Tap and buy: Come 2017, approximately 25% of online retail transactions in the U.S. will take place on mobile devices, more than double the number that occurred in 2012, according to an eMarketer study published Tuesday. That figure excludes purchase of event tickets and travel services. Between last year and this year alone, mobile commerce (mcommerce) sales are expected to increase 82% from $24.81 billion to $38.84 billion. By 2017, mcommerce will generated more than $108 billion in U.S. retail sales. This year, most of sales in the mobile category — more than 60% of them — will take place on tablet devices. That share will increase over time: By 2017, eMarketer estimates that more than 70% of online retail sales will occur on tablets, versus a little more than a quarter on smartphones. Mcommerce penetration will outpace spending.