Formula: (Ca,Mn)(Mg,Fe,Mn)3(PO4)2(OH,F)2
Origin of name: for Decio Thadeu, Portugese geologist, Technical University, Lisbon, Portugal, for his studies on Portuguese ore deposits.
Crystal system: Orthorhombic
Crystal class: 222
Twinning:
Unit cell: a = 6.412, b = 13.563, c = 8.545
Colour: Yellow-orange
Diaphaneity: Translucent
Luster: Vitreous
Habit: Massive, cleavable, coarse-grained
Hardness: 3.5 to 4
Specific gravity: 3.25
Cleavage: 1; {010} good, another perpendicular
Tenacity:
Streak: White
Synonyms/varieties:
Comments: Primary mineral in the margins of hydrothermal veins, formed between 230 to 360 °C and 100-1,000 bars.
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