Lightning storms can occur with volcanic eruptions (first link). There are multiple theories as to how the charge separation which leads to the lightning strike takes place (second link). Some of these processes operate only in conventional thunderstorms, but others occur in volcanic ash clouds. There is enough friction between the countless ash particles to generate a powerful static charge.
Source(s):
http://www.livescience.com/environment/090408-volcano-lightning.html
http://enviromom.us/lightning/lightning%20formation.htm

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