Glauconite occurs as the major constituent of late Cretaceous and Tertiary greensand deposits formed in New Zealand during periods of marine transgression. These glauconites are generally found to be high in potassium content, and X-ray studies show them to be well ordered, some of them falling into the highest classification group for this mineral (1M). These 1M glauconites are not restricted to any particular period, and are usually found associated with calcareous or quartzose sediments. Recent glauconites, now forming off the East Coast of New Zealand, have not yet fully developed the glauconitic structure and are extensively interlayered with an expandable montmorillonitic clay. They form mainly as foraminiferal casts, in sediments which often contain very little clay sized material. Glauconites from different deposits are shown to react differently to both mechanical and chemical wreathing, the effects being largely due to the manner of cementation of the glauconite crystallities. The change in glauconite structure on heating has been studied both by X-ray diffraction and infrared absorption methods. On heating to 1000ºC a spinel is formed; the order of crystallisation of this spinel is found to depend on the Fe : Mg ratio in the glauconite. The origin of glauconite is discussed. X-ray investigation of vermicular pellets, which occur in large quantities in South Canterbury and North Otago, upholds an existing theory of the change of biotite – or other mica – to glauconite. Many other deposits from these areas suggest foraminiferal casts, formed by either the alteration of existing degraded clay particles, or by a method of precipitation from solution. This later method is found to be possible by synthesis on a laboratory scale, and is suggested as a highly probably method of formation for some greensand deposits.