The Quartzofeldspathic Fulgurite of Bustaviejo (Madrid): Cathodoluminescence and Raman Emission

J. Garcia-Guinea1, M. Furio1, Fernandez-Hernan1, M., Bustillo, M.A.1, E. Crespo-Feo1, V. Correcher2, L. Sanchez-Muñoz, 2, E, Matesanz3

1Museo Nacional Ciencias Naturales, Jose Gutierrez Abascal 2, Madrid 28006, Spain, 2CIEMAT, Avenida Complutense 22, Madrid 28040, Spain.

3Centro de Asistencia a la Investigacion de DRX. Univ. Complutense Madrid. 28040, Spain. Correspondence author: mfurio@mncn.csic.es

Abstract. We discover and study a new quartzofeldesphatic fulgurite, sized 12 cm in diameter in the main branch, of Bustarviejo (Madrid, Spain). Here we characterize fulgurite aliquots by Electron Probe Micro-analysis (EPMA), Environmental Scanning Electron Microscopy with a X-ray Dispersive Spectrometry and cathodoluminescence probes (ESEM-EDS-CL), in situ high-temperature X-ray diffraction (HTXRD), Hy-perspectral Raman Microscopy (Raman) and Thermoluminescence (TL). The fulgurite body was formed by lightning strike fusion on arkose sand during a fast process of volatilization and extreme reducing condi -tions at temperatures in excess of 2000 K. The fulgurite is composed by schocked quartz, neo-formed cristobalite, native Si-Fe-Al alloys and aluminum-silicate glass with spherical cavities. Both, the hyper-spectral CL and Raman mappings highlight the cristobalite rim of the schocked quartz grains. The phase transition quartz-cristobalite preserves the spectra CL arising the 380 nm band of the [AlO 4]º / alkali de-fects, conversely the melting process forming glass phase destroys these CL defects (380 nm band) remain-ing other more structural centers, e.g., oxygen efficient centers, responsible of the 300 and 520 nm CL emission bands. In addition we also identify quartzofeldesphatic fulgurite with the stone nº 206, labelled mazintarincan, described in the medievaeal book, The Lapidary of King Alfonso X The Learned, written in the XII Century, at 1250.

Keywords: Fulgurite, Bustarviejo, Madrid, Lapidario, Raman, Cathodoluminescence, Cristobalite, Native Silicon, Schocked Quartz.PACS: 78.60Hk, 78.55Mb, 78.55.-m, 71.70Fk, 71.55

INTRODUCTION

Fulgurites are glassy irregular tubes produced by the fusion of sand, which has been struck by lightning. They have attracted attention from very early times, e.g. in Spain a quartzo-feldes-pathic fulgurite was described and outlined (Fig. 1), in the XII Century, at 1250, in the me-dievaeal book, Lapidary of King Alfonso X The Learned [1], kept in the Escorial monastery main library, as the stone number 206, labelled mazintarincan, i.e., an ancient Chaldean term meaning cold, since in those times they know as fulgurites strike in the cold mountain tops. This ancient text describes accurately, in ancient Spanish Language (Castillian), a quartzo-feldes-pathic fulgurite including the glassy green components (glass) and white grains (schocked quartz) and the external arborescent pipe-like shapes. Quartzo-feldespathic fulgurites formed

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from arkosic sediments are highly vesicular bodies but does not develop tubelike shapes as the case of those formed in quartz sands. The pioneer publication of Myers & Peck at 1925 [2] on the sand-claystone fulgurite of South Amboy (New Yersey, USA) describes quartz centers sur-rounded with a fine grained border of cristobalite and black stained areas attributed to possible iron oxides. The extreme reduction produced by lightning strike fusion and the discovery of sili-con and silicon-iron alloys was later described in natural fulgurites [3-4]. Additional findings of fulgurites from other rocks were also studied with luminescence and stable isotopes techniques [5-7]. Nowadays, modern techniques have also employed analyzing natural fulgurites such as micro-Raman, TL-dating, stable isotopes.

