[Rockhounds] when volcanoes drop bombs: giant crystal payload

[DonH]

Tim Fisher wrote:

> . If it's sunstone it's
> automatically plagioclase, at least until some was analyzed and found to 
> me andesine lol.


This is one of those annoying artifacts of nomenclature.   "Plagioclase" 
is a series that covers albite to anorthite, in order of proportion of 
calcium to sodium in the primary cation site.  I've heard rumors that 
the trend is to do away with the names like andesine, bytownite, etc., 
and simply list the proportions; for example, "plagioclase (An90)", 
where this represents 90% calcium in the crystal.  I doubt the common 
terms will ever go away, especially since the literature is full of 
them, but I think that in formal papers the trend is to simply list the 
composition.

This list might help make more sense of it.

Names for intermediate plagioclases: In order of increasing Ca content:

     * Albite - An0 - An10 (or Ab100-Ab90)
     * Oligoclase - An10 - An30
     * Andesine - An30 - An50
     * Labradorite - An50 - An70
     * Bytownite - An70 - An90
     * Anorthite - An90 - An100

Don