This week, we’re actually getting a three-for-one deal, since K-feldspar is actually just a shorthand to talk about sanidine, orthoclase, and microcline. All three minerals have the same formula (they’re “polymorphs“), but in thin section they vary a bit. Some of the properties will be the same across the board, but I’ll try and highlight the differences.
In plane light, the K-feldspars are very, very similar to quartz. Usually clear, they can alter to clays and instead be cloudy or light brown in color. The K-feldspars have a slightly lower refractive index that quartz, so while they also have low relief it can vary from slightly greater than the mounting epoxy (positive Becke Line) to slightly less than the mounting epoxy (negative Becke Line). Though the K-feldspars have cleavage, their relief is too low to usually see the planes in thin section.
In XPL, we can start to distinguish between the three types. Orthoclase & sanidine will frequently have simple twins (one side goes extinct at a different angle than the other).
In contrast, microcline will frequently have two directions of twinning that look like a cross-hatch or a tartan. (The twins are due to sanidine (highest T/P) & orthoclase (moderate P&T) changing their structure to the low T&P microcline.) Unfortunately, authigenic microcline usually only has simple or no twins, which will make it more difficult to ID.
Also, as you go from high temperatures & pressures to closer to the surface of the Earth, sanidine can start to exsolve (at higher T’s, more Na can fit in the K-feldspar structure than at lower T’s–if the sanidine slowly cools, it may separate into two different minerals: one K-rich and one Na-rich). The exsolved sanidine is called either perthite or antiperthite (former is K-rich surrounding Na-minerals, latter is Na-rich surrounding K-minerals).
(a brief search of the internet didn’t result in an antiperthite photomicrograph–if you see one, comment at the end!)
Ok, how else might we distinguish between the three? Now we get into problematic differences that are difficult to find:
- extinction angles: if you can find an elongate mineral (good luck) or a crystal that has distinct parallel twins, we can measure whether the minerals go extinct parallel to NS or EW or at an angle; microcline ranges from 0 – 15 degrees, orthoclase 0 – 12 degrees, and sanidine 0 – 5 degrees–not much of a variation
- biaxial optic axis figure: if you can find a circular section (even harder than finding an elongate xtal), all three of the minerals are biaxial negative, but they have different 2V’s: microcline is extremely difficult because of the twins, but is usually >65 degrees; orthoclase is normally greater than 40; and sanidine may appear uniaxial due to its very low 2V (<40 degrees)
(If you don’t know what an optic axis figure is, I’ll cover that in two weeks!)
What is K-feldspar usually confused with? Actually, this is the same exact list as quartz, so just look back :)
Next week? Halite… not exactly a common mineral to examine with the microscope.