Below are 3 videos that allow you to compare radii and crystalline structure. Each video highlights a different cation (Fe, Al, Si). Please note that these are ideal models, but naturally occurring Majorite Garnets would exhibit a solid solution of the 3 cations; or would not be pure Fe, Al, or Si but a mixture.
Isometric System: (4/m 3 2/m) Cell Parameters:11.52 Å Dana Number: 51.4.3a.5 Strunz Number: 9.AD.25