The docs for nalgebra say
fn new(axisangle: Vector3<N>) -> UnitQuaternion<N>[−]
Creates a new unit quaternion from the axis-angle representation of a rotation.
That's great, except, that's not an axis angle, that's just an axis. Most axis angle representations have an explicit angle and axis. One could make the assumption that the angle is encoded as the vector length, but I can;t find anything that explicitly says so.