  Pyroxenes with the general formula $AM$Si$_2$O$_6$ ($A$ = mono- or divalent
metal, $M$ = di- or trivalent metal) are shown to be a new class of
multiferroic materials. In particular, we have found so far that
NaFeSi$_2$O$_6$ becomes ferroelectric in a magnetically ordered state below
$\simeq 6$ K. Similarly, magnetically driven ferroelectricity is also detected
in the Li homologues, LiFeSi$_2$O$_6$ ($T_C \simeq 18$ K) and LiCrSi$_2$O$_6$
($T_C \simeq 11$ K). In all these monoclinic systems the electric polarization
can be strongly modified by magnetic fields. Measurements of magnetic
susceptibility, pyroelectric current and dielectric constants (and their
dependence on magnetic field) are performed using a natural crystal of aegirine
(NaFeSi$_2$O$_6$) and synthetic crystals of LiFeSi$_2$O$_6$ and LiCrSi$_2$O$_6$
grown from melt solution. For NaFeSi$_2$O$_6$ a temperature versus magnetic
field phase diagram for NaFeSi$_2$O$_6$ is proposed. Exchange constants are
computed on the basis of {\it ab initio} band structure calculations. The
possibility of a spiral magnetic structure caused by frustration as origin of
the multiferroic behaviour is discussed. We propose that other pyroxenes may
also be multiferroic, and that the versatility of this family offers an
exceptional opportunity to study general conditions for and mechanisms of
magnetically driven ferroelectricity.
