Andesine
Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is (Ca, Na)(Al, Si)4O8, where Ca/(Ca + Na) (% anorthite) is between 30 and 50%. The formula may be written as Na0.7-0.5Ca0.3-0.5Al1.3-1.5Si2.7-2.5O8.
| Andesine | |
|---|---|
Andesine | |
| General | |
| Category | plagioclase, feldspar, tectosilicate |
| Formula (repeating unit) | (Ca,Na)(Al,Si)4O8, where Ca/(Ca + Na) is between 30–50% |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (same H-M symbol) |
| Space group | P1 |
| Unit cell | a = 8.155 Å, b = 12.9 Å, c = 9.16 Å; α = 93.917°, β = 116.3333°, γ = 89.166°; Z = 8 |
| Identification | |
| Color | White, gray, green, yellow, flesh-red |
| Crystal habit | Crystals rare, to 2 cm; commonly massive or granular |
| Twinning | Common following albite, pericline, and carlsbad twin laws |
| Cleavage | Perfect on {001}, good on {010} |
| Fracture | Uneven to conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 6 – 6.5 |
| Luster | Subvitreous to pearly |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.66 – 2.68 |
| Optical properties | Biaxial (+/−) |
| Refractive index | nα = 1.543 – 1.554 nβ = 1.547 – 1.559 nγ = 1.552 – 1.562 |
| Birefringence | δ = 0.009 |
| 2V angle | Measured: 76° to 83° |
| Diagnostic features | Requires optical/chemical analysis |
| References | |
The plagioclase feldspars are a continuous solid solution series and as such the accurate identification of individual members requires detailed optical study, chemical analysis or density measurements. Refractive indices and specific gravity increase directly with calcium content.
It is sometimes used as a gemstone.
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