def convert_composition_dict_to_string( d ):
    """
    Converts a composition dict of stype composition['CH4'] = 0.02,
    into a string like "CH4: 0.02"
    """
    X = ""
    for item in d:
        if d[item] > 0.0:
            X += "%s:%0.16f"%(item,d[item])
            X += ", "
    X=X[:-2]
    return X