02 – THE ARACATI FULGURITES (NORTHEASTERN BRAZIL): MINERALOGY AND PARENT MATTER

Ano 12 (2025) – Número 2 – Fulgurites, Gastroliths, Aluminium, Crônicas Artigos

THE ARACATI FULGURITES (NORTHEASTERN BRAZIL): MINERALOGY AND PARENT MATTER

 

10.31419/ISSN.2594-942X.v122025i2a2MESX

 

Marcondes Lima da Costa^1*

Milson Edmar da Silva Xavier²

Glayce Jholy Souza da Silva Valente³

Pabllo Henrique Costa dos Santos³

 

¹Geosciences Institute and PPGPatri, Federal University of Pará, Belém, Pará, Brazil, marcondeslc@gmail.com

²Applied Mineralogy and Geochemistry Working-Group – GMGA, Geologist, milsonest@gmail.com;

³Faculty of Conservation and Restoration, Federal University of Pará, Belém, Pará, Brazil, phsantosgeo@gmail.com, jholyglayce@gmail.com

  ^*Author for correspondence

Received on January 28, 2025; revised and accepted on February 16, 2025

 

ABSTRACT

Fulgurites were recently discovered on the dune field of Aracati, in the state of Ceará in the domain of beachrocks and even eolianites. Mineralogical analyses by optical microscopy, X-ray diffraction, and scanning electron microscopy and energy dispersive spectroscopy, in addition to morphological observations and density determinations, show the classic patterns for class III fulgurites. They are mainly formed from calcite to Mg-calcite and quartz, in addition to barite. The glassy part is very restricted, located, and represented by lechatelierite and perhaps moissanite. Feldspar grains and calcitic bioclasts were also recognized. The mode of occurrence and mineralogy, as well as the punctual and total semiquantitative chemical composition, demonstrate that eolianites were the main parental material for the formation of Aracati fulgurites, which find a possible parallel with the rhizoconcretions described in these rocks by several articles published on eolianites and beachrocks in the coastal region of northeastern Brazil.

 

INTRODUCTION

Fulgurite (the expression fulgurite comes from the Latin, fulgur, lightning) is the domination given to amorphous to crystalline solid materials formed by the high and instantaneous electrical discharge, originally related to lightning, the cloud-to-ground lightning strike, which reached and still reaches the surface of the Earth, whether on soils, sediments or diverse rocks of different ages. Fulgurites have been known since at least 1250 AD, e.g., from the Lapidarium of King Alfonso X the Wise (1221–1284) (Rappenglück, 2022). They were first reported by Herman in 1706, who recognized them as amorphous and a mineraloid, referred to as lightning stone, petrified lightning or lechatelierite (Carter et al., 2010). However, the term ‘fulgurite’ was coined in 1790 by William Withering (Rappenglück, 2022). Merrill (1886) already describes them in detail in dune fields including chemical composition. Lightning strikes reach the ground on Earth as many as 8 million times per day or 100 times per second. About 90% of these lightning flashes occur over continental landmasses (Lay et al., 2007).

Fulgurites have been researched in depth, concerning their morphology, mineralogical composition, and origin.  They have already been recognized in the Permian and Miocene, also known as paleo-fulgurites, but they are more frequent from the Late Pleistocene, and mainly in the Holocene (Essene & Fischer, 1984; Navarro-González et al., 2007; Carter et al., 2010; Pasek &Pasek, 2018; Martin-Ramos et al., 2019). The formation temperature is above 1800 ºC (Carter et al., 2010; Martin-Ramos et al., 2019). The energy required for the formation of fulgurites is discussed by many authors, and is varies significantly, considering the target material and its peculiarities and the natural environment (Pasek et al., 2012; Elmi et al., 2017; Pasek, 2017; Pasek & Pasek, 2018); cloud-to-ground lightning can cause shockwave pressures in excess of 10 GPa and temperatures above 1700 °C (Kuo et al., 2021). An idea about the energy required is presented by Pasek et al. (2012): “Fulgurite morphology shows that the energy of fulgurite-forming strikes is between 1 and 30 MJ/m of fulgurite formed, suggests heating rates in the order of 1,000 K/s, and lightning channel thicknesses of about 1 mm diameter”.

With the growing interest in the scientific study of fulgurites, fostered by their rich morphological diversity, but mainly by their mineralogy that comprises the complex silicate domain and exotic chemical composition, with an interface with those of impactites and meteorites, it has become necessary to establish a classification, which is contained in the studies by Pasek et al. (2012) and Pasek & Pasek (2018). As for the target material, they recognized five types of fulgurites: Type I – sand fulgurites consisting of thin glass walls; type II – clay fulgurites, consisting of thick, melt rich walls; type III – caliche fulgurites, with thick, glass poor walls;  type IV – rock fulgurites, consisting of glasses with walls from the unmelted rock; type V: droplet fulgurites, although with distinct morphology, this type can be correlated with type II or IV fulgurites. An exogenic fulgurite cools down very quickly, deforms into the air like drops, solidifies, and falls back to the ground as a fulgurite droplet. These are characterized by the double process of ejection into the air and re-entry (Rappenglück, 2022). Phytofulgurite – a proposed class of objects resulting from partial to total alteration of biomass (e.g. grasses, lichens, moss, wood, haycock) by lightning (Lysyuk et al., 2006).  They were excluded from the classification scheme because they are not glasses, so classifying them as a subset of fulgurites is debatable (Pasek et al., 2012).

