Fluor-liddicoatite
| Fluor-liddicoatite | |
|---|---|
Liddicoatite from the Ambesabora pegmatite, Madagascar. Photo Rob Lavinsky
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| General | |
| Category | Cyclosilicate Tourmaline Group |
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Formula (repeating unit) |
Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F |
| Strunz classification | 9.CK.05 (10 ed) 8/E.19-80 (8 ed) |
| Dana classification | 61.3.1.2 |
| Crystal system | Trigonal |
| Crystal class | Ditrigonal pyramidal (3m) (same H-M symbol) |
| Space group | R3m |
| Identification | |
| Formula mass | 945.8 g/mol |
| Color | Usually smoky brown, but also pink, red, green, blue, or rarely white. |
| Crystal habit | Stout prismatic, with a curved convex trigonal outline |
| Cleavage | Poor or absent on {0001} |
| Fracture | Uneven to conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 7½ |
| Luster | Vitreous |
| Streak | White to very light brown |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.02 |
| Optical properties | Uniaxial (-) |
| Refractive index | No = 1.637, Ne = 1.621 |
| Pleochroism | Strong: O dark brown or pink, E light brown or pale pink |
| Other characteristics | Not fluorescent, not radioactive |
| References | |
Fluor-liddicoatite is a rare member of the tourmaline group of minerals, elbaite subgroup, and the theoretical calcium endmember of the elbaite-fluor-liddicoatite series; the pure end-member has not yet been found in nature. Fluor-liddicoatite is indistinguishable from elbaite by X-ray diffraction techniques. It forms a series with elbaite and probably also with olenite. Liddiocoatite is currently a non-approved mineral name, but Aurisicchio et al. (1999) and Breaks et al. (2008) found OH-dominant species. Formulae are
Fluor-liddicoatite was named in 1977 after Richard T. Liddicoat (1918–2002) gemmologist and president of the Gemological Institute of America, who is well known for introducing the GIA diamond grading system in 1953.
Fluor-liddicoatite belongs to the trigonal crystal system, class 3 m, space group R 3m. It has a rhombohedral lattice, with unit cell parameters
Fluor-liddicoatite is isostructural with (has the same structure as) all members of the tourmaline group, which are cyclosilicates with the general formula
For fluor-liddicoatite, the X sites are occupied by Ca, the Y sites by Li or Al and the Z sites by Al, giving the formula
The Y sites are octahedrally coordinated by oxygen O and hydroxyl OH ions; three octahedra surround the three-fold axis at the origin, and each octahedron shares an edge with each of its two nearest neighbours. The silicon Si ions are tetrahedrally coordinated by O, forming SiO4 groups. These tetrahedra form six-membered rings, with two of the four Os in each tetrahedron shared between adjacent tetrahedra. So the formula for the ring is Si6O18. In each Si tetrahedron an O at one free apex is shared with one of the Y octahedra. The boron B ions occur in triangular coordination, each triangle sharing a common apex with two Y octahedra. This composite unit is linked to others like it by aluminum Al ions at the Z sites, and its outer oxygen atoms are also atoms of the aluminum coordination octahedra. The X sites are sandwiched between the units along the c axis.
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