  This study examines the lanthanide calcium oxyborates Ca$_4$LnO(BO$_3$)$_3$
(Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb). The reported monoclinic
structure (space group $Cm$) was confirmed using powder X-ray diffraction. The
magnetic Ln$^{3+}$ ions are situated in well-separated chains parallel to the c
axis in a quasi-one-dimensional array. Here we report the first bulk magnetic
characterisation of Ca$_4$LnO(BO$_3$)$_3$ using magnetic susceptibility
$\chi$(T) and isothermal magnetisation M(H) measurements at T $\geq$ 2 K. With
the sole exception of Ca$_4$TbO(BO$_3$)$_3$, which displays a transition at T =
3.6 K, no magnetic transitions occur above 2 K, and Curie-Weiss analysis
indicates antiferromagnetic nearest-neighbour interactions for all samples.
Calculation of the magnetic entropy change $\Delta S_m$ indicates that
Ca$_4$GdO(BO$_3$)$_3$ and Ca$_4$HoO(BO$_3$)$_3$ are viable magnetocaloric
materials at liquid helium temperatures in the high-field and low-field regimes
respectively.
