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 both senarmontite and valentinite are oxidation products of the antimony sulfide.
Color is colorless, white or gray.
Luster is vitreous, adamantine, greasy or resinous. Crystals are translucent to transparent. Crystal System is isometric. Crystal Habits typically include well formed octahedral crystals but more commonly massive as well as granular and encrusting. Cleavage is imperfect.