre of the glass matrix.

Therefore, the map in Fig. 3c was collected from a regionknown to contain at least two pockets of PAHs, as well asan area containing crystalline quartz and the glass matrix.To generate the map, a combination of two multivariatestatistical techniques was used. In the first instance,alternating least squares multivariate curve resolutionalgorithm (ALS-MCR, Renishaw's Empty Modeling™method) was used to generate reference spectra, see

a b c

Fig. 3 a Optical image of a fulgurite; the red boxes indicate the fourareas that were mapped. b Raman map illustrating distribution ofcrystalline quartz (blue) and one region containing polyaromatichydrocarbons (red) c Raman map of smaller sample region (asindicated by the dashed red line in a) with increased spatial resolutionillustrating the distribution of (1) crystalline quartz (blue), (2)

amorphous glass (green), (3) polyaromatic hydrocarbons (red). Thered arrows in b and c indicate where the regions of polyaromatichydrocarbons are located within the maps. The region in the smallsolid red square in c was mapped at a higher spatial resolution andresults are presented in Fig. 4b

Table 1 Data collection and analysis parameters for the Raman maps collected from the fulgurite sample

Figure number Step size (μm) Binning number (y) Accumulationtime (s) per spectrum

Number of spectra Data analysis

Figure 3b (entire) 93.6 30 0.3 46,388 Signal-to-baseline from 474to 455 cm−1 (quartz)

Signal-to-baseline from 1,237 to 1,188 cm−1 (PAH)

Figure 3c (strip) 31.2 10 0.5 30,602 ALS-MCR Empty Modeling™and DCLS

Figure 4b (bubble) 3.1 1 2.55 15,748 ALS-MCR Empty Modeling™and DCLS

Figure 6a (quartz) 21.8 7 0.39 119,715 Signal-to-baseline from 474to 455 cm−1

Figure 6b (quartz) 21.8 7 0.39 119,715 Curve-fitting of the A1 quartz bandat ∼464 cm−1 then plotting theband position


Fig. 4a. The reference spectra were then used to fit theunknown map data using direct classical least squarescomponent analysis (DCLS).

The interface between the band of crystalline quartz(blue) and the fused silica matrix (green) is quite evident.The glass bubbles are well resolved and there are numerousPAH (red) containing regions. One area of particularinterest is the PAH region near the glass bubble, markedby a solid red box. This is the original area that wasinvestigated using point spectroscopy. Given the size of allthe PAH regions within this map, it was certainlyserendipitous that any PAHs were located without the aidof mapping. Figure 4b presents a map of this glass bubble,

which confirms that the PAH is located at the interfacialzone of the bubble and it occupies an area of ∼50 μm2.Crystalline quartz is also observed at this interfacial zone andprevious research has suggested that a surrounding gas bubblein a fulgurite provides some insulation to quartz, therebyallowing some protection from the heat of the lightning strike[10]. This may also explain the PAH's preservation, most ofwhich were found near the glass bubbles.

Figure 5 presents the scanning electron microscopy(SEM) image and corresponding elemental maps forSi, O, Ti, C, Al, Fe that were collected from a similarregion encompassing the PAH containing bubble mappedin Fig. 4b. The orientation of the SEM and elementalimages are rotated though 90° compared to the Raman mapsin Figs. 3c and 4b. Inductively coupled plasma opticalemission spectrometry has previously established the oxidecomposition of the fulgurite is predominately SiO2 (81.3%)but it also contains TiO2 (1.15%), Al2O3 (8.32%) and Fe2O3

(8.48%) [25]. These fulgurites represent some of the mostiron-rich fulgurites that have been investigated to date, seeTable 2. The elemental maps illustrate that the distributionof Si and O is relatively homogenous throughout thisparticular region of the sample. A similar distribution isalso observed for the elemental maps of Al, Fe, and, to alesser extent, Ti. The carbon is particularly interesting, andon comparison with the other elemental maps, it wouldappear that this map is purely attributable to noise.However, the area circled in this map coincides with thePAH region in the Raman map and EDS spectra collected(data not shown) from this area confirmed the presence ofcarbon. This suggests that there may be many more PAHregions within the sample.

