I read this book for work a long time ago.. Actually, I reviewed it for work, I think we got a free copy. The book was OK, but on page 17 there was a a picture of a bunch of commuter bikes, and the caption claimed "These bicycle frames are made from two of the lightest metals--magnesium and aluminum." They look like cheap steel frames to me.
Magnesium is the 12th element on the periodic table. Its phase is a solid and has an atomic mass of 24.30 amu. It also has a density of 1.74 g/cm. Magnesium was discovered by Joseph Black in 1755.