Mature fault zones appear to be weaker than predicted by both theory and experiment. One explanation involves the presence of weak minerals, such as talc. However, talc is only a minor constituent of most fault zones and thus the question arises: what proportion of a weak mineral is needed to satisfy weak fault models? Existing studies of fault gouges indicate that >30% of the weak phase is necessary to weaken faults - a proportion not supported by observations. Here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt% talc, provided the talc forms a critically-aligned, through-going layer. Observations of foliated fault rocks in mature, large-offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults.