Rhombic Tin

Another modification of tin, occurring in rhombic crystals, is formed when tin cools slowly from the molten state; and it is probable that grain tin owes its peculiar structure to this modification. Rhombic tin is also metastable at atmospheric temperature, the transition temperature between this form and tetragonal tin being variously given as 161° C., 170° C., and 202.8° C. The heat of transformation of tetragonal into rhombic tin is 14.03 ± 0.002 calories per gram, and the volume change 0.00017 c.c. per gram. The density of rhombic tin - slightly impure - is 6.525 to 6.557 at 15.8° C. (Trechmann). The relations between the three varieties of tin may thus be set forth:

1. Grey tin ⇔ tetragonal tin, 20° C.
2. Tetragonal tin ⇔ rhombic tin, 161° C. to 202.8° C.
3. Rhombic tin → molten tin, 231.9° C.

The heat of transformation of white into grey tin at 273° abs. Is 532 calories per gram-atom.