I was able to purchase the spell "Create Desert" without the requisite shards displayed in the "Requirements" Panel. See the screenshot below: It says I have 1 Air and 1 Earth shard available. The spell requires 1 Fire Shard to learn. When I clicked purchase, I was able to learn the spell, despite not having any fire shards available for use.

http://www.screencast.com/users/Cristofyr/folders/Jing/media/f4aba336-1409-447d-b7f1-2695202bd38d

http://www.screencast.com/users/Cristofyr/folders/Jing/media/6f889d11-efec-47c4-9401-4d222caa59cf

EDIT: O.K. This is not a bug. The shard amount displayed as a requirement is what it costs to cast the spell and I am not allowed to do so on the map - so it is technically working as intended. However, I still think this is confusing UI. The proximity of the "Requirements" of the spell to the "Purchase" button can easily lead one to believe that you must have that many shards to actually research the spell rather than use it on the adventure map.

I would prefer that the shard amount was listed directly on the spell icon itelf, maybe one icon per corner of the spell if applicable. I think this would then associate the shard amount with casting the spell rather than with the ability to purchase it.