Rare minerals from pegmatites
Part of Hall of Planet Earth.
AMNH/R.Mickens As molten granite crystallizes, the melt that remains becomes enriched in water and rare elements such as boron, lithium, tantalum, and niobium. These elements concentrate in the melt because they are not taken up by the minerals crystallizing from it. Eventually, enough water builds up that steam forms. Gem and ore minerals containing rare elements form in the pockets of steam- and water-rich melt. In the presence of water, minerals can grow to enormous sizes, forming rocks known as pegmatites.
