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Annite
Annite
	Annite sample
General
Category	Phyllosilicates ; Mica group
Formula; (repeating unit)	KFe32+AlSi3O10(OH,F)2
IMA symbol	Ann
Strunz classification	9.EC.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) ; (same H-M symbol)
Space group	C2/m
Unit cell	a = 5.39, b = 9.33 ; c = 10.29 [Å]; β = 100°; Z = 2
Identification
Color	Brown to black
Crystal habit	Usually in "blocks" of thin sheets or as aggregations of foliated thin scales. Includes large, well formed pseudohexagonal prisms up to 12 cm across and 15 cm long.
Twinning	Contact twins with composition surface on {001} and twin axis {310}
Cleavage	Perfect in one direction {001}
Fracture	Uneven
Tenacity	Flexible
Mohs scale hardness	2.5–3
Luster	Submetallic to vitreous; pearly on cleavage surfaces
Streak	Brownish white
Diaphaneity	Translucent to transparent
Specific gravity	Approximately 3.3
Optical properties	Biaxial (−)
Refractive index	nα = 1.625 – 1.631 nβ = 1.690 nγ = 1.691 – 1.697
Pleochroism	X = brown; Y = Z = dark brown
2V angle	Calculated: 12° to 36°
Other characteristics	Diagnostic: occurrence: in magnesium poor igneous and metamorphic rocks.
References

Annite is a phyllosilicate mineral in the mica family. It has a chemical formula of KFe₃²⁺AlSi₃O₁₀(OH)₂.^[8] Annite is the iron end member of the biotite mica group, the iron rich analogue of magnesium rich phlogopite.^[9] Annite is monoclinic and contains tabular crystals and cleavage fragments with pseudohexagonal outlines.^[6] There are contact twins with composition surface {001} and twin axis {310}.^[10]

Annite was first described in 1868 for the first noted occurrence in Cape Ann, Rockport, Essex County, Massachusetts, US.^[7] It also occurs on Pikes Peak, El Paso County, Colorado.^[8] It occurs in igneous and metamorphic rocks that are deficient in magnesium and is associated with fluorite and zircon in the type locality.^[8]

Properties

The relief of a mineral refers to the way a mineral may stand out in plane polarized light. A mineral may be referred to as having a low or high relief. Minerals with a high relief, such as annite, have sharp grain boundaries and display good fracture and cleavage. When viewed under a microscope, this mineral may appear to stick out of the other minerals in the thin section. Relief primarily depends on the index of refraction of the mineral.^[8] The index of refraction of a mineral is a measure of the speed of light in the mineral. It is expressed as a ratio of the speed of light in vacuum relative to that in the given mineral. Annite has three indices of refraction known to be n_α = 1.625 – 1.631 n_β = 1.690 n_γ = 1.691 – 1.697.^[7] It is also an anisotropic mineral, meaning under the cross polars of a microscope the mineral will become extinct every 90°. However, in plane polarized light, annite appears as a brown or green platy form and is pleochroic, meaning the mineral changes colors under a microscope without the polars being crossed.^[8]

Uses

Annite is a member of the mica group and has very similar properties as other micas such as muscovite and biotite. More importantly, annite is interesting to geologists because it can be used for potassium-argon dating. Because annite contains large amounts of potassium, it can be used to find the absolute age of articles older than 1000 years. This type of dating also preserves a record of the direction and intensity of the local magnetic field, giving field geologists better knowledge of their surroundings.^[11]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Hazen, R.M. and Burnham, C.W. (1973) The crystal structures of one-layer phlogopite and annite. American Mineralogist, 58, 889–900.
^ Deer, W.A., Howie, R.A., and Zussman, J. (1963) Sheet Silicates. Rock-forming minerals Volume 3, 55–84.
^ Eugster, H.P. and Wones, D.R. (1962) Annite. Stability relations of the ferruginous biotite. 3, 82–125
^ Dana, E.S. (1892) Dana's system of mineralogy, (6th edition), 634.
^ ^a ^b Kwak, T.A.P. and Askins, P.W. (1981) Geology and genesis of the F-Sn-W(-Be-Zn) skarn (wrigglite) at Moina, Tasmania. Economical Geology, 76, 439–467
^ ^a ^b ^c Mindat.org
^ ^a ^b ^c ^d ^e ^f Handbook of Mineralogy
^ Deer, W. A., R. A. Howie and Zussman, J., An Introduction to the Rock Forming Minerals, Longman, 1966, p. 212 ISBN 0-582-44210-9
^ Anthony,J.W., Bideaux, R.A., Bladh, K.W., and Nichols, C.M. (1995), Handbook of Mineralogy, Volume II, Silica, Silicates Part 1, p. 31
^ Eugster, H.P. and Wones, D.R.(1962) Stability relations of the ferruginous biotite, annite. 3, 82–125