Mairon and Melkor at the beginning of Arda

At the beginning of Arda.
The world is still very young, there is a lot of time before elves, but differences between Valar and Melkor are giant. He left and built himself a dark corner.
Mairon still was light, but he didn't feel hatred for Melkor. And he didn't understand why Melkor don't like the light. So he found this bright crystal and brought it to the dark Vala to show him that the light is beautiful and not at all horrible. Mairon wanted to understand why there was no peace, why everything was so against Melkor. But when he came to these dark halls, he realized that combination of brightness of a spark of light in a dense darkness is truly beautiful.
Melkor thought that the best spark would be Mairon ...

Not the canon, yep