More recent research demonstrates the presence of several silicides, phosphides, and carbides of Fe, metallic silicon, native copper, amorphous carbon, Si carbide, Fe-Ni alloys, Fe-Al, Fe-Ti-Si, Al-Si, among many others such as Fe-P-S, which are minerals first identified in meteorites, and then in impactites, which provide a terrestrial source for these phases, until then considered as exclusively extraterrestrial materials (Hiltl et al., 2011; Pasek et al., 2012; Elmi et al., 2017; Pasek & Pasek, 2018; Hess et al., 2021). Minerals of this type have also been found in fulgurites (Plyashkevic et al., 2016; 2022). But Çalışkanoğlu et al. (2023), based on their results of experimental development of fulgurites from natural volcanic ash samples of phonolitic composition, which were successful, were unable to fit them into the five types mentioned above. When the protolith consists of incoherent sediments or clastic rocks, this will play a crucial role in the morphology, density, porosity, and proportion of glass in the fulgurite, in addition to variations in the current flow and duration of the lightning strike (Çalışkanoğlu et al., 2023). For more details regarding energy intensity, exposure and reaction time, see Çalışkanoğlu et al. (2023). In a review work Rappenglück (2022) summarizes the natural conditions (energy, temperature, time) for the general formation of fulgurites.

Fulgurites are known in geological literature to form primarily as a result of cloud-to-ground lightning; however, the term is also being applied to describe glassy fuseate that formed during electrical breakdown in circuits (Bussiere et al., 2007; Pasek et al., 2012). To address this distinction, the term natural fulgurites describes those fulgurites formed by lightning; and artificial fulgurites, refer to fulgurites formed by high voltage or high current discharge from a power line (for example the fulgurites of Curuçá, northeast of the State of Pará, formed in front of the external area of ​​the Health Post building, by electrical discharge on the energy transformer, described by Costa et al. (2024, in BOMGEAM 11, No.4); fulgurites with foreign or manmade substances in the target material, such as the Curuçá fulgurites.

Fulgurites generally come in cylindrical, tubular, straight or tortuous, conical shapes, with few to many branches, with a central conduit or not. The dimensions vary greatly, but are generally around 40 to 50 cm, and the external diameter varies from a unit of mm to several cm (Fig.1 and 2). The external appearance is rough, rough, containing binding material, the target material, and the interior is amorphous to crystalline vitreous (Merril, 1886; Pasek et al., 2012; Karadag et al., 2022; Çalışkanoğlu et al., 2023). They also occur in very irregular, porous forms, with highly irregular cavities (Navarro-González et al., 2007; Carter et al., 2010; Pasek & Pasek, 2018).  Fulgurites are recognized in the form of a film or layer on crystalline rocks (Elmi et al., 2017; Kuo et al., 2021), in fractures (Kuo et al., 2021) and even exogenic fulgurites, also referred to as ‘droplet fulgurites’ or as ‘type V’ fulgurites (Martín-Ramos et al., 2019).

 

Figure 1 – On the left a schematic diagram and image of a fulgurite. Source: Block (2011) and Pasek et al. (2012) and). On the right side an anthropogenic fulgurite described by Costa et al. (2024) for comparison.

 

Figure 2 – Example of natural fulgurite and its common morphology with longitudinal and cross-section view.  Source: Çalışkanoğlu et al (2023).

 

Fulgurites, although rare, are found worldwide. However, they stand out more in desert regions and coastal landscapes, where beach sands and dunes, generally quartz, predominate, and are favorable to the formation of glass by the electrical discharge of lightning.  They also form on the surface of hard, crystalline rocks, caliches, various soils, etc., as shown previously. The vast majority of detected are of current and subcurrent origin. In Brazil, fulgurites are being discovered along its coast, especially in the northeast (for example Barreirinha, in the state of Maranhão) and south (São José do Norte, Litoral Norte, Torres, state of Rio Grande do Sul) according to Graminha et al. (1996) and sales websites, in addition to occurring in the interior of the country (São Geraldo do Araguaia, Pará, according to Faria & Nascimento, 2019), but there are still very few articles published about fulgurites in the country (Faria & Nascimento, 2019). However, the prospects for occurrences are high, as the country is the champion in lightning incidents, and still has a large area under tropical control, with high temperatures and humidity (Kimmemgs, 2016). This work describes isolated and unprecedented occurrences of fulgurites on the coast of the municipality of Aracati, Ceará, reported first-hand by Xavier (2024).

 

MATERIALS AND METHODS

Fieldwork and sampling – When describing the geological exposures along the cliffs and dune fields of the Aracati municipality coastline, in the state of Ceará, we came across immature laterite profiles erosively truncated and partially obliterated consistent with the style described by Costa (1991), by fossil dunes (eolianites), fixed, and current fixed and mobile. Additionally, the presence of beachrocks and locally rocky material suggestive of fulgurites was observed. The environment is highly favorable, as in addition to sand-quartz, it is subjected to storms. On this occasion, emphasis was placed on collecting different shapes, sizes and colors of potential fulgurites, both with and without a central conduit, accompanied by captures of various images, to support detailed laboratory analysis.

Density determination – Density was determined using a digital pycnometer Anton Paar Ultrapyc 5000, serial number: 1050059383; software version: 1,002,020, nitrogen gas, target pressure of 19 psi; target temperature: 20 ºC; flow mode: monolith; cell size: nano; preparation method: flow; final mode criteria: < 0.5%; running time: 7.4 minutes. These analyses were carried out in the LAMIGA laboratories of Geosciences Institute, Federal University of Pará.

Optical microscopy – From a thin section, microtextural and mineralogical aspects were observed with the aid of a polarized light optical microscope coupled to a camera and monitored by a LEICA computer application. These analyses were also carried out in the LAMIGA, PPGG/IG/UFPA laboratories.

