Celestine and other minerals from Azaila

The presence of large flint nodules in the Tertiary lands on the border between the provinces of Teruel and Zaragoza has been well known since the 19th century (Cortázar, 1885). These rocky blocks, with sizes of up to 50 cm, have been accumulated on the periphery of cultivated fields in some areas.

Within these flint blocks, we could find cavities that are covered by nice quartz crystals, sometimes of amethyst or smoky varieties, along with some other interesting mineral species. The hardness of these blocks makes searching for minerals an arduous and even dangerous task when flint shards are projected out when hammered, which can cause cuts and serious eye damage if protective equipment is not worn.

Fig. 1 - Source: Google Maps (modified) – Inst. Cartográfico Nacional de España.

The most common mineral we could find, as we have already said, is amethyst quartz. Brilliant crystals covering rock cavities. But the most attractive species for mineral collectors is surely the celestine. This species was already cited in other localities in this area such as Lécera (Zaragoza) by Calvo (1988) and, the same author indicates the Azaila (Teruel) outcrops in later works (Calvo et al., 1998; Calvo, 2018).

Fig. 2 - Quartz, amethyst variety. FOV 4 mm. MOTIC PlanApo ELWD 5x with stacking. Los Pedreñales, Azaila.

Fig. 3 - Flint cavity layered by quartz (var. amethyst) crystals. Size: 8 x 6.5 x 5 cm. Los Pedreñales, Azaila. Collection Joan Rosell.

The Azaila and Lécera deposits are found in a wide Miocene formation formed by gypsum strata, together with other detrital-type materials. These materials were deposited after the evaporation of inland sea waters that occupied the so-called Ebro Tertiary Depression and which was filled, from the beginning of the Cenozoic to the end of the Miocene, by Tertiary continental sediments. The specimens that we present were collected years ago in the area called “Los Pedreñales”, near Azaila, and belonging to the Belchite-Híjar evaporitic zone.

Fig. 4 - Flint with cavities layered by quartz and some celestine crystals. FOV 6 cm. Los Pedreñales, Azaila.
Collection Joan Rosell.

Fig. 5- Rich faceted crystal of celestine on quartz. Interesting growth planes. FOV 4 mm. MOTIC PlanApo ELWD 5x with stacking. Los Pedreñales, Azaila.

Crystals of celestite, strontium sulphate SrSO₄, are found within the cavities of the flint. Sometimes, these cavities are filled by gypsum (selenite), which is very transparent, and allows us to see if there are celestine crystals included. The usual technique consists of leaving the sample in warm water for days until the gypsum dissolves, a very slow process due to its low water solubility. In other cavities, crystals appear without gypsum. Azaila celestine crystals present a remarkable richness of facets, with a transparency, brilliance, and honeyed colour that make them very attractive to mineral collectors. Crystals of up to 20 mm have been found, but usually, they do not exceed 5 mm (Calvo, 2018).

Fig. 6 - Complex crystal of celestine on quartz. FOV 3 mm. MOTIC PlanApo ELWD 5x with stacking. Los Pedreñales, Azaila.

In addition to gypsum, quartz (hyaline, amethyst, and sometimes smoky), and celestite, other minerals can be found in these cavities. Other species are: jasper and chalcedony (cryptocrystalline varieties of quartz); tyuyamunite Ca(UO₂)₂VO₄)₂·5-8H₂, an ore with uranium, very rare in the deposit; sepiolite Mg₄(Si₆O₁₅)(OH)₂·6H₂O, as an alteration product of flint and clays.

We can also find malachite Cu₂(CO₃)(OH)₂, usually as compact green aggregates filling fissures, rarely crystalline, accompanied by minor azurite Cu₂(CO₃)(OH)₂. The origin of these secondary copper carbonates must be related to some black masses, which also fill fissures and accompany them, whose analysis indicates that it is a copper sulphide: covellite (CuS). In the attached photograph we can see a very small sphere of this species.

Fig. 7 - Covellite spherical aggregate on quartz. FOV 1 mm (cropped image). MOTIC 10x. Los Pedreñales, Azaila.

Fig. 8 - Flint fragment with quartz, malachite (green) and black covellite. FOV 3 cm. Los Pedreñales, Azaila.

NOTES

Cortázar, D. (1885): Bosquejo fĭsico-geoloǵico y minero de la provincia de Teruel. Boletín de la Comisión del mapa geológico de España. Impr. M. Tello, Madrid. 345 pp. https://catalog.hathitrust.org/Record/009218027 [on-line, 3/2022].

Calvo, M., Besteiro, J., Sevillano, E., Pocovi, A. (1988): Minerales de Aragón. En col. Temas, Geología. Mira Editores, Zaragoza. 152 pp.

Calvo, M., Gascón, F., Sevillano, E. (1998): “Cristalls de celestina i minerals associats a Azaila (Terol)”. Mineralogistes de Catalunya, vol. 7, núm. 2, pp. 71-78 (Catalan). Revista de Minerales, 1999, vol. 1, núm. 7, pp. 199-206 (Spanish).  http://milksci.unizar.es/miner/remetallica/celestina.pdf [on-line, 3/2022].

Calvo, M. (2018): Minerales de Aragón. Ed. Prames, Zaragoza. 463 pp.