Of all common crustal rocks, only shales and other clay-rich rocks have Al/Si ratios that are elevated relative to average crustal ratios, and sufficiently enriched in Al, to stabilize the alumino-silicate minerals.
(only shales and other clay-rich rocks; have; Al/Si ratios) (Al/Si ratios; are elevated relative; )
only shales and other clay-rich rocks Al/Si ratios
have are elevated relative
Al/Si ratios