>I would imagine that the use of chalk is simply because of the sheer whiteness of the material.<

Very true fitz, and as with all colours (and none colours) there are also variations in 'whiteness'. I first became interested in calcium carbonate because it's the principal white pigment used in Japanese paintings and prints. Chinese paintings generally use white lead so it's quite an easy test, if you're not sure whether a painting or print is Chinese or Japanese, to run an ultraviolet lamp over the white pigment; if it fluoresces the chances are that it's calcium carbonate and therefore Japanese. I seem to recall that some washing detergents employ a similar (though probably chemically different) trick by introducing a slight blueing agent to make the wash look whiter.

I did actually take a small portable UV lamp out to Avebury once to see if the chalk on the eroded pathways at the top of the bank fluoresced at all; it was a bit of a failure because it was tipping down at the time and I was loathed to remove even a small piece of chalk for proper 'indoor' tests. The problem at Avebury is also that even the eroded paths are actually quite dirty and any sort of dirt or growth on the chalk is going to inhibit the fluorescing of the material.

I've walked along the paths of the Avebury banks on moonlit nights and they do appear quite white but that's probably due to them just being white and not to any fluorescing effect. I do find the possibility, though, that Avebury, Thornborough and Silbury may have fluoresced under certain conditions - if they did they would have appeared very, very magical indeed.

>Couple all of this with the chalk being the rock that yields the best quality flint, then we could be heading towards a sacred geology.<

Hit the nail on the head there fitz, and I'd go as far as to say that Avebury is where it is for that very reason.