These two nouns, although related, have very different meanings. (They are related in that they both describe coverings—things placed over other things.)
A mantel is the large piece of wood or stone that is set above a fireplace, on which people often place photographs and other sentimental objects.
Mantle, in contrast, has several meanings:
- a cloak, cape, or piece of cloth worn loosely over other clothes
- a soft layer of tissue between the shell and the body of a mollusk
- a protective casing around a fire
- a lacy cloth-like hood placed over a flame in a lamp, which shines brightly when heated
- the layer of rock within the Earth, covering the Earth’s core (below the “crust” – the part we live on)
- any loose blanket-like covering, such as a mantle of leaves on the ground
Americans sometimes write mantle when they mean mantel, but mantel is more correct for its meaning (the wooden beam above a fireplace).