Cacoxenite is a mineral that is commonly an inclusion in quartz, especially amethyst. The inclusions in amethyst often detract from the amethyst as the brown acicular needles dampen the pure purple color to a less desirable brownish hue. The inclusions of cacoxenite will certainly ruin any chance for the host stone to become a gemstone. This is not to say that some specimens of cacoxenite included quartz have no value, for indeed some are actually enhanced with interesting surreal landscaping effects.
Cacoxenite on its own is appreciated as a scarce phosphate mineral and is known from classic phosphate localities. It is often associated with other attractive and rare phosphates and can therefore represent some very nice mineral specimens. These specimens can be quite popular and attractive with a silky luster and a typical yellow-brown color.
The rather daunting formula of cacoxenite, Fe24(AlO6)(PO4)17(OH)12 - 75H2O, can be represented in a more condensed and simpler version, Fe4(PO4)3(OH)3 - 12H2O. This shorter version is what is often used in some references. The shorter version is approximately one sixth of the larger one except for the addition of one aluminum and an adjustment to the number of oxygens and water molecules. This can be demonstrated:
This change requires the loss of three oxygens in order to balance the formula due to the change in charge from one negative 3 phosphate group to the negative 9 aluminum group. Natural cacoxenite contains one aluminum group for every 17 phosphates making the larger formula more accurate, if not less cumbersome.