Mithril bricks are the silver bricks that are required to forge the Mithril items with. There are 250 Mithril bricks found throughout the game. This will probably be the last achievement you get.
Story mode/Free Play requirements for this are:
Complete each of the 16 Story chapters (16 bricks total)
Collect all 10 minikits in each of the 16 chapters (160 minikits, 16 bricks total)
Collect one schematic in each of the 16 chapters (16 bricks total)
Collect all four treasures items in each of the 16 chapters (64 items, 16 bricks total)
Obtain Master Burglar in each of the 16 chapters (16 bricks total)
80 in Total will be earned throughout story levels and during Free Play.
The remaining 170 Mithril bricks are located in Middle-earth. Some are only available during a certain time of day. Others you need to complete fetch quests to obtain (white question marks) while others can only be obtained by completing puzzles and mini-games (timed or otherwise). Then there are some that are available outright, but require you to reach an area only accessible with a certain character or item. To see how many bricks you need, press . Use your map when needed and set your marker to navigate to them.