If rumors of a mini iPad are true, Apple can’t bring it to market quickly enough. A new study by The Pew Research Center claims that Apple’s iPad fell from 81 percent market share in the United States to just 52 percent by mid-2012, because of strong sales growth in Android-based tablets.

Most troubling for Apple, that shift happened before the two most popular Android-based tablets—the inexpensive Google Nexus 7 and Amazon Kindle Fire HD—were released. So it’s likely that Android tablets have already surpassed the iPad.

“The influx of Android-based devices has changed the tablet market, much as it did for smartphones over the past several years,” the Pew report notes. “Of the 44 percent of adults who have a smartphone, 46 percent have an Android phone, 38 percent have an iPhone, and 10 percent have a BlackBerry.”

Of the Android tablets, Amazon commands the lion’s share, with its Kindle Fire taking 21 percent of the market overall. Samsung’s Galaxy line owns about 8 percent of the market, Pew says, and the rest is split between a mix of other companies.

Pew also notes that customers tend to be platform loyal. About 57 percent of iPad users also own an iPhone, with 32 percent using an Android phone. And 66 percent of users with an Android tablet use an Android phone, compared with just 29 percent with an iPhone.

It will be interesting to see how the market dynamics change in the coming year. Microsoft and its many hardware partners are set to release Windows 8-based PCs and devices and, more important, tablets based on Windows RT that will more closely compete with iPad and Android. And Apple, as noted, is rumored to be launching a so-called iPad Mini in November. That device will have a 7"-to-8" screen similar to the majority of the best-selling Android tablets and, presumably, at a more competitive price point.