The airplane entered an uncontrolled descent and impacted terrain at about 1500 Mountain time, killing the Student pilot, its sole occupant. The airplane was destroyed. Visual conditions prevailed. The Student pilot was flying her third solo flight of the flight training syllabus. The student was to depart, go to the Buckeye Airport, perform at least one landing, taxi back, and takeoff, then proceed to a practice area south of the departure airport, perform maneuvers and return.