To make the sorbet: Combine the sugar and water ina large saucepan and bring to a boil. Reduce the heat and simmer for 10 minutes. Remove from the heat and allow to cool. Meanwhile, combine the orange and lime juices. Puree the pineapple in a food processor fitted with a steel blade or blender along with the mint leaves. Use some of the orange juice to moisten. Combine with the orange-lime juice. Stir the pineapple mixture into the cooled syrup. Either freeze in an ice cream freezer or freeze in a covered bowl. When frozen solid, remove from the bowl and spoon into a food processor fitted with the steel blade. Process until fluffy; this breaks up the ice crystals and results in a smoother texture. To ensure this very smooth texture, repeat the process one more time after a couple of hours (this step is optional). Spoon into individual serving dishes, cover with plastic and then foil, and freeze. Or oil a loaf pan or mold and line with plastic wrap. Pour in the sorbet mixture, cover tightly with plastic wrap and then foil, and freeze. Work quickly so that the sorbet doesn't melt, or ice crystals will form again when it freezes. To make the garnish and serve: Toss together the chopped pineapple and the Cointreau if you using it. Add the mint leaves. Set aside or refrigerate until ready to serve. Twenty minutes before serving, place the sorbet in the refrigerator to soften. If you froze it in the loaf pan, unmold on a platter. Cut slices using sharp knife. IF you froze it in individual molds, unmold into bowls. Serve, topping each serving with the garnish. Advance preparation: This sorbet will hold for weeks, covered tightly, in the freezer. The garnish can be made several hours ahead of serving time and held in a covered bowl in the refrigerator. Calories: 86. Fat: .40 g. Sodium 1 mg. Protein 1 g. Carbohydrate 21 g. Cholesterol 0.