X Ray Diffraction (XRD) – Was used for additional identification of mineralogical phases and assessment of the presence of amorphous materials. The Bruker D2 Phaser diffractometer was used, equipped with a Cu anode and Ni-Kβ filter, from LAMIGA laboratories. The powder method was used, therefore, previously powdered in an agate mortar. The diffractometer was set to θ-θ Bragg-Brentano geometry with a Lynxeye linear detector. Measurements were obtained in reflection mode in the range of 5° to 70° 2θ with a step size of 0.02° and a counting time of 38.4s per step. The mineral interpretation from the diffractograms was carried out with the aid of the HighScore Plus 5.0 software, which uses the database of the Powder Diffraction Archives of the International Center for Diffraction Data (ICDD) of the Mineral Technology Laboratory of PPGG/IG/UFPA.

Partial chemical analyses – For this purpose, Bruker’s LAMIGA portable X-ray fluorescence S1 Turbo was used, which allows the detection of even Mg, the lightest element for this method. The analyses were performed out directly on the flat sample surface obtained by saw cutting. The results are considered semi-quantitative.

Imaging and spot semi-quantitative chemical analysis by Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) – This was followed by image capture and semi-quantitative chemical analysis by SEM-EDS, on bench-top Hitachi TM3000 equipment, coupled with an Swift ED300 Energy Dispersive Spectroscopy (EDS), under accelerating acceleration from 5 to 15 kV and, with SDD detector (161 eV-Kα), also from LAMIGA laboratories. The analysis was performed under low vacuum, without metallization.

 

RESULTS AND DISCUSSIONS

Occurrence and general characteristics – In detailed terms, these aspects were described by Xavier (2024) (BOMGEAM issue 11, number 4). The fulgurites are found both in situ, sub-vertical, and partially fragmented and rolled, accumulated in the lower portion of the sandy floor of palaeodunes (eolianites), partially covered by sand from the ongoing deflation processes. In the uppermost portion of the in fixed dune bodies, with a slightly reddish color, under strong deflation, most of the fulgurites stand out in their original position, as elongated, straight or curvilinear bodies, sometimes strongly branched in a sub-horizontal way, resembling roots. This picture configures a beautiful landscape, given by the emergence of the tubes in the body of the partially deflated fixed dune (Fig. 3).  Surrounding these tubes there are accumulations of various fragments resulting from erosional processes (Fig.3 and 4).

 

Figure 3 – Rapid deflation exposes subvertical fulgurites developed in fixed paleodune (eolianites?), reddish brown in color. In the background, a subcurrent mobile dune with white sand, which covers beachrocks and paleodunes (eolianites).

 

Figure 4 – Clusters of fulgurites of different sizes and shapes, intact and fragmented by erosion, accumulated on the reddish-brown sands of fixed paleodunes (eolianites ?). The quartz pebble in the upper left corner is an anthropic accumulation, found in profusion in the concrete of nearby civil constructions, in addition to being part of the beachrocks, which emerge isolated along stretches of the beach.

 

In general, the fulgurites  are elongated, branched, straight, and tortuous, 20 to 60 cm long, with a diameter of 1 to 4 cm, with a central channel, in which the external zone is rough, irregular, with parent material (sand), and the interior crystalline to glassy appearance (Fig.5 and 6), with glass sound. There are tabular specimens, 10cm wide, some dense, brown to white in color, with or without a longitudinal duct/channel. Others are tabular, platy, found in a sub-horizontal position.

 

Figure 5 – Different morphological aspects of Aracati fulgurites. Images A and B show the same specimens in front and back, while C forms straight cylindrical shapes and D in cross section. The scale for all images is indicated by the coin present in A and B.

 

Figure 6 – On the left, a platy fulgurite with a reticulated surface, on the center a cylindrical, almost solid surface, and on the right, a longitudinal section of the previous one, showing an almost massive outer zone and a very porous inner zone with a tendency to become fibrous.

 

Density – Density is relatively high, varying between 2.68 and 2.72, on average 2.70 (Tab.1). In other words, it is between the density close to quartz and calcite, which, as will be described in the next topics, are the two minerals that form the fulgurites investigated.

 

Table 1 – Density values for five fulgurite samples coming from Aracati. 

Measurements	1	2	3	4	5	Average
Density g/cm3	2.7053	2.7023	2.6834	2.6927	2.7179	2.7003

 

XRD Mineralogy – The most frequent minerals in decreasing order of abundance are quartz, calcite (Mg-calcite (?)), kaolinite, and feldspars (microcline and sanidine) (Fig. 7). The typical glass domain was not identified. Calcite is more abundant in the inner zone of the fulgurite, closer to the channel, when present. The strong presence of calcite in these fulgurites allows them to be classified as type III according to Pasek et al. (2012), demonstrating that they were formed from target material rich in calcium carbonates, which in their classification presupposes caliche. In this condition, they are poor in glass. In the present case, they are dunes, which must contain bioclasts of different coastal and continental marine organisms, whose bodies are formed from Ca carbonates (aragonite and/or calcite), in correlation with the eolianites of western Ceará described by Arias (2015, 2020). The strong presence of calcite justifies the high density, on average 2.70.

 

 

Figure 7 – XRD identified minerals in five distinct samples for Aracati fulgurites. The most frequent minerals in the outer zone are quartz, calcite, kaolinite and feldspar (microcline and/or sanidine).