Shocked quartz—location and origin

One aspect of the maps presented in Fig. 3 worthy ofadditional investigation are the two large white crystals(∼5 mm long×∼2–3 mm wide) located within the bottomhalf of the semi-circular crystalline band near the glassmatrix. Spectroscopically, these crystals have beenidentified as quartz and the crystal morphology is reasonablyevident in these maps at first glance. However, closeexamination of Fig. 3 reveals that the Raman map is not anexact match to the optical image and much of the area thatshould be blue (indicative of a quartz signal) is in fact black(indicative of no quartz signal).

This region was mapped using a step size of ∼22 μm andthe results are presented in Fig 6. Figure 6a is a mapillustrating the distribution of crystalline quartz and wascreated using the direct analysis method by calculating thesignal-to-baseline of the A1 mode of quartz (474–455 cm−1).The figure confirms that there is no signal originating fromthe lower half of the largest crystal. In the optical image, its

1400 1200 1000 800 600 400 200 Raman Shift (cm-1)

Inte

nsity

(A

rbitr

ary

Uni

ts)

i

ii

iii

a

b

Fig. 4 a Reference Raman spectra of the components from the mixedspectral data produced using the Empty Modeling™ algorithm (1)Polyaromatic hydrocarbon (2) crystalline quartz (3) amorphous glass.b A map collected from a small region of the sample, as indicated bythe red square in Fig. 3c, with a higher spatial resolution. Raman mapillustrating the distribution of (1) polyaromatic hydrocarbons (red), (2)amorphous glass (green), (3) crystalline quartz (blue)


O

C

Si

Fe

AlTi

Fig. 5 Scanning electron micro-graph of the bubble mappedwith Raman spectroscopytogether with correspondingelemental maps obtained usingenergy dispersive X-rayspectroscopy. The sample isrotated 90° to the left comparedto Fig. 4, the three dark spots(circled in red) were found tocontain high levels of carbon


shape is very distinctive being wide, long, and graduallynarrowing to a tip. However, this tip is not observed in anyof the corresponding Raman maps, see Figs. 3b and 6a. Inaddition, two areas containing anatase were also located(magenta color and location indicated by red arrow).

This sample was known to contain at least one region ofshocked quartz, however its size and location wereunknown [25]. Shocked quartz is typically associated withquartzose rocks that have experience extreme, suddenpressure changes, like those generated during impactcratering [27]. Shocked quartz is characterized by planardeformation features and fractures, and sometimes zones ofglass and high-pressure polymorphs of quartz like coesite[28]. Shocked quartz is almost exclusively associated withimpacts or nuclear explosions, and the discovery of itspresence in fulgurites (see also [29]) suggests that thismaterial is not exclusive to impacts. Some impacts havebeen identified almost solely on the presence of shockedquartz [30]; the present work suggests other routes for theformation of shocked quartz must be eliminated to prove animpact. To locate the shocked quartz regions within thismap, spectra were subjected to curve-fitting and theposition of the A1 mode was plotted, see Fig. 6b. Thisfigure clearly shows that the two large crystals are amixture of normal (i.e., unshocked) and shocked quartz.The shocked quartz is surrounded on one side by a numberof smaller unshocked quartz crystals and on the other by thefused silica matrix.

