Gordon introduced a class of matroids $M(n)$, for prime $n\ge 2$, such that $M(n)$ is algebraically representable, but only in characteristic $n$. Lindström proved that $M(n)$ for general $n\ge 2$ is not algebraically representable if $n>2$ is an even number, and he conjectured that if $n$ is a composite number it is not algebraically representable. We introduce a new kind of matroid called {\it harmonic matroids}, of which full algebraic matroids are an example. We prove the conjecture in this more general case.