  Crystal and magnetic structures of the mineral centennialite
CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O are investigated by means of synchrotron
x-ray diffraction and neutron diffraction measurements complemented by density
functional theory (DFT) and pseudofermion functional renormalization group
(PFFRG) calculations. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O crystallizes in the
$P\bar{3}m1$ space group and Cu$^{2+}$ ions form a geometrically perfect kagome
network with antiferromagnetic $J_1$. No intersite disorder between Cu$^{2+}$
and Ca$^{2+}$ ions is detected. CaCu$_3$(OH)$_6$Cl$_2\cdot0.6$H$_2$O enters a
magnetic long-range ordered state below $T_\text{N}=7.2$~K, and the
$\mathbf{q}=\mathbf{0}$ magnetic structure with negative vector spin chirality
is obtained. The ordered moment at 0.3~K is suppressed to
$0.58(2)\mu_\text{B}$. Our DFT calculations indicate the presence of
antiferromagnetic $J_2$ and ferromagnetic $J_d$ superexchange couplings of a
strength which places the system at the crossroads of three magnetic orders (at
the classical level) and a spin-$\frac{1}{2}$ PFFRG analysis shows a dominance
of $\mathbf{q}=\mathbf{0}$ type magnetic correlations, consistent with and
indicating proximity to the observed $\mathbf{q}=\mathbf{0}$ spin structure.
The results suggest that this material is located close to a quantum critical
point and is a good realization of a $J_1$-$J_2$-$J_d$ kagome antiferromagnet.
