Carminite
| Carminite | |
|---|---|
Carminite from the Alto das Quelhas do Gestoso Mines, Gestoso, Manhouce, São Pedro do Sul, Viseu District, Portugal. Picture width 1.5 mm.
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| General | |
| Category | Arsenate minerals |
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Formula (repeating unit) |
PbFe3+2(AsO4)2(OH)2 |
| Strunz classification | 8.BH.30 (10 ed) 7/B.28-40 (8 ed) |
| Dana classification | 41 Anhydrous phosphates, arsenates and vanadates containing hydroxyl or halogen |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Cccm |
| Unit cell | a = 16.591 Å, b = 7.58 Å, c = 12.285 Å; Z = 8 |
| Identification | |
| Formula mass | 639.87 g/mol |
| Colour | Carmine red |
| Crystal habit | Typically bladed crystals, also acicular crystals, in spherical or tufted aggregates and as fibrous or drusy masses |
| Cleavage | Distinct on {110} |
| Tenacity | Brittle (D,All) |
| Mohs scale hardness | 3½ |
| Lustre | Vitreous, pearly on cleavages |
| Streak | Reddish yellow |
| Diaphaneity | Translucent |
| Specific gravity | 5.03 - 5.18 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 2.070, nβ = 2.070, nγ = 2.080 |
| Birefringence | 0.010 |
| Pleochroism | strong, X= pale yellowish red; Y=Z= dark carmine red |
| Solubility | Slowly soluble in HCl with the separation of PbCl2, and totally soluble in HNO3 |
| Other characteristics | Carminite is not radioactive. No piezoelectric effect could be detected |
| References | |
Carminite (PbFe3+2(AsO4)2(OH)2) is an anhydrous arsenate mineral containing hydroxyl. It is a rare secondary mineral that is structurally related to palermoite (Li2SrAl4(PO4)4(OH)4).Sewardite (CaFe3+2(AsO4)2(OH)2) is an analogue of carminite, with calcium in sewardite in place of the lead in carminite. Mawbyite is a dimorph (same formula, different structure) of carminite; mawbyite is monoclinic and carminite is orthorhombic. It has a molar mass of 639.87 g. It was discovered in 1850 and named for the characteristic carmine colour.
Carminite belongs to the orthorhombic crystal class (2/m 2/m 2/m) and has space group C ccm or C cc2. The structure consists of linked octahedra of iron surrounded by oxygen and hydroxyl which are aligned parallel to the c axis. They are connected together in the direction of the an axis by arsenate tetrahedra (arsenic surrounded by 4 oxygen). Coordination about the lead atoms is eight-fold. The edges of the unit cell have lengths a = 16.59 Å, b = 7.58 Å and c = 12.295 Å. There are 8 formula units in each unit cell (Z = 8).
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