The presence of shocked quartz, while surprising, maybe expected from the energy of a lightning strike.Temperatures of the heated column of gas or plasmawithin a lightning strike can exceed 30,000 K; from thisvalue, we can estimate the pressure of that heatedcolumn of gas. If the gas fills the central void space of

a fulgurite and is not allowed to immediately escape(which is justifiable as several fulgurites have largecentral voids that are disconnected with no clear escapetube), and if the gas behaves ideally, then the pressureinside the void will be equal to P=nRT/V. The number ofmoles, n, would be equivalent to the number of molescreated when the material in the void spaces is vaporized.For a fulgurite, this material includes the oxide constitu-ents of the rock, which should break down into atomic/ionic species at 30,000 K. Hence, the number of moleswill be equal to the mass of material vaporized (equal tothe density of the soil—about 2 g/cm3-times the volume)divided by the average atomic weight of the gaseousspecies evolved during vaporization. The atomic weightcan be estimated given the bulk composition of thefulgurite (Table 2), and by adding 7 wt.% water,consistent with the water content of the soil prior to thelightning strike. Therefore, the average atomic weight isestimate as 17.85 g/mole. Solving for the pressure at30,000 K within this tube assuming complete dissociationof the oxides to their atomic constituents gives a pressureof 27 GPa, within the range of pressures expected for thepresence of shocked quartz. This is in accordance with ourprevious findings where it was estimated from shifts in theposition of a number of Raman bands that the quartz wassubjected to pressures ranging from between 22 and30 GPa [25, 26].

Thermal diffusion

The fulgurite is characterized by a central void, sur-rounded by a region of glassy melt, which is in turnsurrounded by a gray, sintered crust. These regions shouldcorrespond to specific temperature regimes, with the void

Table 2 Comparison of the bulk oxide abundance (wt.%) of the fulgurite investigated in this work compared to other literature values

Bulk abundance (wt.%)

Oxide This work Navarro-Gonzalez [16] Essene [11] Crespo et al. [14]

SiO2 81.3 98.70 81.5 85.0 88.2 96.7 98.9 73.0 73.23 72.04 75.13 76.41 75.03

TiO2 1.15 <0.060 0.2 0.4 0.5 1.1 0.0 0.1 0.1 0.15 0.24 0.12 0.13

Al2O3 8.32 0.65 10.3 5.6 7.4 1.6 0.0 15.7 15.41 15.47 13.42 14.11 14.88

Fe2O3(total) 8.48 0.17a 1.9 1.8 2.3 0.5 0.2 1.6 1.93 3.11 2.20 0.99 1.12

MnO 0.28 NR ND ND ND ND ND 0.04 0.07 0.04 0.04 0.2 ND

MgO 0.10 0.02 1.2 0.4 0.6 0.1 0.0 0.22 0.22 0.35 0.34 0.12 0.13

CaO 0.15 0.08 1.8 0.9 1.4 0.2 0.0 0.53 0.47 1.00 1.84 0.43 0.52

Na2O 0.04 0.07 1.5 0.8 0.8 0.3 00 3.17 3.31 2.72 2.22 2.68 2.97

K2O 0.18 0.26 2.4 1.4 2.1 0.6 0.0 5.18 5.13 4.87 4.36 4.8 4.97

P2O5 0.01 <0.005 0.0 0.1 0.1 0.1 0.0 0.26 0.25 0.26 0.21 0.17 0.22

NR not recorded, ND not determineda Not indicated as a total amount (total)


a

b

Fig. 6 Raman maps illustratingthe: a distribution of crystallinequartz (blue) and anatase(magenta) and b position of thequartz band (464 cm-1, A1 mode),the unit of the scale bar is cm−1.The area mapped can be locatedon Fig. 3a marked by the reddash-dot-dot line ( )


representing vaporization of the target material, the glassindicative of complete melting of the target, and the crustindicative of melting only along grain boundaries. Thesetemperature regimes roughly parallel the mineralogicaland elemental features of the fulgurite. We therefore setout to model the temperatures experienced by specificareas of the fulgurite using a thermal diffusion model, tocompare expected temperature regions with mineralogicaland petrologic data.

