Both minerals have the exact same chemistry, but they have different structures. Senarmontite is isometric and valentinite is orthorhombic . It is similar to the situation between diamond and graphite . Valentinite is associated with senarmontite as well as stibnite . Actually this association is no surprise since both senarmontite and valentinite are oxidation products of the antimony sulfide.
Color is colorless, white, yellow, reddish or gray.
Luster is adamantine or pearly. Crystals are translucent to opaque in more massive specimens. Crystal System is orthorhombic. Crystal Habits include prismatic to fibrous and compact aggregates forming tufts and radiating sprays. Also massive and granular.