01 – NATIVE ALUMINIUM IN CHALCEDONIAN PEBBLE FROM SÃO MIGUEL, AZORES, PORTUGAL

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

NATIVE ALUMINIUM IN CHALCEDONIAN PEBBLE FROM SÃO MIGUEL, AZORES, PORTUGAL

 

10.31419/ISSN.2594-942X.v122025i2a1MLC

 

Marcondes Lima da Costa¹

Glayce Jholy Souza da Silva Valente²

Susana Horta³

 

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

     ²Faculty of Conservation and Restoration, Federal University of Pará, Belém, Pará, Brazil

     ³Av. António Medeiros Almeida 4H 2AB, Rosário, 9560 Lagoa Açores, Portugal

                 ^*Author for correspondence

 

Submitted on January 27, 2025; after revisions accepted on February 25, 2025

 

ABSTRACT

The chemical element aluminium, the third most abundant in the Earth’s crust, is part of the chemical composition of more than a thousand species of minerals. However, as a native element, it is very rare, with only 22 known occurrences in the world to date. While the chemical element was discovered in 1825, the industrial production of the metal began in 1856. The discovery of the first occurrence of native aluminium only occurred in 1978, but it was only actually accepted in 1983. The penultimate discovery was made in Tolbachik in Russia and this work presents the occurrence of a micro flake of native aluminium in chalcedonian pebble from São Miguel, Azores, Portugal.

 

INTRODUCTION

Although the chemical element aluminium (Al) is the third most abundant element in the Earth’s crust (8.23% by mass), behind only oxygen and silicon, its discovery was only announced in 1825 by the Danish physicist Hans Christian Ørsted (1777 – 1851). The name was given by Sir Humphry Davy, who discovered the element (https://www.mindat.org/min-107.html, accessed on 26.01.2025). However, the type localities for natural occurrence of native aluminium are the Billeekh intrusion and the dike OB-255, Sakha Republic (Oleinikov et al., 1978). In a gabbro-dolerite of the Billeekh intrusion it occurs with copper, zinc, tin, lead, cadmium, iron, antimony and moissanite”. (https://www.google.com/search?q=native+aluminum&rlz=1C1FCXM_pt-PTBR1000BR1000&oq=native+aluminum&aqs=chrom e..0i19i512j69i57j0i19i512l3j0i19i22i30l3j0i10i19i22i30j0i19i22i30.853308828j0j15&sourceid=chrome&ie=UTF-8, accessed on 5, May 2023; https://en.wikipedia.org/wiki/Native_aluminium, accessed on 13, January 2025).

The first industrial production of aluminium metal was started by the French chemist Henri Étienne Sainte-Claire Deville (1818 – 1881) in 1856, but it only became widely available in 1886 with the discovery of the Hall–Héroult process. This was the beginning of its mass production, which, for this reason, depended on a large amount of electrical energy. Aluminium metal thus became part of everyday life in the developed world. During the First and Second World Wars, this metal became even more crucial, having become a strategic metal for the military industry, especially for aviation. From the 1950s, it became the most produced and consumed non-ferrous metal, supplanting copper. Currently, aluminium metal is widely used in ​​transportation, construction of all sizes, packaging, etc. In addition, in the form of oxides and salts, it is widely used in the chemical industry.

However, this abundant chemical element, with atomic number 13, mass 26.981 and stable isotope 27Al, whitish gray in color, metallic luster, hardness 2–3.5, density 2.707, and belonging to the nickel group, so useful for the development and well-being of humanity since the 19th century, was completely unknown as a native element until half a century ago. In other words, it would not occur spontaneously free in nature,  on the Earth. The chances of this element occurring as a native element were somewhat slim and expected, given its strongly amphoteric character, in which its main ion, Al³⁺, is strongly reactive with oxygen, acids and bases. Given the exceedingly high reducing conditions to form, the aspects above make the native Al findings unconvincing and controversial, and raised questions that are still awaiting answers, expressed Paar et al. (2019).

