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In surprising news last night, Square’s chief operating officer and No. 2 exec Keith Rabois announced that he was leaving the mobile payments startup. His actual reasons for leaving are still unclear, though AllThingsD reports that it could be the results of disagreements between Rabois and Square CEO Jack Dorsey.

It’s a significant change for Square, especially now as it’s breaking into the mainstream via Starbucks and sitting on a new $200 million funding round at a $3.25 billion evaluation. The company started out offering phone dongles for credit card payments, but it’s quickly transformed into a company that’s also looking ahead to the future of payments.

Square’s chief financial officer, Sarah Friar, will serve as acting COO. AllThingsD notes that it’s unusual for an exec to land such a role without much operational experience, but Friar’s financial background could be just as useful for Square.

Rabois offered the following cryptic statement about the move to AllThingsD:

It is amazing what Square has accomplished since August of 2010. When I joined, there were 17 engineers all reporting flatly to Jack. The local coffee shop served as our interview room. Leading our amazing crew has been the most rewarding professional journey of my life. I am forever grateful to Jack, for his confidence in me and to each and every member of the team for allowing me to learn from them.

But every day matters. And it is better at this point for me to be doing something different every day.

As a result, I’ve decided to resign from Square. I am very excited about what lies ahead for the company. Square could not be better poised for greatness.

I will have more to share about my next opportunity soon.

Dorsey didn’t waste many words with his statement to the site:

Today I accepted Keith’s resignation from Square. When he joined, we had fewer than 30 employees and under 1000 active merchants. Today, over 3 million individuals and businesses are able to accept credit cards with Square, processing over $10 billion annually. We couldn’t have done it without him and we wish him well in his next opportunity.