def eq_xtl(
    Cl, D, F,
):
    """
    eq_xtl calculates the composition of a trace element in the remaining liquid after a certain amount of
    crystallization has occured from a source melt when the crystal remeains in equilibrium with the melt
    as described by White (2013) Chapter 7 eq. 7.81. It then calculates the concentration of that trace element
    in a specific solid phase based on partition coefficient input.
        Inputs:
        Cl = concentration of trace element in original liquid
        D = bulk distribution coefficient for trace element of crystallizing assemblage
        F = fraction of melt remaining

        Returns:
        Cl_new = concentration of trace element in the remaining liquid

    """
    Cl_new = Cl / (D + F * (1 - D))
    return Cl_new