>Black Rock Phosphate" this mined product is rated at the top of the list.
>It is black in color and is shipped out, with minimal processing, in
>exactly the condition it is in when excavated, as a fine sand. PCS used to
>grind it to a powder but found that that did not speed release
The primary factor in the 'release' of rock phosphates is the amount of
carbonate they contain as a structural substitution in the
non-phosphate anionic section of the apatite-group minerals comprising
most of the rock. The more carbonate, the more easily available.
Moroccan phosphates (the best in the world) often contain 5% carbonate
(or even more), making them readily available in acid soils.
Hard-rock phosphates are typically tricalcium phosphates with no
carbonate, and as a result they take just about forever to release.
Text-based systems aren't very good for chemical formulae, but if you
take all numbers as chemical subscripts the following is the general
formula for generic apatites.
Ca2Ca3[PO4]3(OH,F,CO3) is the simple version.
For you real keeners out there, the more detailed formula is for an
apatite-group mineral called francolite, itself a common component of
many of the best phosphate rocks.
Phosphate mineralogy is an absolute blast, and if anyone wants more
information they should contact me off list.
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