- journal_title：American Mineralogist
- Contributor：Stuart J. Mills ; Giovanni Ferraris ; Anthony R. Kampf ; Georges Favreau
- Publisher：Mineralogical Society of America
- Date：2012-
- Format：text/html
- Language：en
- Identifier：10.2138/am.2012.3984
- journal_abbrev：American Mineralogist
- issn：0003-004X
- volume：97
- issue：2-3
- firstpage：415
- section：Articles
We describe the first documented case of {101̄0} twinning by reflection (or by twofold rotation about [100]) or merohedry (class II) in a member of the apatite supergroup. Twinning about [100] had previously been noted for the apatite supergroup but not confirmed. Pyromorphite crystals from Puech de Compolibat, Combret, Aveyron, France, were studied by single-crystal X-ray diffraction [a = 10.0017(19), c = 7.3413(16) Å, and V = 636.0(2) Å3, in P63/m], where twinning was confirmed with the approximate twin fraction 62:38. Subsequent inspection of the morphology confirmed the nature of the twinning. The pyromorphite crystals are typically elongate and show the faces: (21̄1̄0), (2̄110), (0001), (0001̄), (101̄0), (1̄010), (101̄2̄), and (1̄012̄).