Bytownite
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CategoryTectosilicate minerals
Formula(Ca,Na)[Al(Al,Si)Si2O8] (Na:Ca = 10:90 to 30:70)
| Bytownite | |
|---|---|
 A somewhat rounded, alluvial crystal of bytownite from the Dorado Mine, Casas Grandes, Chihuahua, Mexico (size 3.7 × 2 × 1.3 cm) | |
| General | |
| Category | Tectosilicate minerals |
| Group | Feldspar group |
| Series | Plagioclase feldspar series |
| Formula | (Ca,Na)[Al(Al,Si)Si2O8] (Na:Ca = 10:90 to 30:70) |
| IMA status | Variety of anorthite |
| Strunz classification | 9.FA.35 |
| Dana classification | 76.01.03.05 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) |
| Space group | P1 (no. 2) |
| Unit cell | a = 8.178 Å, b = 12.870 Å, c = 14.187 Å; α = 93:5°, β = 115:9°, γ = 90:63°; Z = 8 |
| Identification | |
| Color | Colorless, white, gray |
| Crystal habit | Rarely as crystals flattened on [010], commonly as cleavable masses or anhedral grains in massive aggregates |
| Twinning | Common Albite, Carlsbad, and Pericline twinning |
| Cleavage | Perfect on [001], good on [010], imperfect on [110] |
| Fracture | Uneven to conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 6 – 6.5 |
| Luster | Vitreous, pearly on cleavages |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.72 – 2.74 |
| Optical properties | Biaxial (+/−) |
| Refractive index | nα = 1.563 – 1.572 nβ = 1.568 – 1.578 nγ = 1.573 – 1.583 |
| Birefringence | δ = 0.010 – 0.011 |
| 2V angle | Measured: 86°, Calculated: 80° to 88° |
| Dispersion | r > v strong |
| References | [1][2][3] |
Bytownite is a calcium rich member of the plagioclase solid solution series of feldspar minerals with composition between anorthite and labradorite. It is usually defined as having between 70 and 90%An (formula: (Ca0.7−0.9Na0.3−0.1)[Al(Al,Si)Si2O8]). Like others of the series, bytownite forms grey to white triclinic crystals commonly exhibiting the typical plagioclase twinning and associated fine striations.
The specific gravity of bytownite varies between 2.74 and 2.75. The refractive indices ranges are nα=1.563 – 1.572, nβ=1.568 – 1.578, and nγ=1.573 – 1.583. Precise determination of these two properties with chemical, X-ray diffraction, or petrographic analysis are required for identification.
