Why should sample rows of monazite or titanite be thinner than zircon rows?
Zircons are typically imaged with cathodoluminescence (CL), which gives good detail at even fairly low magnification (100X). Monazite and titanite, however, require back-scattered electron (BSE) imaging which is usually more subtle and requires higher magnification to bring out the detail. At even moderate magnifications (~200X), a thick row will not fit into the field of view, so imaging becomes inefficient and laborious. For these minerals, we recommend a single row, where sequential grains are slightly offset, as in a saw-tooth pattern. Incidentally, very old, U-rich zircons (often purplish in color) are oftentimes also best imaged with BSE and thus also may benefit from this sample arrangement.
