Mineral Information and Data:

Halloysite-7A
Al2Si2O5(OH)4 [CNMNC approved formula]

Named in 1826 after the Belgian geologist Jean Baptiste Julien d'Omalius d'Halloy (1783-1875) . He studied in detail the Tertiary deposits of the Paris Basin, and ascertained the extent of the Cretaceous and some of the older strata, which he for the first time clearly depicted on a map (1817). The terms halloysite(7Å) and and halloysite(10Å) were approved by IMA for usage in 1975.

IMA status : Valid species (Pre-IMA 1826) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
71.01.01.04  
  (71) Phyllosilicate Sheets of Six-Membered Rings
  (71.01) with 1:1 layers
  (71.01.01) Kaolinite group
  
Nickel-Strunz
(10th edition) :
09.ED.10
  (09) Silicates
  (09.E) Phyllosilicates
  (09.ED) Phyllosilicates with kaolinite layers composed of tetrahedral and octahedral nets
  
Crystal system: Monoclinic System
Point group (H-M): m — domatic
Unit cell: a = 5.14 Ǻ, b = 8.90 Ǻ, c = 14.9 Ǻ, β = 101.9°
Crystal Habit: Tubular (on microscopic scale); massive, compact or mealy
  
Color: White; gray, green, blue, yellow, red from included impurities
Diaphaneity: Semi-transparent
Luster: Pearly, waxy, or dull
Hardness (Mohs): 2 to 2.5
Measured Density: 2 to 2.2 g/cm3
Cleavage: {001}, probable
Tenacity: Brittle
Fracture: Conchoidal
  
Polymorphism and Series: Polymorphous with dickite, kaolinite, and nacrite;
Geologic Setting: A product of hydrothermal alteration or surface weathering of aluminosilicate minerals, as feldspars. Formed by dehydration of endellite above 110 °C.
Mineral Association: Kaolinite, endellite, allophane, alunite
  
Synonyms/varieties: Dehydrated halloysite, Metahalloysite
Comments: Note: halloysite-7Å is not a polytype of halloysite. It is however an IMA-approved (grandfathered) species distinct from halloysite-10Å.

 

Halloysite-7A
Halloysite-7Å
Origin: Biela Hora, Michalovce, Slovak Republic
Sample size: thumbnail
Photo: Diederik Visser

 

Halloysite-7A
Origin: Utah, U.S.A.
Sample size: 7 x 5 x 3 cm
Photo: Dan Weinrich

 

Halloysite-7A
(zoom-in of above)

 

Halloysite-7A
Blue colored (by Cu) compact masses, with conichalcite
Origin: Glory Hole area of the Gold Hill Mine, Tooele Co., Utah, USA
Sample size: 4 x 4 cm
Photo: Diederik Visser

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