1. Chill 13 x 9 x 2-inch nonreactive metal baking pan in freezer for at least 30 minutes.
2. Puree raspberries in blender or food processor. Force though sieve with rubber spatula to remove seeds (you should have 1 cup puree).
3. Puree pineapple, sugar and lemon juice in blender or food processor until smooth. Stir in raspberry puree.
4. Pour into chilled baking pan. Freeze until ice crystals form around the edges, 30 to 60 minutes. Using a fork, stir well to incorporate ice from edges into whole mixture. Return to freezer. Every 30 minutes, mash mixture and stir, until all the liquid is uniformly frozen, about 3 hours.
5. Spoon granita into chilled bowls and serve. The granita should be slightly soft. If it freezes solid, place pan in refrigerator for a few minutes and mash again before serving. Garnish with fresh mint, if desired. Makes 6 servings.