Dickite

Dickite is a clay mineral that shares the same formula as kaolin, nacrite, and halloysite but has a distinct crystal structure[1]. It is often found intergrown with other clay minerals and requires x-ray diffraction for positive identification, which can lead to its easy oversight[1]. Dickite can occur in various geological settings, including soils, shales, and as a hydrothermal precipitate[1].

The mineral was first discovered in Almwch, Island of Anglesey, Wales, UK, and is scattered across Wales, forming occurrences in vein assemblages and as a rock-forming mineral[2]. It is also found in other locations worldwide, such as Ouray, Colorado, US; San Juanito, Chihuahua, Mexico; and St. George, Utah, US[2].

Dickite is a phyllosilicate clay mineral named after the metallurgical chemist Allan Brugh Dick, who first described it[2]. It is chemically composed of 20.90% aluminum, 21.76% silicon, 1.56% hydrogen, and 55.78% oxygen[3]. The mineral has a monoclinic crystal system and can appear in various colors, including white, blue, colorless, gray, yellow-brown, due to impurities[3].

Dickite is an essential component of certain rock names and can be an accessory component in others[4]. It is a valid IMA mineral species, grandfathered, and is a secondary mineral usually of hydrothermal origin in conjunction with veins, derived in part from the alteration of aluminosilicate minerals[4]. Dickite also occurs as an authigenic sedimentary phase[4].

In Wales, dickite is found in various localities, including Aberfan, South Wales, where it is associated with nacrite, and at the Pant-y-Gaseg Mine, Anglesey, which is the type locality for the mineral[5]. It forms colorless to white hexagonal plates or stacks of plates up to 0.1 mm across, on quartz and dolomite[5]. Dickite is also present in clay ironstone nodules found within the Coal Measures in South Wales[5].

Citations:

[1] https://home.wgnhs.wisc.edu/minerals/dickite/

[2] https://en.wikipedia.org/wiki/Dickite

[3] https://webmineral.com/data/Dickite.shtml

[4] https://www.mindat.org/min-1287.html

[5] https://museum.wales/mineralogy-of-wales/database/?mineral=87&name=Dickite