I'm tempted to say that washed sand is what you get when you add a dash
of washing up liguid to the mortar to make it easier to work. Really
it means that there is little or no clay and silt mixed in with the
sand. This is not neccessarily an advantage. When making mortar with
sand and lime, as oposed to Portland cement, a sound mortar can be
obtained with less lime if there is some silt and clay in the
aggregate.

Limonite it not a kind of clay but is a hydrated iron oxide, usually
amorphous or cryptocrystalline and often closely combined with
colloidal silica, phosphates, clay minerals and organic decomposition
products. It's a product of the weathering of iron bearing minerals
and is responsible for mush of the yellow/brown colour of sand.
There's not much limonite in silver sand!

Geological pedant.