The saga of the discovery of native aluminium began formally in 1978 with the publication of Oleinikov et al. (1978), in which they described Al^o flakes in crushed rocks and argued that it was native aluminium. Although contested, it was submitted to the CNMMN (Commission on New Minerals and Mineral Names) of the IMA (International Mineralogical Association) under protocol numbered IMA 1980-085. After two consecutive proposals (IMA 1980-085a and IMA 1980-085a), the new mineral and its name, native aluminium, were approved in 1983 (CNMMN Chairman J. Mandarino’s memorandum, in Dekov et al., 2009; Warr, 2021).  Even after approval, the contestations continued, but new articles were published about new discoveries of native aluminium. Korzhinsky et al (1995) described native Al and Si, amont other metals, in hot volcanic gas jets associated with Kudriavy volcano on lturup island in the Kuril arc. In recent years, there have been more than 20 articles on the occurrence of native aluminium in different rocks and geological environments (Chen et al., 2011; Paar et al., 2019). A citation was found regarding the discovery of native aluminium by Deng et al. (1983) in fault zones embedded in granodiorites. However, it was not possible to access the article, only the citation. Dekov et al. (2009) describe metallic Al flake protruding from a rock specimen with phlogopite and emerald in a pegmatite vein in the Rila Mountain (Bulgaria). They discuss and suggest two geological processes for the formation of native aluminium from this occurrence: (1) desilication of the pegmatite vein, and (2) serpentinization of the ultramafic body, and demonstrate that, due to its chemical composition, a possible anthropogenic origin, which is normally imputed, is ruled out.

Chen et al (2011) highlight that although native aluminium has already been described in more than twenty different occurrences under distinct geological conditions, until then, there were still many controversies about the natural conditions of its formation, much to be elucidated. They describe the occurrence of native aluminium, as well as provide a detailed characterization of this mineral that was found at cold seeps in the northeastern of the South China Sea. According to these authors, the Al^o (native aluminium) particles occur as spherules, irregular plates and elongated forms with typical lamellar structures and their chemical compositions are 95.07 – 99.84% Al, containing very small amounts of Si, Fe, Ti, S, Zn, Mg, Ca, K, Na, Cu, Co and P. This composition is similar to that of other Al° particles found in the East Pacific Rise (Dekov et al., 1995) and the Central Indian Basin (Iyer et al., 2007), but differs greatly from those of other localities, where native aluminium particles were found in basic and ultrabasic rocks, non-carbonate zeolitic pelagic clays, kimberlite tube, tungsten deposit, and jarositized quartzitic rock (Chen et al. 2011). These authors discuss the source of aluminium and address different hypotheses for its formation. Another notable occurrence of native aluminium, in addition to the more than 20 of Chen et al (2011) is presented by Paar et al (2019). It was found in 2004 at Hochwurten, in the Land Carinthia’s, NW Austria. Native aluminium occurred in situ on the surface of a brownish weathered gneiss. A small “satellite grain” (200 μm length) of native aluminium was found adjacent to the large aggregate (Paar et al., 2019). A new occurrence of native aluminium occurred in the Tolbachik volcano on the Kamtschaka peninsula, Russia (Silaev et al., 2019). Native aluminium is the second most abundant native compound in the volcanic ejecta from Tolbachik, Al0.99–1Co0–0.01, after native iron, Fe0.73–1(Si,Al,Mn,Cr,Zn,Mo) 0–0.2, associated with the explosive atmospheric electrogenic paragenesis (Silaev et al., 2019).

A new occurrence of a native aluminium microparticle is described in the present work on the island of São Miguel, in the Azores, Portugal. This is probably the first occurrence of this mineral in the Azores Archipelago, since no mention of it was found in the references consulted. not even in Mindat (https://www.mindat.org/min-107.html, accessed on 26.01.2025) and not in “Rocks and Minerals in the Azores, PORTUGAL” (https://www.oficina70.com, last accessed on 26.01.2025), RTP3/RTP AÇORES. 2018.

