Size  9cm x 3cm
Glendonite is a term used for these unusual pseudomorphs. The speciman was originally an Ikaite crystal, Ikaite crystals only form in near freezing waters and under warmer temperatures, Ikaite is unstable and converts into calcite. So technically, these are calcite pseudomorphs after Ikaite. They were originally found near Glendon in NSW, Australia, and so inherited the name Glendonite.
Don't see many of these around these days.