We conducted laboratory-drilling experiments in Berea sandstone blocks of two different porosities in which borehole breakouts were induced under several truly triaxial far-field stress conditions. We used a petrographic microscope to study thin sections of the drilled boreholes and observed the mechanism of failure that ultimately resulted in breakout formation. In lower porosity Berea sandstone (17%) we identified a breakout failure mechanism that is similar in its final V' shape to that observed in granite and limestone, and occurring along the ah spring line. This shape results from successive spalling of slabs initiated by dilatant intragranular microcracking subparallel to a~ direction, The cracks extend intergranularly due to the much lower toughness of the rock matrix, and reach the borehole wall along tortuous diagonal paths, forming a wide-angle V'- shaped notch. A dramatically different failure mechanism was observed in the higher porosity sandstone (22.5%), a more weakly cemented but still competent Berea sandstone. Breakout failure occurs at the same locations around the borehole as above, but its final shape is best described as a narrow linear slot or fracture perpendicular to aft direction. Our interpretation of the mechanism forming this type of breakout is that it is related to the weak bonding between the quartzitic grains as inferred from the high porosity of the rock, which also implies less cementing matrix material between the grains. Under sufficient far-field stresses, high stress concentration around the hole (along the ah springline), causes the collapse of the weak and sparse matrix, and partially or totally debonds the quartz grains. The disaggregated grains spall off in non-dilatant fashion, creating a higher stress concentration behind them. This in turn facilitates further disintegration of the cementing matrix, and removal of additional grains along the ah spring line. The newly observed breakout type, different from any previously experienced, could have major significance in studying instability problems in wellbores intersecting poorly consolidated sandstones.