 

MATERIAL AND METHODS

Sampling – Among the various samples of chalcedony pebbles collected by S. Horta in São Miguel, one was occasionally selected that had a clear bark or skin and a gray interior with an intense greasy shine and the native aluminum flake was found in it.

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. For more details see Costa et al (2025, in this Bomgeam edition, 12 (2)).

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. For more details see Costa et al (2025, in this Bomgeam edition, 12 (2)).

Partial chemical analyses – For this purpose, Bruker’s LAMIGA portable X-ray fluorescence S1 Turbo was used. The analyses were performed out directly on a natural flat sample surface.

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 a Swift ED300 Energy Dispersive Spectroscopy (EDS), under accelerating acceleration from 5 to 15 kV and, with SDD detector (161 eV-Kα). For more details see Costa et al (2025, in this Bomgeam edition, 12 (2)).

 

RESULTS AND DISCUSSIONS

Pebbles measuring less than 1 cm to 5 cm in size, naturally polished by ocean waters (North Atlantic), frequently appear on the beaches of the southern coast of São Miguel Island, Azores. They are notable for their brown, cream and dark grey colour, subspherical to drop-shaped shapes, sometimes with an outer film lighter than the inside, subrounded to well-rounded surfaces, with a vitreous to greasy luster, which make them very attractive (Figure 1A). XRD analyses and density measurements, as well as optical microscopy analyses, show that they are predominantly formed by microcrystalline quartz, chalcedony type (Fig. 1 B, C and D) (Costa et al., 2025 under submission). One of the pebbles, cream-colored on the outside and gray on the inside, formed mainly by SiO₂ (Table 1), when analyzed by SEM-EDS, showed several metallic inclusions (Fig. 1.E), among them one with an elongated shape, flake type, which would correspond to native aluminium (Fig. 2).

 

Table 1 – A semiquantitative chemical composition of chalcedonian pebble obtained by portable XRF carried out direct over the natural sample (Analyses A and B).

 

Wt.%	A	B
MgO	0,11	0,19
Al2O3	1,46	1,50
SiO2	100,00	100,00
S	0,09	0,10
Cl	0,07	–
K2O	0,09	0,04
CaO	0,05	0,07
Fe2O3	0,50	0,42

 

 

Figure 1 – (A): Natural rounded and polished chalcedonian pebbles from São Miguel, Azores; (B): Optical microscope photograph get by thin section on the border of the pebble; (C) optical microscope photograph showing the domain of microcrystalline quartz (chalcedony) with a fibrous rosette texture (quartz); (D) optical microscope photograph displaying a microcrystalline quartz with acicular and opaque mineral inclusions; (E): SEM microphotograph showing native copper inclusions in quartz-rich (chalcedony) pebble.

 

The singular flake found in the microcrystalline mass of chalcedonian pebble investigated,  shows the aluminium dominance (Fig. 3) in the SEM-EDS chemical spectrum. In addition to Al, it also contains O, C and Si, with less expressive concentrations of Mg and Ag (Table 2). Considering carbon as an elemental form, it would be an aluminosilicate. However, this composition is not compatible with the polymorphic minerals of AlOSiO4 (kyanite, andalusite and sillimanite), which are not compatible with acidic volcanics. It would also not be correlated with the minerals of Al₂Si2O₅(OH)₄, represented by 1:1 clay, such as kaolinite (dickite and nacrite), which could be present as a hydrothermal product. Assuming that C, O and Si are reflecting the composition of the surrounding chalcedony, which is very plausible, demonstrated by analyses 4 and 5 and even 6, as shown by Costa et al. (2025 under submission) for the chalcedonian pebble from the Azores, the elongated, flake-like shape would correspond to native metallic Al (?), which is very rare, as demonstrated previously. In turn, the matrix (analyses 4, 5 and 6) does not contain or has only a low content of Al, which reinforces the singularity of Al, and likewise does not contain Ag (Table 2). It is probably native aluminium with little Ag. In the publised native aluminium, Ag was not identified, but other metals such as Si, Fe, Ti, S, Zn, Mg, Ca, K, Na, Cu and Co in native aluminium from Bulgaria (Chen et al., 2011) and Zn, Mg, Fe, Si, Cr and Mn in that from Austria (Paar et al., 2019). From a crystallochemical point of view, Ag is compatible with Zn, Cu and even Co, therefore it could participate as a metallic alloy with Al.