FIGURE 1. LEFT. El Lapidario Book held in the Main Library of the Monasterio de El Escorial (Madrid) built from 1563 to 1584. RIGHT. Stone number 206 so-called Mazintarincan, i.e., stone coming from cold areas at to

mountain top. Note the characteristic arborescent look of the quartzofeldespathic fulgurites.

Here we analyze a new granite fulgurite of Bustaviejo (Madrid) by Electron Probe Microanalysis (EPMA), Environmental Scanning Electron Microscopy with a X-ray Dispersive Spectrometry and cathodoluminescence probes (ESEM-EDS-CL), in situ high-temperature X-ray diffraction (HTXRD), Hyperspectral Raman Microscopy and Thermoluminescence (TL).

FULGURITE SAMPLES AND EXPERIMENTAL

We found, and study, a glassy quartzofeldspathic fulgurite, formed recently on arkose sediment which covers granite rocks of Bustarviejo village surroundings (Madrid, Spain). Quartzofelds-pathic fulgurites formed from arkose sediments are highly vesicular but does not develop pipelike shapes as the case of those formed in only quartz sands. The EPMA analyses were per-formed in a Jeol Superprobe JXA-8900M and by ESEM-EDS in a Inspect-S ESEM of the FEI company with a direct optical coupling to chamber mounted monochromator MonoCL3 of Gatan Co. This CL device includes a retractable detachable, diamond turned CL mirror, in-line spectral calibration lamp, PA3 manual remote unit, standard PMT (185-850nm), panchromatic and monochromatic imaging, photon counting and CCD serial spectroscopy and a parallel array spectroscopy (IR) and a standard IRPMT nitrogen cooled. The in situ HTXRD fulgurite patterns were recorded from room temperature (RT) to 1200ºC using a Panalytical X-Pert PRO MPD diffractometer (Cu Kα radiation, 45 kV, 40 mA) equipped with a high-temperature Anton Paar

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HTK1200 camera and a fast detector X’Celerator with Ni β filter. During circa 17 h, we per-formed 76 HTXRD profiles with gaps of 100ºC from RT to 500ºC between each XRD profile and then of 10ºC up to 1200ºC. The hyperspectral Raman quartz and cristobalite distribution was explored with a ThermoFischer Raman Microscope which has a point-and-shoot Raman capabil-ity of one micron spatial resolution using a laser source at 532 nm. The TL measurements of ful-gurite were performed using an automated Risø TL system model TL DA-12 with an EMI 9635 QA photomultiplier. The emission was observed through a blue filter FIB002 in which the wave-length is peaked at 320–480 nm.

RESULTS AND DISCUSSION

The Bustarviejo fulgurite is made of aluminum-silicate glass, schocked quartz and neo-formed cristobalite. In addition, the spherical cavities, sized from micron to centimeter, exhibits black spot of native Si-Fe-Al alloys, which unmixed from aluminum-silica-rich melt. (Figure 2). In despite that figure 2d display crystallographic silicon trigons along (0001) orientation) we also analyze under ESEM other crystallographic habits with high amounts of Al and Fe. Addi-tional studies by micro-Raman have been scheduled to clarify the molecular structures of these Al-Si-Fe phases.

FIGURE 2. Quartzofeldesphatic fulgure of Bustarviejo (Madrid): (a) Samples collected by authors, (b) Slice exhibiting glass (green), schocked quartz and cristobalite (white), Fe-Si-Al alloys (black spots) and cavities. (c) Detail of a schocked quartz.

(d) Native silicon crystals along the crystallographic (0001) orientation.

We infer that the spherical cavities stem more from gases formation mainly produced by precur-sor water (H2O vapour) or air (N2, O2, CO2) that from possible alkali leakages from feldspar phases (Na+, K+), further laboratory driven experimental must also be performed to clarify these

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assumptions. The EPMA analyses of glasses display the following compositional ranges (%): SiO2 (63–70.68), Al2O3 (19.00–23.63), FeO (0.06–2.09), CaO (0.04–0.74), Na2O (0.03–0.17), K2O (0.09–0.96), P2O5 (0.01-0.016) as the most representative elements. The top analyzed amount CaO (0.74) and Na2O (0.17) stem from the same sample, i.e., a single former grain of plagioclase.

