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We present the variability study of broad absorption lines (BALs) in auniformly radio-selected sample of 28 BAL quasars using the archival data fromthe FIRST Bright Quasar Survey (FBQS) and the Sloan Digital Sky Survey (SDSS),as well as those obtained by ourselves, covering time scales $\sim 1-10$ yearsin the quasar's rest-frame. To our surprise, 5 quasars showing strongvariations are all belong to a special subclass of overlapping iron lowionization BAL (OFeLoBAL) quasars, however, other 4 non-overlapping FeLoBALs(non-OFeLoBALs) are invariable except one case with weak optical depth change.Meanwhile, we also find 6 typical variations of high-ionization andlow-ionization BALs in this BAL quasar sample. Photoionization models suggestthat OFeLoBALs are formed in a relative dense ($n_e>10^6$ cm$^{-3}$) outflowsat a distance from the subparsec to the dozens of parsecs from the continuumsource. They differ from those of non-OFeLoBALs, which are likely produced bylow-density gas, locating at a distance of hundreds to thousands parsecs. Thus,OFeLoBALs and non-OFeLoBALs, i.e., FeLoBALs with/without strong BAL variations,perhaps represent the bimodality of Fe II absorption, the former is located inthe active galactic nucleus environment rather than the host galaxy. We suggestthat high density and small distance are the necessary conditions that causeOFeLoBALs. As suggested in the literature, strong BAL variability is possiblydue to variability of the covering factor of BAL regions caused by cloudstransiting across the line of sight rather than ionization variations.

Strong Variability of Overlapping Iron Broad Absorption Lines in Five Radio-selected Quasars