Micro crystal, possibly plagioclase feldspar in obsidian
Rectangular crystal at 100X magnification showing first order white to light gray interference colors and inclined extinction. Plagioclase exhibits birefringence from 0.007 to 0.013. The crystal above would be a 0.007 on the Michel- Levy chart. Another smaller, rectangular crystal was located in the glass matrix that exhibited a birefringence of 0.012 0r 0.013. This would be the upper range of birefringence for plagioclase. Both crystals had inclined extinction, as does plagioclase. Neither crystal had any features that were distinguishable,
such as cleavage or twinning.
Micro crystal, possibly plagioclase feldspar in obsidian
Rectangular crystal at 100X magnification showing first order white to light gray interference colors and inclined extinction. Plagioclase exhibits birefringence from 0.007 to 0.013. The crystal above would be a 0.007 on the Michel- Levy chart. Another smaller, rectangular crystal was located in the glass matrix that exhibited a birefringence of 0.012 0r 0.013. This would be the upper range of birefringence for plagioclase. Both crystals had inclined extinction, as does plagioclase. Neither crystal had any features that were distinguishable,
such as cleavage or twinning.