Disclosed herein are crosslinker compositions for aqueous hydroxyl functional polymers. The compositions include at least one borate species, about 0.05 wt % to 10 wt % of a synthetic layered silicate, and a water source. The compositions are characterized by shelf stability, wherein the sparingly soluble borate does not undergo sedimentation. The crosslinker compositions are usefully added to aqueous polymer solutions of hydroxyl functional polymers to form injectable compositions for hydraulic fracturing.