The diffusion of heat during a lightning strike can bemodeled using the formula solving diffusion in cylindricalcoordinates:

@T

@t¼ k

@2T

@r2þ 1

r

@T

@r

� �

where it is assumed there is no diffusion along the z axis. Inthis formula, k is thermal diffusivity, set equal to 10−6m2/s,r is the radial distance from the source of heat (the lightningbolt), T is temperature in Kelvin, and t is time in seconds.Solving for T with respect to t and r (Fig. 7) demonstratesthat heat energy diffuses out rapidly from the line oforigination. The initial temperature of the line of originationis set equal to 30,000 K, the temperature of ionized airwithin a lightning bolt [9].

Since this model does not factor-in phase changes, thetemperatures shown here are estimates. Phase changes, likethe transformation of soil to melt, and of melt to vapor,likely occur at distinct radial locations, depending on a

variety of factors like composition, water content of thesoil, and grain size. The soil from which the fulguriteformed is estimated to melt at temperatures between 1,200and 2,000 K and higher which suggests a region of glassbeginning at 1.5–2 cm inward to 0.5–0.7 cm, where vaporis estimated to form, corresponding to temperaturesbetween 2,500–3,000 K. These model results roughlyparallel the structure of this fulgurite.

Conclusions

Raman spectroscopy has long been a powerful analyticaltool used to investigate a wide array of samples and themost recent technological development has enabled rapidmapping. The rapid mapping capabilities allow for large-scale survey maps to be collected with smaller higherspatial resolution maps obtained once a region(s) ofinterest has been located. This technology has been usedto investigate the nature and spatial relationship ofvarious inorganic and organic components found withina fulgurite.

Quartz plays an important role in the field of geosciencesit is an indicator of extraterrestrial impacts, as well as theprincipal shock barometer in the terrestrial environment.The location and distribution of two forms of crystallinequartz (normal and shocked) in the fulgurite were mapped.Shocked quartz was found within the boundary regions andis proposed to have resulted from contact with vaporizedmaterial during the lightning strike. The discovery ofshocked quartz suggests that its formation is not exclusiveto the impact process and this work suggests other routesfor the formation of shocked quartz must be eliminated toprove an impact has occurred.

Mapping the entire sample allowed numerous regionscontaining anatase and polyaromatic hydrocarbons to belocated, which would have been impossible to achieve usingpoint spectroscopy. The presence of anatase, a low-temperature polymorph of TiO2, and polyaromatic hydro-carbons implied that regions existed within the fulgurite thatwere not subjected to temperatures of 1,200 to 2,000 K thatare required to melt quartz. The anatase was located inregions containing unshocked crystalline quartz and thepolyaromatic hydrocarbons were found within the glassymatrix typically in the vicinity of a glass bubble. Thepreservation of the polyaromatic hydrocarbons may also bedue to a surrounding gas bubble providing insulation fromheat of the lightning strike. To compare expected temperatureregions with mineralogical and petrologic data, a thermaldiffusion model was developed. The results of the modelsuggested a region of glass beginning at 1.5–2 cm inward to0.5–0.7 cm and were found to parallel the structure offulgurite.

Fig. 7 A cylindrical thermal diffusion model (temperature vs. radiusvs. time) illustrating the varied temperature regimes provided by theheat of lightning. Inset temperature legend


Acknowledgments This research was supported by the AustralianResearch Council (International Linkage and LIEF grants), as well asthe USYD/NHMRC Major Equipment Funding Scheme, and grantNNX07AU08G from NASA Exobiology and Evolutionary Biology(MAP). The authors are grateful to Virginia Pasek who developed theprogram to solve the thermal diffusion equation. EAC would like tothank Sarah Kelloway for brainstorming, reading and editing thismanuscript, and her photographic skills. The authors also acknowledgethe facilities as well as scientific and technical assistance from staff inthe AMMRF (Australian Microscopy & Microanalysis ResearchFacility) at the Electron Microscope Unit, The University of Sydney.

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Rapid Raman mapping of a fulgurite