Matrices over orders in algebraic number fields as sums of $k$-th powers

David R. Richman proved that for $n \geq k \geq 2$ every integral $n \times n$ matrix is a sum of seven $k$-th powers. In this paper, in light of a question proposed earlier by M. Newman for the ring of integers of an algebraic number field, we obtain a discriminant criterion for every $n \times n$ matrix $(n \geq k \geq 2)$ over an order of an algebraic number field to be a sum of (seven) $k$-th powers.