US mobile payments firm Square has announced that Keith Rabois (pictured), the company’s chief operating officer, has left the company in what is an unexpected move.

Rabois was Square’s number two executive behind CEO and founder Jack Dorsey. Frequently interviewed, he had a high profile. Rabois was with the company since August 2010.

No reason was given for his resignation, although AllThingsD said disagreements between Rabois and Dorsey were part of the reason for his leaving, citing sources.

The change seems particularly surprising since Square is undergoing major growth and has established itself as the leader in offering the processing of card payments to small businesses, via its card reader.

It is also looking to branch out, for instance by pushing its app that enables consumers to make payments in retailers. It also signed a major strategic agreement with Starbucks last summer.

Rabois is replaced by CFO Sarah Friar on an interim basis until a new COO can be found.