Fluor-liddicoatite
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 Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F
- Elbaite Na(Al1.5Li1.5)Al6(BO3)3Si6O18(OH)4
- Olenite NaAl9B3Si6O27O3(OH)
| Fluor-liddicoatite | |
|---|---|
Liddicoatite from the Ambesabora pegmatite, Madagascar. Photo Rob Lavinsky | |
| General | |
| Category | Cyclosilicate Tourmaline Group |
| Formula (repeating unit) | Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F |
| IMA symbol | Fld |
| 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 | 7+1⁄2 |
| 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 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.
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