Lavendulan
| Lavendulan | |
|---|---|
Intense blue lavendulan with green cornwallite on matrix. The circlet of lavendulan is about 0.5 cm across. Locality: Dolores prospect, Pastrana, Region of Murcia, Spain.
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| General | |
| Category | Arsenate Minerals |
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Formula (repeating unit) |
NaCaCu5(AsO4)4Cl.5H2O Usually with some K, Co and Ni |
| Strunz classification | 8.DG.05 |
| Dana classification | 42.9.4.2 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | P21/n |
| Identification | |
| Formula mass | 1,062.00 g/mol |
| Color | Blue or greenish blue |
| Crystal habit | Thin botryoidal crusts of minute radiating fibers or thin rectangular, pseudo-orthorhombic plates |
| Twinning | Common |
| Cleavage | Good on {010}, distinct on {100} and {001} |
| Fracture | Uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 2½ 2½ to 3 |
| Luster | Vitreous to waxy, satiny in aggregates |
| Streak | Light blue |
| Diaphaneity | Translucent |
| Specific gravity | 3.54 3.84 |
| Optical properties | Biaxial (-), nearly uniaxial (–) |
| Refractive index |
Nx = 1.645 Ny = 1.715 Nz = 1.725 |
| Pleochroism | O = pale blue to pale greenish blue, E = blue to greenish blue |
| Solubility | Easily soluble in hydrochloric acid |
| Other characteristics | Not radioactive |
| References | |
Nx = 1.645 Ny = 1.715 Nz = 1.725
Nx = 1.660 Ny = 1.715 Nz = 1.734
Nx = 1.66 Ny = 1.715 Nz = 1.734
Lavendulan is an uncommon copper arsenate mineral, known for its characteristic intense electric blue colour. It belongs to the lavendulan group, which has four members:
Lemanskiite and lavendulan are dimorphs; they have the same formula, but different structures. Lemanskiite is tetragonal, but lavendulan is monoclinic. Lavendulan has the same structure as sampleite, and the two minerals form a series. It is the calcium analogue of zdenĕkite, and the arsenate analogue of sampleite.
Lavendulan was originally named for the lavender color of the "type" specimen, which has since been determined to be a mixture with no relationship to modern lavendulan. The mineral which is now called lavendulan is not a lavender blue color, and has no relationship to the “type” material from Annaberg. It often contains potassium, cobalt and nickel as impurities.
Although lavendulan is monoclinic, the angle β is very close to 90°, making the mineral pseudo-orthorhombic. Most references describe the lavendulan unit cell as an orthorhombic cell containing 8 formula units (Z=8) but Mindat.org describes a monoclinic unit cell with the length of the c axis halved, and only 4 formula units per unit cell (Z=4) and space group P21/n. Unit cell parameters are reported as
Lavendulan is a blue or greenish blue translucent mineral, with a vitreous to waxy luster, satiny in aggregates, and a light blue streak. It occurs as thin botryoidal crusts of minute radiating fibers or as thin rectangular, pseudo-orthorhombic plates, with cleavage in three directions, nearly perfect perpendicular to the b crystal axis, and distinct perpendicular to the a and c axes. Twinning is common. The mineral is brittle, with an uneven fracture. It is quite soft, with hardness 2.5, between gypsum and calcite, and relatively dense; its specific gravity is 3.84, close to that of topaz, and much denser than quartz (specific gravity 2.5 to 2.7). It is easily soluble in hydrochloric acid.
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Wikipedia