 

Optical microscopical features and mineralogy – In a thin section longitudinal to the body of the fulgurite, covering the external and internal zone, grains of quartz cemented by calcite, sub micritic to micritic, in the external zone and practically only sub micritic and micro spathic calcite in the internal zone is observed, and partially to completely filling microcavities (Fig. 8). The cavities are notable and more abundant in the inner zone (Fig. 8D). In this area they are larger and elongated, parallel to the cylindrical body, giving rise to a parallel “banded” appearance, like a fibrous feature, but which may be partly modicum features, inherited from fragments of bivalve organisms (Fig. 8D).

 

Figure 8 – General microscopic appearance of Aracati fulgurites from the outer zone with quartz grains dispersed in the submicritic calcitic matrix (A, B) to the inner zone (C, D), where C represents the transition between these zones. Polarized light with crossed Nicols.

 

Quartz

Quartz appears irregular to subspherical grains, angular to subrounded in general (Figs. 9 and 10). Most exhibit irregular contours, like a micro gulf (Fig. 9), with fringes of micritic to micro spathic calcite and with undulating extinction. Other grains exhibit features of a polycrystalline nature, resembling submicron to micro granular brittle shocked quartz (Figs. 9 and 10). In SEM images with EDS analysis, these grains can be distinguished, surrounded by both calcite and kaolinite films (Fig.11). It is possible to delineate the cement, calcite and kaolinite films (halos) through EDS analyses, with calcite being the most abundant. Even though the analysis is semi-quantitative, the high carbon contents draw attention, which are in excess for the constitution of calcite, and even in its absence, the carbon contents persist. Another carbon compound is present and will be discussed below.

 

Figure 9 – Detail of the external zone in which irregular quartz grains (qtz), with corrosion gulfs and possible bioclasts, are immersed in sub micritic calcite cement. Possible fragments of mollusks (Cc1) with sub-spathic and micritic calcite or even gastropod fragments (Cc2) given by the oval outline. Possible glassy features like lechatelierite (Gl) are small and scattered, sometimes confusing with deformed quartz. Polarized light with crossed Nicols.

 

Figure 10 – Two grains of rounded quartz (A, B), on the left monocrystalline and on the right polycrystalline, with a fringe of microspathic calcite. It can be correlated to shocked quartz, which is best represented in C. D: Lechatelierite, possibly. Polarized light with crossed Nicols.

 

Figure 11 – SEM microphotographs and EDS chemical composition from quartz grains and surrounding (aureoles, films and cement) composed of calcite and kaolinite.

 

Glass (lechatelierite)

Features suggestive of glass (Gl), like lechatelierite (Fig. 8D), are present, always a few microns long, and can be confused with modified quartz grains, sometimes completely. Several grains of quartz, perhaps thermally deformed, describing bands, halos or “inclusions” suggestive of glass (amorphous, considered lechatelierite) were also detected (Fig.9; 10 D; 12 D). Lechatelierite is a mineral typical of fulgurites whose parental matter involves quartz sands (Carter et al., 2010; Pasek et al., 2012). A domain of glassy, ​​amorphous mass, typical of many fulgurites, was not observed, demonstrated by the absence of shoulders in the XRD spectra (Fig. 7). Spot chemical analyses by SEM/EDS in a calcite or Mg-calcite domain generally show high Si contents, without the corresponding Al content, which would justify the presence of aluminosilicate, and therefore admitted as amorphous silica, lechatelierite type. These glasses, like lechatelierite, are surrounded by calcite, which serves partially to glue the grains together (Pasek et al., 2012). The thermal history of these fulgurites could not be elucidated using thermodynamic codes, as CaCO₃ is not expected to be stable over the temperature range of fulgurite formation at 1 bar of pressure and should instead form CaO and CaSiO₃. Since this is not the case, the heating rates of these fulgurites may be too fast for equilibrium chemistry, that is, these fulgurites may be the result of non-equilibrium processes, perhaps as a result of shock chemistry (Ivanov and Deutsch 2002 in Pasek et al. 2012).

 

 

Figure 12 –  A) A possible grain of quartz (Q) partially vitrified (G), or a shocked quartz grain sectioned by glass (dark band) and with an “inclusion” of moissanite (M), with a glass halo and calcite fringe; B) another quartz grain similar to the previous one, but intact and without “inclusion”, with the same edge and glassy fringe in a mass of micritic calcite and full of cavities; C) another quartz grain three times larger than the previous ones, but also with partial evidence of vitrification/transformation; glassy forms (G) are visible in the micritic calcite mass at the bottom of the image; many cavities (V, void);  D) irregular grain of lechatelierite in a mass of micritic calcite. Polarized light with crossed Nicols.

 

Calcite, “Mg-calcite”, and Calcitic Bioclasts

As mentioned, calcite is the other very abundant mineral in the Aracati fulgurites, and its presence was not expected, given the “sandy” environment in which they are found, dominated by dune fields. It constitutes the “vitreous”, microcrystalline mass of these fulgurites, giving it consistency and the appearance of glass (which is conferred by its micritic nature), and micritic calcite, quartz, and deformed vitrified quartz are found. These minerals are surrounded by calcites in the external zone, while in the internal zone calcite is the dominant mineral, with rare grains of quartz, but glasses are present (Fig. 13). Calcite appears sub spathic on the periphery of cavities (Fig. 13), and as fringes of quartz and feldspar grains (Fig.13), and in the structure of bioclasts (Fig. 14). Micritic and sub spathic calcite were delineated in the SEM images and their composition assessed by EDS (Fig.15). All analysis shows the presence of Mg in the calcite domain, varying between 0.3 and 1.8% and with no positive or negative relationship with the Ca content. The general literature would describe Mg-calcite, which crystallochemically is not so compatible. In the beachrocks of the northeast region of Brazil, high Mg-calcite was recognized (Vieira & Ros, 2006; Vieira et al., 2007; Ferreira et al., 2011). Submicrometer inclusions of other carbonates are very likely, such as dolomite or magnesite (less likely).  Long et al. (2014) address the biogenic formation and synthesis of high Mg-calcite, highlighting its thermodynamic instability and the submicron to nanometric nature of the crystallites. The success of synthesis generally considers amorphous starting material. They mainly emphasize biogenic from Echinodermata. Coralline algae may also contain high Mg-calcite, or as magnesite and dolomite (Long et al. 2014). Another important aspect is that the carbon content is always well above the stoichiometric value for calcite, admitting all Ca content, such as calcite. Only the analysis of massive calcite (Fig. 15E) reaches values ​​compatible with stoichiometry and suggests 100% (calcite + another carbonate). It also contains Si and Al, with the former always in excess for kaolinite, suggesting the presence of quartz and/or glass (lechatelierite?) and kaolinite in the calcite domain. Calcite is, however, a frequent mineral in fulgurites, with emphasis on type III fulgurites derived from parental matter containing carbonates, such as caliche soils, in which it is associated with quartz, feldspars, as well as hematite, barite, among others (Pasek et al.  2012).

