  The applications of quaternion in physics are discussed with an emphasis on
elementary particle symmetry and interaction. Three colours of the quark and
the quantum chromodynamics (QCD) can be introduced directly from the invariance
of basic equations under the quaternion phase transformation (quaternion gauge
invariance). The generalized quaternions obey the SU(3) symmetry. QCD is
essentially the quantum quaternion dynamics. The further generalization of
SU(3) quaternion to G2 octonion is worked out. We demonstrate that the G2
octonion contains seven tri-generator sets of SU(2) symmetry and three of them
form SU(3) subgroup. A model of the elementary particle classification and
interaction based on octonion gauge theory is proposed. The model unifies the
colour and flavour of all particles. It provides a framework for the unified
description of four kinds of elementary particles (quarks, leptons, gauge
fields and Higgs bosons) and their interactions.
