[R] create a "square buffer" around a coordinate for a raster

[Dana Benz]

Hello all, 

I'm working with two landsat scenes from 2011 and 1987 with different extents and want to run a cpa over a square with a side length of 10 km. I unfortunately only have the coordinates for the center point. I tried to use a buffer to create the extent but have no idea how to make a square buffer. As I don't have the coordinates for the corners of the square using extent, mask or crop is not possible either - at least to my knowledge. 

Could someone please point me in the right direction? I tried to look for this on google and the mail archives but have found nothing. Also, I'd like to mention that english is not my first language so please excuse my errors and any inaccuracies in describing my problem.

With thanks,
Dana