A. ALKALI FELDSPARS
1. SANIDINE (K, Na)AlSi3O8 BH-250-15
Sanidine is a common constituent of silicic, potassium-rich volcanic rocks.
It is a high temperature form of Orthoclase. It shows the feldspar cleavage
{001} perfect and {010} distinct, intersecting at 90°, has R.I. values (1.52-1.53),
low 2V (0-12o), maximum extinction angle 5° to {001} cleavages and simple,
non-lamellar Carlsbad twinning (or it may be untwinned).
2. ORTHOCLASE (K,Na)AlSi3O8 (BH-250-13)
Orthoclase is a common potassium feldspar in silicic plutonic and high grade
metamorphic rocks and is also present in detrital sedimentary rocks. It lacks
the lamellar albite twinning of plagioclase and the grid twinning of microcline,
having instead the simple twinning like sanidine. Its 2V is higher than sanidine
and unlike the clear sanidine crystals orthoclase is often clouded by clay and
other alteration products.
Look at the orthoclase in the orthoclase slides (all granitic rocks) and become
familiar with this feldspar.
3. MICROCLINE (K,Na)AlSi3O8 (BH-250-13)
Microcline is a K-feldspar that displays "grid" or "tartan"
twinning or double set of polysynthetic twins. Microcline is common in silicic
plutonic and high grade metamorphic rocks. Its composition lies between sanidine
and albite and it is twinned by albite and pericline twin laws. Re-examine Leucite (1,2,3)
slides and see how the twinning in microcline is different from that of leucite?
4. PERTHITE
Perthite is an intergrowth of albite in a host of orthoclase or microcline.
It can apparently form either by replacement or exsolution. If the perthitic
structure is only visible with a microscope it is termed microperthite. If the
structure is finer than can be resolved with a microscope it is termed cryptoperthite.
An intergrowth of orthoclase or microcline in plagioclase is termed antiperthite.