 

 

Figure 13 – Textural aspects of the second most abundant mineral, calcite, which appears sub micritic to micritic in the general mass and micro spathic bordering cavities and as a fringe of quartz grains and partly in bioclasts. A) general view of the “banded” arrangement of micritic calcite with elongated cavities in the internal zone; B) the same previous image under crossed Nicols; C) detail of the transition from the outer to the inner zone, with angular quartz grains of varying sizes in the micritic calcite matrix, which contains several small and irregular glass (Gl) features, possibly lechatelierite; Nicols X;  D) zoom of the previous image showing sub spathic calcite in the walls of cavities (V) and micritic in the general framework; crossed Nicols. Polarized light with crossed Nicols.

 

Figure 14 – Descriptions of possible bioclast relicts in fulgurites. A) it could be a possible fragment of bivalve mollusk; B) inferred as a gastropod fragment in subpathic calcite and mass of micritic calcite and many cavities, some angular, misshapen, highly fractured quartz grains; C) could be remains of algae, by correlation with images from Arias (2015, 2021), coralline algae (Corallina officinalis) by correlation to Mesquita et al. (2016) Fig.7 p.790; rounded quartz grains (qtz) with subspathic calcite fringe, many cavities (V) and opaque mineral (Op); D) suggests an ovoid-shaped fossil body (mollusk ?)  in subspathic calcite, alongside features of glass like lechatelierite (Lt); E) possible relics of algae; F) possible fragment of bivalve mollusk, in subspathic calcite. Polarized light with crossed Nicols.

 

Figure 15 – Distinct textural features from calcite after SEM images and its EDS chemical composition. A) subspathic around micro void; B) spathic in the cement; C) microdrusic; D) micro prismatic; E) massive.

 

The bioclasts were inferred from the images obtained by optical microscopy and by comparison with the images of bioclasts described by Arias (2015, 2020) in the eolianites of northwestern Ceará, rich in carbonate and calcitic bioclasts, as well as quartz grains. The carbonate content from bioclasts in these eolianites is 10 to 25% (Arias, 2015). Fulgurites with little or no glassy constitution and rich in carbonates, such as those investigated here, would fit into type III of Pasek et al. (2012), which would have as parental matter, caliche, not known in the region investigated. In addition to eolianites, other late Pleistocene to Holocene formations in the region, including in the Aracati fulgurite area, are beachrocks, but not yet investigated.

However, beachrocks in the region were researched by Vieira & Ros (2006), Vieira et al. (2007), Cabral Neto (2011), Mesquita et al. (2016), Freire (2017), Otávio et al. (2017), and Arias (2020), which also involve everything from quartzose to arcosean sands, with bioclasts, and cement to calcite and aragonite, the beachrocks of Aracati can probably be comparable to those described by these authors. The bioclasts identified by Arias (2015, 2020) represent fragments of mollusks (bivalves), bryozoans, foraminifers, spongiaria, bryozoans, echinoderm spines, crinoids, barnacles, coral, red algae, peloids (algae + clay), among others. The contribution of beachroks, whether directly or more as a source of fragments (grains) for the formation of paleodunes (eolianites), becomes evident due to their richness in bioclasts, reinforced by the mineralogical constitution dominated by calcite, in particular Mg-calcite, which is present in the Aracati fulgurites. It is possible to identify a fragment of a bivalve mollusk (Fig. 14 A, F), inferred as a gastropod fragment in calcite subspathic and mass of micritic calcite and many cavities, some angular, misshapen, highly fractured quartz grains; in addition to possible remains of algae (Fig. 14 C, E). Algae stands out for high Mg-calcite (Long et al., 2014).

 

Feldspars

Feldspars were identified by XRD, with some diffractograms signaling sanidine. It is likely to be a Neogene mineral from fulgurites. Under the optical microscope, feldspar grains were also delineated (Fig. 16), however, it was not frequently recognized, possibly due to the size of the grains and possible changes and abundance of micritic calcite.

 

Figure 16 – Feldspar grains (F) in micritic calcite (Cc) cement associated withm quartz (Q) and glass (or vitrified quartz) (G) and plenty of voids (V). Polarized light with crossed Nicols.

