Not sure how much truth is here, but if we forget the fireballs, ufos and orbs for a moment...this guy said that fire suddenly flared up burning with a purple flame.
He's jumped to the conclusion that salt burns this colour...don't think so, but anything containing potassium/potash does.
So, it could be Potassium Chloride.
"It occurs naturally as the mineral sylvite and in combination with sodium chloride as sylvinite."
Scrolling down to the heading "Production" shows the crystalline forms.
[link to en.wikipedia.org]
Would there be a lot of this form of salt in these domes?
This fire is 57 mls from the sinkhole.
[link to lasinkhole.wordpress.com (secure)]
I did mean to include this...a view of burning methane, the above would be adding the purple hue.
[link to www.youtube.com
Haven't been able to verify the above from any other source, so there may be nothing to it.