  We set up a harmonic lattice model for 2D defect melting which, in contrast
to earlier simple-cubic models, lives on a triangular lattice. Integer-valued
plastic defect gauge fields allow for the thermal generation of dislocations
and disclinations. The model produces universal formulas for the melting
temperature expressed in terms of the elastic constants, which are different
from those derived for square lattices. They determine a Lindemann-like
parameter for two-dimensional melting. In contrast to the square crystal which
underwent a first-order melting transition, the triangular model melts in two
steps. Our results are applied to the melting of Lennard-Jones and electron
lattices.
