def METHOD_NAME(df: pd.DataFrame):
    """
    Chemical Index of Weathering (molecular) [#ref_1]_
    Parameters
    ----------
    df : :class:`pandas.DataFrame`
        DataFrame to calculate index from.
    Returns
    --------
    :class:`pandas.Series`
        Alteration index series.
    References
    ----------
    .. [#ref_1] Harnois L (1988). The CIW index: A new chemical index of weathering.
           Sedimentary Geology 55:319–322. doi:
           `10.1016/0037-0738(88)90137-6 <https://dx.doi.org/10.1016/0037-0738(88)90137-6>`__
    """
    return 100.0 * df.Al2O3 / (df.Al2O3 + df.CaO + df.Na2O)