Oligoclase, a rock-forming mineral, is a member of the plagioclase feldspars. It has intermediate characteristics between albite and anorthite. The mineral was named by August Breithaupt in 1826 after the Greek words ‘oligos’ and ‘clasein’ which mean little and to break, respectively.
Properties of Oligoclase
The following are the key properties of Oligoclase:
- Cell Data
- Space Group: C1 (low)
- a = 8.152
- b = 12.821
- c = 7.139
- α = 93.99°
- β = 116.46°
- γ = 88.58°
- Z = 4
- Space Group: C1 (high)
- a = 8.163
- b = 12.875
- c = 7.107
- α = 93.39°
- β = 116.27°
- γ = 90.29°
- Z = 4
- Crystal Data
- Triclinic
- Point Group: 1
- Crystals uncommon, flattened and tabular along [010], to 6 cm
- More commonly cleavable to compact, massive
- Twinning: Typically twinned according to the Albite, Carlsbad, and Pericline laws.
- X-ray powder pattern: 3.18 (100), 4.03 (80), 3.20 (80), 3.76 (70), 2.93 (70), 6.38 (60), 3.69 (60)
- Chemical Composition
| Elements |
Content 1 |
Content 2 |
Content 3 |
| SiO2 |
64.92 |
66.04 |
60.76 |
| Al2O3 |
22.20 |
21.26 |
24.82 |
| Na2O |
9.72 |
10.57 |
8.12 |
| CaO |
2.64 |
2.13 |
6.30 |
| K2O |
0.68 |
- |
- |
| H2O |
0.09 |
- |
- |
| Total |
100.25 |
100.00 |
100.00 |
- Optical Properties
- Optical Class: Biaxial (+) or (-)
- Dispersion: r > v; weak
- α= 1.533-1.545
- β= 1.537-1.548
- γ= 1.542-1.552
- 2V(meas.) = 84° to -87° to 90°(low); -52° to -73° (high).
- Estimated Properties
| Electron density |
Bulk density (electron density)=2.63 g/cm3
note: Specific gravity of Oligoclase =2.65 g/cm3 |
| Photoelectric |
PEOligoclase = 1.98 barns/electron
U= PEOligoclase x ρElectron density= 5.19 barns/cm3 |
| Fermion index |
Fermion index = 0.01
Boson index = 0.99 |
Radioactivity
|
Oligoclase is not radioactive
|
How to Identify Oligoclase
Oligoclase can be identified in greenish, gray, yellowish, brown or colorless forms. This mineral has transparent to translucent appearance. It is a fluorescent mineral with a vitreous lustre, a white streak and perfect cleavages. The fractures on the flat surfaces are uneven. Its granular crystals appear as anhedral to subhedral crystals in a matrix. An excellent external pattern is developed with the well-formed euhedral crystals. The oligoclase has an average density of 2.65 g/cm3 and relative hardness of 7.
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A cluster of tabular Oligoclase crystals. Image source:http://en.wikipedia.org/wiki/File:Oligoclase-4jg47a.jpg Original Image: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0
Global Distribution
Oligoclase is distributed in the following places:
- Danviken, near Stockholm, Sweden
- Arendal, on TromÄoy Island at Alve, Tvedestrand
- Ytterby at Sweden
- KemiÄo (Kimito) Island, Finland
- Silberberg, near Bodenmais, Bavaria, Germany
- Tenerife, Canary Islands
- Fine and Macomb, St. Lawrence Co., NewYork
- Corundum Hill, Chester Co
- Media, Delaware Co., Pennsylvania
- Bakersville east, near Hawk, Mitchell Co., North Carolina
- Lake Harbour, Baffin Island, Northwest Territories, Canada.
Occurrence of Oligoclase and Useful Mineral Association
Oligoclase occurs in amphibolite facies metamorphic rocks, gneiss, serpentinite, andesite, rhyolite, sedimentary rocks as clastic grains, syenite, granite and nepheline syenite. It is closely associated with corundum, tourmaline, sanidine, orthoclase and quartz.
References
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