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Brochantite, Cu₄SO₄(OH)₆

Dipartimento di Scienze della Terra, Via S.Maria 53,I–56126 Pisa, Italy

^* Email: merlino{at}dst.unipi.it

^** corresponding author: natale{at}dst.unipi.it

Single-crystal X-ray diffraction studies of brochantite demonstrate its Order-Disorder (OD) character. The OD structures of the brochantite family can be described as built up by equivalent OD layers with symmetry Pn2₁m. Two MDO polytypes are possible in this family, and both have been identified in specimens of brochantite from various localities.

The MDO₁ polytype corresponds to "normal" brochantite P12₁/a₁, a = 13.140(2) Å, b = 9.863(2), c = 6.024(1), β = 103.16(3)°, whereas the newly discovered MDO₂ polytype is monoclinic, P2₁/n11, a = 12.776(2) Å, b = 9.869(2), c = 6.026(1), = 90.15(3)°. Given their polytypic relationships, the two MDO₁ and MDO₂ polytypes can be designated as brochantite-2M₁ and brochantite-2M₂, respectively.

The refined structure (R₁ = 0.049) of brochantite-2M₁ substantially agrees with the model of Cocco & Mazzi (1959) and confirms the assignment of the OH bands due to Schmidt & Lutz (1993). On the basis of the OD relationships between the two polytypes, a structural model for the newly discovered 2M₂ polytype was derived and subsequently refined up to R₁ = 0.062.

Key-words: brochantite, OD structures, 2M₁ and 2M₂ polytypes, structure refinement..

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