Twin lamellae occur in both calcite and dolomite, especially in rocks that have been deformed, such as marbles.  In calcite, such lamellae can occur both parallel to the rhomb edges (A and B) and parallel to the long diagonal of the rhomb, but not the short diagonal.  In dolomite, however, twin lamellae can occur parallel to both the long and short diagonals of the rhomb.  Twin lamellae parallelling the rhomb are illustrated for calcite in A and B and those parallel to the short diagonal of the dolomite rhombs are illustrated in C and D.  And B are from a phlogopite-bearing calcite marble from Dutchess County, New York and C and D are from a dolomite carbonatite from an unknown location.  All images are 2.2 mm across. A, C ppl, B, D x-nicols.