Sussexite is a manganese borate mineral MnBO₂(OH). Crystals are monoclinic prismatic and typically fibrous in occurrence. Colour is white, pink, yellowish white with a pearly lustre. It has a Mohs hardness of 3 and a specific gravity of 3.12.

It is named after the Franklin Mining District in Sussex County, New Jersey,USA where it was first discovered in 1868.

Sussexite also occurs in France, Italy, Namibia, North Korea, South Africa, Switzerland, and the US states of Michigan, New Jersey, Utah and Virginia.

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