Kësterite

Well-documented kësterite on matrix from the classic Gunheath Pit, St Austell granite district, Cornwall, United Kingdom. The specimen shows dark grey to steel-grey, metallic, fine-grained kësterite aggregates disseminated through a light quartz–feldspar pegmatitic matrix, typical for Cornish tin-bearing deposits.

Kësterite is a rare tin–copper–iron sulfide, belonging to the sulfide mineral group and structurally related to stannite. It typically forms in hydrothermal tin deposits, often as a late-stage phase in granitic and greisen-related systems. Cornwall is one of the historically important regions for tin mineralization in Europe, and Gunheath Pit represents a classic locality for tin sulfides.

The mineral formed from hydrothermal fluids associated with granite-related mineralization, precipitating sulfides in fractures and replacement zones within the host rock. Kësterite is frequently overlooked due to its similarity to other tin sulfides, making confirmed specimens from well-documented localities especially desirable for advanced collectors.

Material from Gunheath Pit was mainly recovered during the 20th century, during and after active quarrying associated with china clay and tin mining in the St Austell area.

Kësterite is stable under normal collection conditions, requiring no special storage beyond standard care for sulfide minerals.

At Gunheath Pit, kësterite is commonly associated with quartz, feldspar, cassiterite, stannite, chalcopyrite, and other tin-bearing sulfides.