 

Barite

This mineral was only identified through SEM/EDS analyses and found sporadically in Mg-calcitic micritic cement, generally as isolated but concentrated crystallites (Fig. 17). Barite also displays some crystallites (<50 um) dispersed in cement or massive calcitic mass. Spot analyses show intergrowth with Mg-calcite and SiO₂ minerals (quartz and/or lechatelierite). The carbon contents, as before, are in stoichiometric excess, requiring a phase for this element, in addition to calcite. Barite, with about 8%, was found only in the central part of iron-rich fulgurite from the Mongolian Gobi Desert (Karadag et al., 2022), the same situation as those from Aracati, although it is rare. Even with this high content, they were unable to delineate it using electron microscopy, but it was confirmed by Raman spectroscopy, XRD, and SEM/EDS. These iron-rich fulgurites are mainly formed by K-feldspars and albite (up to 60%) and quartz (up to 52%), in addition to hematite (goethite) and barite (Karadag et al., 2022). Barite is found in type III and other fulgurites (Block, 2011; Pasek et al., 2012; Elmi et al., 2017). Therefore, barite is a frequent accessory in fulgurites.  The presence of barite exclusively in the inner zone or core of the fulgurite suggests that the sulfur present in the outer zone was volatilized due to the higher temperature and very rapid cooling than in the core, in correlation with that of the Mongolian Gobi Desert (Karadag et al., 2022).

 

Figure 17 – Micrometric barite aggregates and crystals dispersed in the Mg-calcite cement in the internal zone of Aracati fulgurites. SEM/EDS analyses.

 

Cu and Zn Minerals (Zinc-rich Native Copper, Natural Brass)

Micrometric aggregates and crystallites of Cu and Zn minerals were also identified in the Mg-calcite cement of the investigated fulgurites (Fig. 18). According to SEM/EDS analyses, the average Cu/Zn ratio is around 2.0 (1.7 to 2.5). They cannot be sulfides or sulfates, as the S contents are very low (< 3%), while Cu varies from 30 to 40%) and Zn from 8 to 16% (Fig. 18). They could be carbonates, but in analysis, there is no carbon, even though this element is almost always present in microanalyses. When present, the Ca content is significant, which would correspond to calcite, and always with a large excess of carbon. In another analysis with high Ca content carbon was not detected. The most obvious deduction is that it could be a Cu-Zn type alloy, or carbide or silicide, which are mineral compounds that have been identified in fulgurites, that are not supported by the Si and C contents. Metallic alloys, and metals, such as Fe, Cu , Al, etc., and silicides, carbides and phosphides of these elements were first identified in extraterrestrial bodies, such as meteorites, impactites, and now in fulgurites (Hiltl et al., 2011; Pasek et al., 2012; However, the most likely is that it is an alloy of Cu and Zn, like natural brass, that is, native copper rich in zinc. Zn-rich native copper with Zn content in the order of 33-34% was first discovered in basalt and breccia samples extracted from the moon by Apollo 11 and 12 (Novgorodova et al., 1980), while native copper on Earth, generally contains Fe, Ag, Pb, Hg, Bi, Sb, V, Au. Native copper is cited by Moller (1973) in anthropogenic fulgurites in southern California. Kvasnytsya et al. (2008) describe native copper in fulgurites, found in a quarry in south-western Donbas. It is a conical, small (1 x 7 cm), classic fulgurite, dominated by SiO₂, partly molten and glassy, containing traces of Fe, Cr, Al, Ca, Mg, and K. But it was only in 1971 that native copper was discovered with 32-34 % Zn in gold quartz-carbonate veins of the upper horizons of a Transcaucasian deposit. A second occurrence was found in lump samples of gold—quartz ore from a deposit in the South Urals (Novgorodova et al., 1980). Like the native gold, copper, and zinc in association with it, zinc-rich copper native mineral forms films (occasionally thin platy segregations) along the surfaces of cracks in the quartz vein. The segregations are no larger than 0.1-0.2 mm; sometimes platy or dendritic (Novgorodova et al., 1980). In the recent volcanic eruptions of Kamchatka and on the Kuril Islands in Russia, Cu-Zn alloys, equivalent to brass, were also identified (Silaev et al., 2019). This zinc-rich native copper mineral, however, was not found in fulgurite. In turn, the stoichiometric formula obtained for the zinc-rich native copper from Aracati fulgurites is in full agreement with zinc-rich native copper described by Novgorodova et al. (1980) found in mineral deposits of South Urals and Transcaucasus (Tab. 2), not in fulgurites, ranging from 29 to 37% of Zn contained in native copper in Aracati fulgurites.

 

Table 2 – Composition of zinc-rich native copper found in mineral deposits in Urals and Transcaucasus (Novgorodova et al., 1980) compared to those one from Aracati fulgurites.

 

	South Urals		Transcaucasus	Aracati fulgurites (SEM/EDS analyses) this paper
Cu	66.87+1.41	67.09+1.05	63.49+0.99	68	71	63	69	71
Zn	32.66+0.39	33.42+0.51	34.65+0.53	32	29	37	31	29
Formulas	Cu2,7Zn1,3	Cu2,7Zn1,3	Cu2,6Zn1,4	Cu2,7Zn1,3	Cu2,8Zn1,2	Cu2,5Zn1,5	Cu2,8Zn1,2	Cu2,8Zn1,2

 

 

Figure 18 – SEM/EDS chemical analyses from zinc-rich copper micro-areas. Formulas: Cu_0,68Zn_0,32; Cu_0,71Zn_0,29; Cu_0,63Zn_0,37; Cu_0,69Zn_0,31; Cu_0,71Zn_0,29.

