TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The siliceous zeolite is present in the catalyst composition in an amount of 0.1-10 wt.%. The siliceous zeolite has a silica to alumina framework molar ratio of at least 200. The siliceous zeolite is silicalite. The silicalite has an average crystallite size of less than 500 (preferably less than 100) nanometers. The large pore zeolite, the medium pore zeolite, and the siliceous zeolite are separately contained in catalyst particles of the catalyst composition. The siliceous zeolite catalyst particles of the catalyst composition comprise the siliceous zeolite in an amount of 20-75 wt.%. The siliceous zeolite catalyst particles of the catalyst composition comprise a silica binder in an amount of 15-40 wt.%. The siliceous zeolite catalyst particles further comprise a kaolin filler. The large pore zeolite has 12-membered ring pores with openings of greater than 0.7 nanometers (nm). The large pore zeolite is Y zeolite. The medium pore zeolite has 10- or fewer-membered ring pores with openings of less than 0.7 nm. The medium pore zeolite is ZSM-5 zeolite. The medium pore zeolite is present in the catalyst composition in an amount of 4-40 wt.%.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The FCC reaction zone as a temperature of 450-700 degrees C (842-1292 degrees F) and a pressure of 0.07-3.4 barg (1-50 psig). Preferred Components: The heavy hydrocarbon feedstock comprises vacuum gas oil (VGO). The heavy hydrocarbon feedstock comprises hydrocarbons (at least 95 wt.%) boiling at a temperature of 343-593 degrees C (650-1100 degrees F). EXAMPLE - A comparative fluid catalytic cracking (FCC) catalyst composition containing a blend of two types of catalyst particles, namely one type containing zeolite Y and another type containing ZSM-5 zeolite, was used as a reference catalyst composition. This composition having a total content of the ZSM-5 zeolite component of 6-8 wt.%. A second (test) catalyst composition was prepared by adding a catalyst particles (5 wt.% based on the total composition) containing nano-silicalite zeolite (40 wt.%) having a silica to alumina framework molar ratio of 740 (i.e. 370 Si:Al atomic ratio). The comparative catalyst composition and test composition containing the additional siliceous zeolite component were tested in a pilot plant for product yields obtained from cracking a commercial refinery vacuum gas oil (VGO) sample. Conversion to gasoline and lighter hydrocarbons (i.e. hydrocarbons boiling below 218 degrees C (425 degrees F) was varied during the testing of each catalyst composition by varying the catalyst to oil ratio. Product yields at range of conversions bracketing 75 wt.% were obtained, in order to estimate the yields at 75 wt.% conversion. The product yield (wt.%) based on coke, hydrogen, methane, ethane, ethylene, propylene, propane, butane and gasoline for test/comparative compositions was found to be 3.30/3.90, 0.15/0.20, 0.78/0.93, 0.48/0.58, 1.92/1.93, 13.2/13.2, 1.66/1.92, 17.1/17.7 and 36.4/34.6, respectively. The results showed that, the use of the siliceous zeolite additive reduced catalyst coke formation by 15% and total dry gas (hydrogen, methane, ethane, and ethylene) yield by 8.5%. Gasoline yield was increased and no loss of propylene yield was observed.