 

 

Figure 2 – A SEM microphotograph showing an 85-um large aluminium flake inclusion in a chalcedonian pebble from São Miguel Island, Azores and the three analytical points carried out by SEM/EDS (Table 2).

 

Figure 3 – SEM-EDS analytical spectra illustrating the possible native aluminium flake in a chalcedonian pebble from São Miguel, Azores. Analytical point position indicated in the Fig.2.

 

Table 2 – SEM/EDS chemical composition of the elongated flake mineral inclusion in a chalcedonian pebble from São Miguel Island, Azores, compared to native aluminium from Bulgaria and Austria. The analytical points are indicated in figure 2. Points 1,2, and 3 refer to aluminium flake; 4 and 5 refer to matrix and 6 to possible micro-amygdala.7. Aluminium flakes from Bulgaria (Dekov et al., 2009). 8. Native aluminium from Austria (Paar et al., 2019), besides Cr and Mn, less than 0,12 Wt.%.

Chemical elements (Wt.%)/Analytical points	Aluminium flake domain			Matrix domain			Bulgaria	Austria
	1	2	3	4	5	6 Amygdala	7	8
Carbon	12.484	9.224	10.500	12.096	36.165	–
Oxygen	17.013	16.702	20.605	50.651	33.737	57.634
Magnesium	0.460	0.443	0.454	–	–	–		1.08
Silicon	11.961	13.102	13.817	37.253	30.098	35.847	7,51	0.18
Aluminum	57.270	59.847	53.943	–	–	0.562	87,5	93.46
Silver	0.813	0.681	0.681	–	–
Calcium						1.558	1,04
Sodium						0.793
Sulfur						1.608	0,17
Chlorine						1.998	0,20
Iron							1,55	0.19
Potassium							1,82
Phosphor							0,17
Titanium							0,035
Zinc								4.48

 

 

The 20 distinct occurrences of native aluminium cited by Chen et al. (2011) have already been surpassed, at least by the occurrence of Tolbachik in the Kamtaschaka Peninsula (Silaev et al. 2019) and the one described in this article. The Tolbachik occurrence appears as a contorted flake (Fig. 4), with a slight similarity to that of the Azores (Fig. 2) but is much larger than the latter. The native aluminium particles described by Chen et al. (2011) in cold seep sediments from the northeastern South China Sea are also on the same order of magnitude as those of Tolbachik but have different shapes. However, lamellar particles predominate, with 50 to 1100 μm in length and 10 to 50 μm in width (Fig. 5), and the chemical composition is much more complex, as previously presented. The native aluminium from Austria occurs as flakes associated with albite, reaching up to several mm in length (Fig. 6) and contains different micrometric and nanometric inclusions of metal alloys, Pb, (Al,Si)19Fe4, and Bi oxides, tellurides or sulfides (Paar et al., 2019). Although the native aluminium morphology tends to present itself in most occurrences as flakes, its composition is quite variable. The specimen from Azores, unfortunately, for now, is limited to just one particle, which does not allow for further consideration. From a chemical point of view, it would possibly be Al-(Ag),