 

Fe, Cr and Ni Minerals

Minerals based on Fe-Cr-Ni were detected by SEM/EDS in the Aracati fulgurites, and for now only in the quartz or partially vitrified quartz domain (?), indicated by the high Si and O contents (Figs. 19 A and B), but possibly intergrown with calcite, based on the Ca and C values. They are rare and appear as irregular fillets or clouds of “grains” or micrometric to submicrometric crystallites dispersed in specific areas in the quartz grain (Fig. 19 A), or even as an isolated “grain” (Fig. 19 B).  Fe contents (4.5 to 13%) are 4 to 6 times those of Cr (1.2 to 1.9%) and Ni contents are below 0.8% in the analyses (Fig. 19). The absence of S eliminates sulfides, which can occur in fulgurites (Pasek et al 2012), but could be oxides (hematite, even wüstite), which are the most common forms. In fulgurites, as previously mentioned, these elements are generally described as native metals, metallic alloys or even as silicides and/or carbides and/or phosphides, but also as iron oxides (Novgorodova et al., 1980; Pasek et al., 2012; Plyashkevich et al., 2016). Cohenite, (Fe, Ni, Co)₃C, a possible mineral in fulgurite, meteorites, and terrestrial native iron (Plyashkevich et al., 2016) could be a possibility to explain the Fe-Cr-Ni in Aracati, diverging into Cr and Co. Chemical analyses always show the presence of carbon, which is in great excess compared to the amount of Ca, in the associated calcite. It could also be naquite, FeSi, found in ophiolites in Tibet (China) and reported in fulgurites from Tiedra, Valladolid, Spain (Martin-Ramos et al., 2019) and as FeSi₂ (equivalent to gupeiite) and FeSi (equivalent to naquite) in the New Port Richey fulgurites (Rappenglück, 2022; Bindi et al., 2023). Fe, Cr, Ni, and Co, are chemical elements that are very compatible with each other, both in meteorites, in ultramafic and mafic rocks, and even in fulgurites, of course, and industrial smelters to be deduced from geochemistry in general and from recent articles covering the same (Block, 2011; Plyashkevich et al., 2016;), in which the levels can vary greatly, and locally, one of them may be absent.  In the recent volcanic eruptions of Kamchatka and on the Kuril Islands in Russia, native Fe was also identified, and several Fe alloys, such as Fe_0.73(Si,Al,Mn,Cr,Zn,Mo)_0.23, as well as naquite and gupeiite (Silaev et al., 2019).

 

 

Figure 19 – SEM/EDS chemical analyses from Fe-Cr-Ni minerals included in quartz domain in the Aracati fulgurites.

 

Moissanite

Moissanite silicon carbide, SiC, was initially found in meteorites from the Canyon Diablo meteorite, Meteor Crater, Coconin County, Arizona, USA; but it is currently known in kimberlites and associated volcanic and pyroclastic rocks, well represented by recent Eruptions in Kamchatka and on the Kuril Islands (Silaev et al., 2019); among many others, generally containing inclusions of iron silicides (Rappenglück, 2022), and produced on a large scale as an abrasive, but also synthesized with gem. Furthermore, it has been frequently recorded in fulgurites associated with Al–Si glass, α-cristobalite, native iron with a phosphorus admixture, nickel-less shreibersite (?), troillite, and possibly cohenite (Plyashkevich et al., 2016); SiC in fulgurites from an archaeological site in Tiedra, Valladolid, Spain (Marin-Ramos et al., 2019), such as SiC in fulgurites from Glen Ellyn, Illinois, USA (Hess et al., 2021) and moissanite with iron silicides (Feng et al., 2021). Silaev et al. (2019) described moissanite intergrown with non-crystalline carbonaceous substance, sometimes with metallic silicon, in explosive ejecta from Kamtchatka volcanoes.  In Aracati fulgurites, moissanite was inferred from optical microscopy images (Fig. 12 A) associated with partially vitrified shocked quartz.

 

Carbonaceous Substances; PAH (Polyaromatic hydrocarbon)

Carbonaceous substances (CS), amorphous carbon, graphite, fullerenes and PAH (polyaromatic hydrocarbon) have been frequently recorded in fulgurites as inclusions in newly formed minerals, in products such as glass, shocked quartz and carbonates (Daly et al., 1993; Heymann,,1996; Navarro-González 2007; Ende et al., 2012; Plyashkevich et al., 2016; Gieré et al., 2015; Holm-Alwmark et al., 2021; Hermes et al., 2023). In the same way as moissanite, lechtalierite, and the various silicides, carbides, and phosphides that were initially found in meteorites, volcanic rocks, and fulgurites, this is also the case for carbonaceous substances, including diamond present in recent volcanic events (Silaev et al., 2019a; Li-Wei-Kuo et al., 2021; in Hess et al., 2021).

Polyaromatic hydrocarbons were found in an interfacial zone of a glass bubble, suggesting that some regions of the fulgurite specimen were not subjected to temperatures of 1800◦C, which are attained when lightning hits the surface of sand or rock (Carter et al., 2010).

In almost all SEM/EDS analysis carried out on Aracati fulgurites, carbon is always found at values ​​well above those for stoichiometric calcium in calcite, when present and in some cases, Ca values ​​are very low or not detected. In the SEM images, the dark gray inclusions are generally dominated by carbon and oxygen (Fig. 20), interpreted as CS associated with lechatelierite or moissanite and calcite. The presence of these carbonaceous compounds is quite plausible, given these data and the fact that they are in fulgurites.

 

Figure 20 – SEM/EDS chemical analyses from possible occurrence of carbonaceous substances (CS) or PAH in glass and shocked quartz in the Aracati fulgurites.