FIGURE 3. Quartzofeldesphatic fulgure of Bustarviejo (Madrid): (a) X-ray diffraction of the fulgure bulk showing cristo-balite presence, (b) Raman spectra of the main phases, (c) Quartz-Cristobalite boundary under the Raman DXR Microscope

and (d) Hyperspectral Raman mapping exhibiting the spatial distribution of the cristobalite phase.

The thermal variations of low quartz and low cristobalite were observed by HTXRD from 200º and 300ºC, the low-high cristobalite phase transition occurs; at 540ºC the low-high quartz and fi-nally beyond 1100ºC the quartz peak at 26,47 2θ splits arising a little peak which disappears be-yond 1180ºC. The experimental TL glow curve of the fulgurite is composed by a broad band peaked at 233ºC--123a.u. and two peaks at 460ºC--500a.u. and 494ºC—1500 a.u., unfortunately, these values do not allow us to perform TL dating since the surrounding sediment. i.e., quartz plus alkali feldspars display a huge TL emission peak at 223ºC—571 000 a.u. and 274ºC – 417 000 a.u., only let us to confirm that must be a geologically recent strike older than circa 30 years which is the time being observed by the surrounding population. The main part of the glassy ma-trix exhibits a molecular-Raman structure similar to a synthetic gel of potassium alumino-silicate obtained experimentally via sol-gel. The hyperspectral Raman contour plots of quartz grains out-line its cristobalite outer comparing characteristic maxima peaks at 465 Raman shift cm -1

(Quartz) and 415 Raman shift cm-1 (Cristobalite) (Fig. 3). Figure 4 concerns to a schocked quartz grain with neoformed cristobalite in the outer and fissures. The panchromatic CL image highlight the cristobalite-rich areas since cristobalite exhibit’s the highest CL emission in com-

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parison with the surrounding naturally annealed phases glass and schocked, the most plausible explanation is that cristobalite has a new crystallographic structure neoformed during the quenching process including a physical size reduction of the formation of new structural CL de-fects. (Figure 4d). From the CL point of view of the emission-defects linkages it is interesting to observe that the polymorphic phase transition quartzcristobalite preserves the spectra CL aris-ing the 380 nm band of the [AlO4]º / alkali defects, conversely the melting process forming the glass fulgurite phase destroys this CL defects remaining other more structural centers, e.g., oxy-gen efficient centre or Si—O bonds responsible of the 300 and 520 nm CL emission bands. These cristobalite boundaries of the inlaid quartz grains let us to infer on the short time of the lightning strike run.

FIGURE 4. Quartzofeldesphatic fulgurite of Bustarviejo (Madrid): (a) Backscattering ESEM image and studied zone, (b) Boundary Schocked Quartz versus Cristobalite versus fulgurite glass. (c) Panchromatic CL image depicting the most intense

CL emission from the cristobalite boundary zones. (d) CL spectra of Cristobalite, Quartz and Fulgurite Glass.

This fulgurite sized 12 cm in diameter in the main branch, was formed by lightning strike fusion on arkose sand during a fast process of volatilization and extreme reducing conditions at temper-atures in excess of 2000 K [4].

References: [1] Alfonso X El Sabio El Lapidario Book. Esc. Trad. Toledo. 1250. Spain. Issue in El Escorial Monastery lib-

rary. [2] Myers W.M. and Peck A.B. Amer. Miner. 10, 152–155 (1925)[3] Blaschke R. and Pfefferkorn G. Fortschr. Miner 61, 27 (1983)[4] Essene E.J. and Fisher D.C. Science, 234, 189–193 (1986)

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[5] Ablesimov N.E., Tsiurupa A.I. and Lipatov V.G. Dokl. Akad. Nauk 290, 1454–1458. (1986)[6] Navarro-Gonzalez R., Mahan S.A., Singhvi A.K., Navarro-Aceves R., Rajot J.L., Mckay C.P., Coll P. and

Raulin F. Geology 35, 171–174 (2007)[7] Champagnon B., Panczer G. and Chemarin C. Chem. Erde Geochem. 57, 290–296 (1997)

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