I love all the colors in quilt! It looked to me like a variation of a wedding ring. I have a copy of Barbara Brackman's Encyclopedia of Pieced Quilt Patterns. I couldn't find anything really close but think the center X was made like the block she identifies as the Greenberg Wedding Quilt or Wedding Chain no.1032 on page 145. I think the person who made this liked wedding ring pattern but also wanted to combine in the Wedding Chain. The wedding ring patterns in the book show a more circular center but this quilt has more of an octagon shape center so the sewer had to adapt the wedding ring pattern to fit the block. To bring the circles together, she used the mint green triangles.
I purchased my copy of Barbara's book on sale a few years ago and from what I've been told its the best resource for tracing down some of the older block patterns. Hope this helped, Chris