Barquillite

Chemical formula: Cu₁+₂(Cd₂+,Fe₂+)Ge₄+S₂-₄

Barquillite is a rare, metallic sulfide of copper, cadmium, iron, and germanium, occurring as microscopic grains in zinc deposits.

## Characteristics Barquillite is a very rare mineral from the stannite group, belonging to the sulfide class. It occurs in the form of anhedral (irregular) grains not exceeding 200 micrometers in size, most often intergrown with other minerals. It is usually dispersed within a sphalerite matrix. Due to the microscopic size of its crystals, visual characteristics observable with the naked eye cannot be determined; it is observed only under a microscope in reflected light, where it has a grayish color with a slight, creamy tint. ## Physical Properties As an opaque mineral with a metallic luster, barquillite is characterized by a high density of approximately 4.89 g/cm³. Its Mohs hardness is estimated at 4. It exhibits no cleavage, and its fracture is uneven. No luminescence has been observed. ## Colors and Varieties Observed under a microscope in reflected light, barquillite is gray with a creamy tint. Some sources mention a greenish tint when compared to co-occurring sphalerite. No varieties of this mineral have been distinguished. ## History and Name The mineral was discovered in the "Fuentes Villanas" mine near the village of Barquilla in Salamanca province, Spain. Its name is derived directly from this location. It was described and approved as a new mineral species by the International Mineralogical Association (IMA) in 1997. The discovery was made by A. Murciego, I.S. Oen, E. Pascual, and J.L. Martín-Izard.

Properties

Mohs hardness: 4
Luster: Metallic
Streak: Black
Density: 4.89
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of barquillite is possible only through advanced laboratory techniques. In reflected light under a polarizing microscope, it is weakly anisotropic. Crucial for identification is performing chemical composition analysis using X-ray microanalysis (EDS/WDS), which confirms the presence of copper, cadmium, iron, germanium, and sulfur in appropriate proportions. X-ray diffraction (XRD) allows confirmation of its crystal structure. ## Differentiation from Similar Minerals Under the microscope, barquillite can be confused with other sulfides from the stannite group, such as kesterite, stannite, or briartite, which have a similar appearance and often co-occur with it. Definitive differentiation requires precise chemical analysis with an electron microprobe. ## Crystal Forms Barquillite forms exclusively microscopic, anhedral (irregular) grains, typically 20 to 200 micrometers in size. It occurs as inclusions or intergrown with other minerals, mainly sphalerite.

Geological environment

## Genesis Barquillite forms under hydrothermal conditions. In its type locality in Spain, it is associated with zinc-germanium mineralization in Cambrian dolomites. Its crystallization occurred at relatively low temperatures, below 200°C, in an environment rich in sulfur and metals. ## Mineral Associations This mineral most commonly co-occurs with sphalerite (especially its germanium-rich variety), galena, pyrite, chalcopyrite, stannite, briartite, renierite, marcasite, as well as dolomite and quartz. ## Localities The only confirmed and thoroughly described occurrence of barquillite in the world is the "Fuentes Villanas" mine near Barquilla in Salamanca province, Castile and León region, Spain. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its microscopic nature, barquillite does not form specimens in the traditional collector's sense. Its value is purely scientific and primarily interests specialized institutions and "micromineral" collectors. Any properly documented and analyzed fragment of host rock (most often sphalerite) with confirmed barquillite presence is considered valuable. The quality of the specimen in this case is synonymous with the quality of the research confirming its identity.

Care and storage

## Cleaning Due to its microscopic nature and occurrence as inclusions within other minerals, individual barquillite specimens are not subject to cleaning. Any attempts to clean the host rock should be very careful and limited to mechanical dust removal with a soft brush. ## What to Avoid Avoid using any chemical agents, acids, ultrasonics, or sudden temperature changes, which could damage both barquillite itself and the surrounding minerals, especially sphalerite. ## Storage Specimens containing barquillite should be stored under stable conditions, away from moisture and chemical contaminants. It is best to keep them in closed collector's boxes to protect them from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)