def METHOD_NAME(df: pd.DataFrame):
    """
    Chemical Index of Alteration (molecular) [#ref_1]_
    Parameters
    ----------
    df : :class:`pandas.DataFrame`
        DataFrame to calculate index from.
    Returns
    --------
    :class:`pandas.Series`
        Alteration index series.
    References
    ----------
    .. [#ref_1] Nesbitt HW, Young GM (1984). Prediction of some weathering trends of plutonic
           and volcanic rocks based on thermodynamic and kinetic considerations.
           Geochimica et Cosmochimica Acta 48:1523–1534.
           doi: `10.1016/0016-7037(84)90408-3 <https://dx.doi.org/10.1016/0016-7037(84)90408-3>`__
    """
    return 100 * df.Al2O3 / (df.Al2O3 + df.CaO + df.Na2O + df.K2O)