Removing potassium chloride crystals from a mineral composed of manganese dioxides
$\ce{MnO2}$ is insoluble in water -- I would expect with a good deal of confidence that the mineral cryptomelane is also insoluble. $\ce{KCl}$ by contrast is highly soluble. 

I would just treat the sample with cold water and filter out the dissolved $\ce{KCl}$ (e.g. with a Buchner funnel and filter paper). Then dry with heat.

I would choose a fine-grain filter paper though because, though insoluble, $\ce{MnO2}$ particles could still be small enough to filter through course paper, like coffee filters (from experience).