  The magnetic and lattice properties of the $S$=1/2 quantum-spin-chain
ferromagnet, CuAs$_2$O$_4$, mineral name trippkeite, were investigated. The
crystal structure of CuAs$_2$O$_4$ is characterized by the presence of
corrugated CuO$_2$ ribbon chains. Measurements of the magnetic susceptibility,
heat capacity, electron paramagnetic resonance and Raman spectroscopy were
performed. Our experiments conclusively show that a ferromagnetic transition
occurs at $\sim$7.4 K. $\textit{Ab initio}$ DFT calculations reveal dominant
ferromagnetic nearest-neighbor and weaker antiferromagnetic next-
nearest-neighbor spin exchange interactions along the ribbon chains. The ratio
of $J_{\rm nn}$/$J_{\rm nnn}$ is near -4, placing CuAs$_2$O$_4$ in close
proximity to a quantum critical point in the $J_{\rm nn}$ - $J_{\rm nnn}$ phase
diagram. TMRG simulations used to analyze the magnetic susceptibility confirm
this ratio. Single-crystal magnetization measurements indicate that a magnetic
anisotropy forces the Cu$^{2+}$ spins to lie in an easy plane perpendicular to
the $c$-axis. An analysis of the field and temperature dependent magnetization
by modified Arrott plots reveals a 3d-XY critical behavior. Lattice
perturbations induced by quasi-hydrostatic pressure and temperature were mapped
via magnetization and Raman spectroscopy.
