Thadeuite occurs as a massive coarse-grained yellow-orange mineral in specimens from selvages of the hydrothermal vein system at Panasqueira, Portugal. It is associated with fluorapatite, wolfeite, the OH-bearing equivalent of isokite, althausite, topaz and quartz. It is optically biaxial, negative, α = 1.568, β = 1.597, γ = 1.600, 2V = 33°, non-pleochroic. The averaged formula, normalized to a total of six cations, is Mg(Ca0.96Mn0.042+⁠)(Mg1.20Fe0.582+Mn0.242+⁠) Р1.98O7.94(OH1.54F0.46). The density is 3.25 g cm−3 (obs), and the hardness is slightly less than 4. The mineral is orthorhombic, space group C2221 with a = 6.412(3), b = 13.563(8), с = 8.545(5)A. The strongest lines in the powder pattern are: 3.61(16), 3.38(100), 3.00(31), 2.793(31), 2.626(23), 2.158(18). The name is in honor of Professor Décio Thadeu of the Instituto Superior Técnico, Lisbon, Portugal.

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