Vladimir Trifonov Geometric modification of quaternionic quantum mechanics (79K, Latex2e) ABSTRACT. We present an axiomatic modification of quaternionic quantum mechanics with a possible worlds semantics capable of predicting essential "nonquantum" features of an observable universe model - the topology and dimensionality of spacetime, the existence, the signature and a specific form of a metric on it, and certain naturally distinguished directions (vistas) in spacetime unrelated to its metric properties.