In a method of synthesizing a mostly CHA-type silicoaluminophosphate sieve, a reaction mixture comprises sources of water, silicon, aluminum, phosphorus, and a template. In one aspect, when the reaction mixture Si/Al2 ratio is less than 0.33, crystallization is induced at least 165° C. Advantageously, the sieve so crystallized exhibits a Si/Al2 ratio less than 0.33 and/or at least 0.10 greater than the synthesis mixture Si/Al2 ratio. In another aspect, the aluminum and phosphorus sources are first combined to form a primary mixture that is aged. The silicon source and template can then be added to form the synthesis mixture. After inducing crystallization, in this aspect, both the synthesis mixture and crystallized sieve exhibit a Si/Al2 ratio less than 0.33 and/or the crystallized sieve has an average crystal size not more than 3.0 μm. The molecular sieve from both aspects can be used in a hydrocarbon (oxygenates-to-olefins) conversion process.