Preheat oven to 350°F In a small bowl, combine graham cracker crumbs and 1/4 cup sugar. Stir in butter. Combine until all crumbs are coated. Pat the mixture into the bottom and sides of a 9" springform pan. Bake for 8 minutes.
In large bowl, with an electric mixer, beat cream cheese, mascarpone and remaining sugar. Beat in egg. Add lime juice. Pour cheese mixture into pan. Bake at 350F for 10 minutes. Reduce heat to 250°F Bake for 1 hour or until center is firm yet barely set. Cool completely, then refrigerate, covered.