 

Parental Matter for the Formation of Aracati Fulgurites

As previously suggested, the data presented demonstrates, both at field and laboratory levels, the Aracati fulgurites show strong parental affinity with dune sediments. The mineralogical and microchemical data in turn indicate that these sediments were rich in calcium carbonates, as cement, and possibly in the form of bioclasts. Eolianites, therefore dune deposits with carbonate cement (Mg-calcite), as well as beachrocks, are abundant in the region around Aracati, at the mouth of Aracatimirim (Freire, 2017), with the occurrence of rhizoconcretions, which seem to refer to fulgurites, and are not so far away in the coastal zone from Piauí to western Ceará with rhizolites (Arias, 2015; Mesquita et al., 2016) and eastern of Aracati on the coast of Rio Grande do Norte (Cabral Neto, 2011) extending to Pernambuco (Ferreira Junior et al., 2011). It is possible that a large part of the rhizoconcretions described by these authors could be fulgurites. The mineralogy of the Aracati fulgurites eliminates the possibility of their being correlated with rhizoconcretions, in turn. Semi-quantitative chemical analyses (Table 3) in turn show that the parental material was rich in calcium, silicon, aluminum, and magnesium, comprising the minerals calcite, quartz, feldspars, and clay minerals (kaolinite). Another striking aspect is the dominance of silicon and aluminum in the outer zone of the fulgurites and calcium and magnesium in the inner zone, in compatibility with mineralogy. The superficial red sands surrounding the fulgurites analyzed are not rich in calcium, but are rich in silicon and aluminum, in the order of magnitude of the fulgurites.

Certainly, eolianites must have been the main parental materials for the formation of fulgurites from Aracati those along and the northeastern Brazil coast. They are widespread and well investigated in this region as highlighted in the previously cited papers.

 

Table 3 – Semiquantitative XRF_handheld chemical composition of investigated fulgurites and in field close related red sand dune.

Chemical compounds or elements	Fulgurites				Dune red sand
	Inner zone		Outer zone		1	2
SiO2	14.1	12.9	45.7	43.3	52.2	69.4
Al2O3	17.9	14.5	19.9	5.3	18.2	22.9
Fe2O3	0.4	0.4	0.4	0.2	0.6	0.7
MgO	9.1	6.4	2.0	–	2.2	–
CaO	47.3	41.7	5.9	7.0	0.13	0.28
K2O	–	–	0.24	0.01	0.43	0.48
TiO2	0.24	0.024	0.23	–	0.33	0.32
P2O5	0.8	0.6	–	–	–	–
MnO	–	–	0.02	–	–	–
Cl	0.1	0.1	0.06	0.02	0.01	0.01
Sr	–	0.06	0.01	0.03	–	–
Zr	–	0.02	0.07	0.03	0.08	0,04

 

The total incidence of lightning in Brazil in 2022 was 78 million. The highest incidence occurs in the center, southeast, and south of the Country, particularly, in the interior, far from the coastal region. Along its coast, the incidence stands out between the states of Espírito Santo and Rio Grande do Sul. In the northeast region, where Aracati fulgurites and potentially many others can be found, the incidence is relatively low, ranging practically zero to 16,400 strikes (Fig.21). However, in figure 21, although not immediately evident, a high incidence line stands out (269,400 to 471,400) following the coastline, exactly where the Aracati fulgurites and the rhizoconcretions described in the several published and already cited articles, are found. These thizoconcretions could in part also represent fulgurites.  Obviously, the data presented on the incidence of lightning, as highlighted, are current, while the fulgurites investigated are linked to eolianites, although they are not contemporary with these rocks, but they are not all from the present, as they are generally covered by younger sediments (generally eolian: dunes and paleodunes), including in Aracati. The eolianites from the western coastal region of Ceará are 2.4 and 1.0 Ka old (Vasconcelos, 2014; Mesquita, 2015) and 1049 to 4422 years BP (¹⁴C) for Piauí and Ceará (Arias, 2015). These ages show that the Aracati fulgurites and the region’s potentials have been formed over the last five millennia and even in the present.

 

Figure 21- Lightning incidences in Brazil in the 2018-2019 biennium. Source:

http://www.inpe.br/webelat/homepage/menu/infor/incidencia.de.descargas.no.pais.php, accessed on December 8, 2024. Location of Aracati fulgurites indicated.

 

CONCLUSIONS

The fulgurites from Aracati, in northeastern Brazil, are quite significant and type III in the general classification, as their main mineral is calcite and/or Mg-calcite, also containing quartz from the parental matter. Other minerals present are feldspars, kaolinite, and barite. The presence of deformed and/or partially vitrified quartz is common, sometimes it is possible to indicate the presence of lechatelierite and moissanite. The presence of mollusk relict shells and algae (bioclasts composed of Mg-calcite), outlined in optical microscopy images, is common. Rare minerals such as zinc-rich native copper (equivalent to bronze) and Fe-Cr-Ni alloy were identified, the first in the calcite matrix and the second associated with deformed quartz (?). It was also possible to assume the presence of carbonaceous substances, PAH, amorphous carbon, etc. The parental matter of these fulgurites were eolianites, cemented by calcium carbonate, widely distributed in the region, which were fed with fragments of mollusks and even algae from extensive strips of beachrock, very well studied according to current literature, these fragments being responsible for the high content of calcium carbonates and carbon in fulgurite. The potential for new occurrences of fulgurites in the northeastern region of Brazil is great, demonstrated by the extensive mention of rhizoconcretions described by several articles cited in the previous items, which may in part be fulgurites equivalent to that of Aracati. In addition to the wide distribution of parental matter along the coast, the hot, sometimes very humid and even very dry climatic conditions, in recent times, have favored and continue to favor the formation of these special and rare rocks.

 

Acknowledgements

To CNPQ for financial support through bench fees granted to M.L. Costa (No. 304967/2022-0). To Professor Alan Albuquerque for his careful review of this manuscript.

 

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