Cornwallite, conicalcite and other minerals from the Reconquistada prospect, Dolores mine, Pastrana, Mazarrón, Murcia

The “Reconquistada” claim is made up of small extractive mining works near the “Dolores” iron mine, in the municipality of Pastrana (Murcia, Spain). In this area there are various exploitations that benefited copper ores. In the case of the “Reconquistada” mineral species, the presence of arsenic in the mineral composition made copper extraction difficult.

From a scientific point of view, these mining works have been important due to the numerous species found. Three of them were identified for the first time worldwide in this mines: barahonaite-(Fe), barahonaite-(Al) and cobaltarthurite (Jambor et al., 2002; Viñals et al., 2008).

But there are other interesting and very colourful minerals. Two of them are very similar in appearance, but distinguishable under magnification: cornwallite and conichalcite. Both are copper arsenates, but conichalcite contains calcium. Cornwallite appears as spherical globules, with a smooth and very brilliant surface, usually with a lighter green color than conichalcite. Conichalcite tends to have a rough surface and a more intense colour.

In this specimen of the photograph, various botryoidal aggregates of conichalcite could be observed, with the characteristics described before, together with an isolated sphere of cornwallite. They are disposed on a matrix with yukonite (an iron and calcium arsenate).

Globular aggregates of conichalcite with internal radiated crystals

Zeolites from La Crosa de Sant Dalmai volcano

La Crosa volcano, also called “La Crosa de Sant Dalmai”, shares the space between the Catalan municipality of Vilobí d’Onyar (in the lands of Sant Dalmai) and those of Bescanó and Aiguaviva. Located in the NE of Catalonia, in Girona.

It is a wide volcanic maar or crater (over 1.200 m, one of the largest in Europe) and low altitude (156 meters above sea level) formed by a phreatomagmatic eruption, produced by an explosion caused by groundwater in contact with ascending magma from the main chamber. The violent explosions have formed a ring of pyroclastic projections, formed both by juvenile materials (of basaltic composition, chalk, tuff and some volcanic bombs) and by fragments of subterranean rocks (igneous and metamorphic fragments). The phreatomagmatic origin of the Crosa maar differentiates it from the rest of the volcanoes in the area, in which strombolian eruptions predominate. It must be said, however, that within this phreatomagmatic volcano we find a smaller cone of the Strombolian type.

The eruptive period of La Crosa is between 7.9 and 1.7 million years (My), characteristic of the volcanic processes of the La Selva depression. The oldest in the Empordà region date from 12 to 8 My and the most recent, from La Garrotxa, are between 500,000 and 10,000 years old.

Figure 1. View of La Crosa de Sant Dalmai volcano, from the Can Guilloteres viewpoint.

Awaruite, a controversial and almost extraterrestrial mineral

Awaruite is a rare terrestrial nickel-iron alloy, with the formula Ni3Fe, which was found, in the form of nuggets, in black sands and of great specific weight in the Gorge River, on the West Coast of New Zealand.

Font: Google maps

The species was determined by W. Skey, a researcher of the New Zealand Geological Survey Office, in 1885. His analytical results indicated it was this rare alloy, with an element composition more typical of meteorites than terrestrial rocks. The samples those Skey studied, came from a collection of minerals sent by the Warden of the Jackson’s Bay District (Okahu in Māori), also on the West Coast of New Zealand.

Unfortunately, Skey and his director J. Hector did not know the exact location of the founding of these black sands. The information accompanying samples indicated that they had been brought by placer gold prospectors from Barn Bay.

They gave it the name of Awaruite after Awarua Bay (Māori name for Big Bay), where the Awarua River flows out, thinking that this was the correct locality. This mineral has never been found in that bay. All samples came from the Gorge River area, about 15 km north of Big Bay and about 20 km south of Barn Bay.

View of the Gorge River mouth, West Coast, New Zealand. © Long family, Gorge River. (Font)

Sideronatrite from Cabo de Palos (Cartagena, Murcia), a reversible mineral

In the easternmost part of the Murcia region we find the well-known Spanish tourist locality of La Manga del Mar Menor. This inland sea closes to the south by Cabo de Palos, in the municipality of Cartagena.

The hills those define it are found within the Regional Park of Calblanque, Monte de las Cenizas and Peña del Águila. This particular area contains the remains of mining works that began with the Punic period, continued during the Roman Empire and continued until the 20th century.

On one of its highest hills, El Talayón (178 m), we found the “Ferruginosa” mine. This mine has been extensively studied by Morales and Rosell (Morales et al., 2020; Rosell et al. 2017,) due to its particular mineral paragenesis, where chlorides and sulphates stand out.

One of these sulphates is Sideronatrite. This hydrated sodium and iron sulphate, Na2Fe(SO4)2(OH)·3H2O, is found in arid climates. It is found as efflorescences formed by small crystals, of an intense yellow colour, on the walls of the mine galleries. Lanceolated, transparent crystals with a very esthetic orange-yellow colour are observed under magnification.

Dogtooth Calcite

Calcite (CaCO3) is a mineral found in many different crystal forms. Dogtooth Calcite are calcite crystals that grow in a certain crystal form. This shape is long, pointed and resembles a dog's canines. This crystal shape comes in a wide variety of sizes, and of course the small ones are more common than the big ones. The growth of very large crystals, also called Dogtooth Spars, takes an extremely long time in undisturbed, calcite-rich water. Dogtooth Calcite crystals are quite common. Normally they are quite small, say a few millimeters.

In the photos we are introduced to the fluorescent properties of this mineral. Long pass fluorescence filter blocks were used for the exposure of the mineral sample: DAPI 375 nm excitation wavelength (UV) and MB 480 nm excitation wavelength (blue light).

Seeing the effect of radiation damage

The rock matrix contains quartz, plagioclase feldspar, microcline feldspar and biotite mica. The greenish brown flake of biotite shown in the centre exhibits many dark brown circular features resembling cigarette-type burns. These features are known as pleochroic halos and are a diagnostic feature of biotite. These halos are interesting as they are isotropic compared to the surrounding material which is anisotropic. The halo in the centre of field shows a small brightly coloured crystal of zircon which causes the dark brown effect due to a process called metamictization. This process is the radioactive decay and destruction of the crystal structure surrounding the zircon and the affected area becomes amorphous and hence isotropic.

Old but indispensable

A polarization microscope can be used to identify the mineralogical composition of geological materials in order to help reveal their origin and evolution. Some of the properties and techniques used include: refractive index, birefringence, Michel-Lévy interference color chart, extinction angle, conoscopic interference pattern (interference figure), Becke line test, wave plate etc.