Mooihoekite Cu9Fe9Si16 occurs with haycockite Cu4Fe5S8 in polished sections from the hortonolite dunite (replacement) pegmatite, Mooihoek Farm, Lydenburg District, Transvaal, South Africa. A second occurrence of mooihoekite and haycockite is in the Duluth Gabbro complex, Minnesota, U.S.A.

Under reflected light mooihoekite has the color of chalcopyrite, but it tarnishes in air and is weakly anisotropic. Indentation micro-hardness VHN50 = 261 kg/mm2. The mineral is tetragonal: a = 10.58(1), c = 5.37(1) Å from powder data, with space group P42m and Dx = 4.38 g/cm3. Synthetic mooihoekite has a 10.585(5), c 5.383(5) Å from single crystal data, Dm = 4.36 g/cm3, Dx = 4.37 g/cm3. Both powder patterns agree closely with the γ-phase of Hiller and Probsthain (1956).

aycockite has the color of chalcopyrite under reflected light; it does not tarnish easily. It is weakly to moderately anisotropic. Indentation micro-hardness VHN50 = 263 kg/mm2. The mineral is orthorhombic, pseudo-tetragonal: ab = 10.71(2), c = 31.56(6) Å from single crystal data, with Dx = 4.35 g/cm3.

The X-ray powder data and some physical properties of mooihoekite and haycockite are compared to those of chalcopyrite and talnakhite.

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