I've heard of this before. Since it would be in orbit and near weightless, it would be better built with thin film reflective mylar and aluminum tubes. It should also have some kind of focusing motors on it for beam manipulation. It needs to be aimed FIRST and then focused as a near concentrated beam would burn a track to the intended target. Focus control would also allow for intensity modulation at the target.
If it really could be made 300 miles in diameter and focused, it could melt down mountains. If used on a volcano, it could melt the solidified lava plug and set it off.
One of the Gundam TV series used something called a Sol Array which was made out of thousands of smaller rectangular mirrors and focused at the enemy. It was portrayed as being devastating to its target, but it was also very difficult to move so it was easy to destroy.
Moon nazis are well known.