In the Azores, the environment is simply dominated by volcanic activity initially underwater and in more recent times then on land. Costa et al (2023) summarized the geology of São Miguel island based on the most recent literature: “On the island of São Miguel, five active volcanic systems are recognized: three central volcanoes with caldera (Sete Cidades, Fogo and Furnas) in the western portion of the island separated by two basaltic fissure systems (Picos and Congros) and two extinct volcanic systems (Povoação and Nordeste) that constitute the eastern part of the island (Gaspar et al., 2015a). In historical times the Fogo and Furnas volcanoes erupted explosively in 1563, and 1439/1443, and 1630, respectively, with smaller eruptions in the Picos fissure system in 1563 and 1652 (Gaspar et al., 2015b). The volcanic stratigraphy of Sete Cidades, on the western end of the island, comprises two groups: the Lower Group, older than 36 ka, composed of basaltic and trachytic lavas and few pyroclastic deposits (Ellis et al. 2022); the Upper Group, which includes all deposits aged less than 36 ka, comprises six formations: the Risco Formation (36 ka), Brittany (29 ka) and Santa Bárbara (16 ka), dominated by pyroclastic interspersed with the Ajuda and Lombras, made up of pyroclastic and basaltic lavas. The Lagoas Formation is the youngest in Sete Cidades and comprises the Cascalho Negro Member (16–5 ka), with pumice deposits, lava domes and deposits associated with ash flow and blocks, lava flows basalt and scoria cones, and the Pepom Member (< 5 ka) composed mainly of trachytic pumice deposits from the caldera (Ellis et al. 2022). Native aluminium occurrence in a gabbro-dolerite of the Billeekh intrusion is associated with copper, zinc, tin, lead, cadmium, iron, antimony and moissanite (https://en.wikipedia.org/wiki/Native_aluminium, accessed on 13, January 2025). Moissanite has been described in Água de Pau Volcano (São Miguel, Azores, in association with peralkaline syenite (Nazzareni et al., 2018).
This fact strengthens this possible occurrence of native aluminium in the Azores.

On the island’s coast, among beaches with gravel and sand, angular fragments of olivine, peridot olivine, augite, feldspars, magnetite, various slags, pumice and locally rounded to sub-rounded pebbles that resemble lapilli stand out and are dominated by carbon-rich chalcedony in brown to gray tones (Costa et al., 2025, under submission). All appear to be disaggregated products of the rocks and pyroclastics mentioned. Therefore, the native aluminium described here shows a strong relationship with the volcanic environment, probably underwater. Native metals like Al and Alcan form in reducing oceanic or mud volcanic environments (Chen et al., 2011), which are associated with upward migration of basaltic magma (Iyer et al., 2007), hydrothermal activity, magmatic or metamorphic process or high-temperature hydrocarbon-enriched fluids (Dekov et al., 1995; Chen et al., 2011). Numerous alternatives for the formation of native Al are discussed, such as exsolution of metal-rich fluids in the magma and degassing of magmatic vapors during submarine eruptions may lead to reduction of some elements to their metallic state (Chen et al., 2011).

 

 

Figure 4 – Native aluminium from Tolbachik, Russland. Silaey et al 2019. https://doi.org/10.1134/S074204631906006X.

 

Figure 5 – Native aluminium particles and EDS analytical spectrum, from Bulgaria. Chen et al. 2011. https://doi.org/10.1016/j.jseaes.2010.06.006

Figure 6 – Intergrowth of native aluminium with albite (a, b), from Austria. Source: Paar et al. (2019), https://doi.org/10.1007/s12210-019-00760-5.

 

CONCLUSIONS

The data presented indicate the occurrence of native aluminium associated with chalcedonian pebble from a beach on São Miguel, Azores. It is a micrometric particle, in the order of magnitude of natural aluminium particles, with a compatible shape, a flake, and chemical composition within the broad chemical spectrum described for most of them, in the specific case of the Al-(Ag) type. Its formation must certainly be related to the strong reduction of aluminium composing other minerals of the volcanic lavas of the last Ka that led to the formation of the largest island of the archipelago, associated with upward migration of basaltic magma, hydrothermal activity, magmatic process or high-temperature hydrocarbon-enriched fluids.

 

Acknowledgments

The authors thank the support of LAMIGA laboratories and the first author financial support from CNPQ (Grants: 304.9672022-0).

 

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PTBR1000BR1000&oq=native+aluminum&aqs=chrom e..0i19i512j69i57j0i19i512l3j0i19i22i30l3j0i10i19i22i30j0i19i22i30.853308828j0j15&sourceid=chrome&ie=UTF-8