Schmiederite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Lead copper selenide selenate hydrate
Chemical Formula: Pb2Cu2(Se+3O3)(Se+6O4)(OH)4
Method(s) of Verification: Llechweddhelyg Mine – XRD & EMPA at the NHM (Rust, 1990); Frongoch Mine – XRD & EMPA (Green et al., 1996) substantial selenium is recorded, but exact proportions are unsure; Esgair Hir Mine - XRD (NHM, 7157F).

Chemical Group:

  • Selenites, Selenates, Tellurites and Tellurates

Geological Context:

  • Supergene: post-mining oxidation & weathering deposits
Introduction: schmiederite is a secondary mineral found in the oxidized zone of lead- and copper-bearing ore bodies. It has a similar X-ray diffraction pattern to linarite, with which it forms a series, therefore electron microprobe analysis is required for definitive identification.
Occurrence in Wales: schmiederite has been reported from a number of mines in the Central Wales Orefield and from Machen Quarry in South Wales, but in many instances the actual mineral is linarite which forms a series with schmiederite and which is similar in appearance and crystal structure. Llechweddhelyg Mine represents the only fully verified locality. Re-examination of specimens and X-ray films from Eaglebrook Mine, Dyfngwm Mine and Machen Quarry suggest that linarite is more likely.

Key Localities:

  • Esgairhir Mine, Tal-y-bont, Ceredigion: samples produce a similar X-ray pattern to that from Llechweddhelyg Mine, but no chemical analyses have been performed.
  • Frongoch Mine, Devil's Bridge, Ceredigion: mid-blue spherules similar to those from Llechweddhelyg Mine occur within the post-mining suite. Green et al. (1996) describe the presence of substantial selenium within linarite-type specimens placing them in the linarite-schmiederite solid solution, although no quantitative analyses have been made.
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: minute, mid-blue spherules sometimes forming aggregates, are a sulphatian variety of schmeiderite (Rust, 1990).

References:

  1. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  2. Rust, S.A., 1990b. Susannite and sulphatian schmeiderite from Llechwedd Helyg Mine, Tir-y-Mynach, Dyfed, Wales. U.K. Journal of Mines and Minerals, 8, 48