THE ACFT DEPARTED DURING A BREAK IN THE WX AFTER HAVING WAITED FOR A SPECIAL VFR DEPARTURE. HIS DEPARTURE RADIO CALL WAS THE LAST COMMUNICATION FROM THE FLT. THE WX REMAINED MARGINAL WITH ICING IN THE CLOUDS AND FREEZING DRIZZLE. THE ACFT WAS ON A FLT TO TRANSPORT A DENTIST & A TECHNICAL ASST TO AREA VILLAGES FOR U.S. HEALTH DEPT CLINICAL WORK. THERE WAS NO PRESSING OR URGENT NEED TO ADHERE TO A SCHEDULE IF THE WX DICTATED A POSTPONEMENT. ACFT WRECKAGE WAS LOCATED ON THE SIDE OF A MT AT 650 FT ABOVE MSL. IT HAD COLLIDED WITH AN UPGRADE WHILE IN A DESCENDING TURN. THE ACFT BROKE APART AND LEFT WRECKAGE FOR A DISTANCE OF 493 FT BEFORE COMING TO REST. THE PLT & ONE PASSENGER REMAINED IN THE WRECKAGE. THE RT FRONT PASSENGER WAS EJECTED AND LOCATED NEAR THE WRECKAGE. WITNESSES AT THE CRASH SITE NOTED ICE 1/8 TO 3/4 INCHES THICK ON THE BROKEN WINGS & AIRFRAME PARTS DURING THE RECOVERY.