I have seen the "raw" Amelia Albite that Dr. Charles Taylor had in his stash. It is a clear phase intergrown with an opaque colourless phase. No way I would personally choose this as a "primary" calibrator. I was always told that there was beam induced Na drift...I think a synthetic high temperature-high pressure mineral would be a better "primary" calibrator. No?
I agree with crystalgrower completely. These natural materials with their inclusions, beam instability and heterogeniety might have been the best we could do 20 or 40 years ago, but if (for example) we consider that what people call "San Carlos Olivine" in their standard collections is actually a distribution of compositions, as documented by John Fournelle, it seems to me that we are living in the "pre-enlightenment" period of microanalysis. That is, analogous to before we had a global meter and kilogram standard. This is intolerable.
We (as professional scientists) need to start a global effort to create high purity, synthetic stoichiometric standard materials in the hundreds of gram quantities, so that every SEM/TEM and EPMA lab in the world has access to primary standard materials that we *know* are exactly the same.
Only then perhaps, can we begin a "consensus" k-ratio database. I propose starting with high purity MgO, high purity Al2O3 and high purity MgAl2O4 synthetic materials grown in several hundreds gram quantities. From this we can obtain two k-ratio measurements (Mg ka in MgAl2O4/MgO and Al Ka in MgAl2O4/Al2O3) from every microanalysis lab in the world. Then we would actually be doing modern global science.
There are many other suitable materials that would qualify for such "global primary standard" status, e.g., Mg2SiO4, SiO2, etc.
Just my 2 cents.