Pennantite (original) (raw)

Formula:

Mn2+5Al(AlSi3O10)(OH)8

Colour:

Orange-red, red-brown, brown, dark red, dark green, black

Specific Gravity:

2.89 - 3.07

Name:

Named in 1946 by Walter Campbell Smith, Frederick Allen Bannister, and Max Hutchinson Hey in honor of Thomas Pennant [14 June {Old Style}, 1726 Whitford, Flintshire, Wales, United Kingdom - 16 December 1798 Whitford, Flintshire, Wales], author, naturalist, and antiquary.

Forms solid solution series with clinochlore and chamosite.
However, Abrecht (1989) gave evidence for a miscibility gap between pennantite and clinochlore.

Compare also gonyerite, a chemically similar, but Al-free Mn-member of the chlorite group.

RI values:

_n_α = 1.615 - 1.664 _n_β = 1.661 _n_γ = 1.622 - 1.673

δ = 0.007 - 0.009

Image shows birefringence interference colour range (at 30µm thickness)
and does not take into account mineral colouration.

Comments:

X = pinkish orange, orange-buff, reddish brown
Y = Z = orange, yellowish orange, very dark brown

Comments:

Absorption: Y = Z > X.

Cell Parameters:

a = 5.45 Å, b = 9.50 Å, c = 14.40 Å
α = 90°, β = 97.3°, γ = 90°

Ratio:

a:b:c = 0.574 : 1 : 1.516

Unit Cell V:

739.52 ų (Calculated from Unit Cell)

Morphology:

Lamellar crystals and flakes.

Comment:

Point Group: 1 or 1; Space Group: P1 or P1.

General Appearance of Type Material:

Very poorly crystallized as patches and small specks in the matrix. Less commonly it occurs as minute flakes in the ore and sometimes as a constituent of very thin veins which traverse the ore. In some specimens the mineral is better developed forming orange-brown micaceous flakes to 0.5 mm. Patches of the mineral in the matrix frequently enclose very small garnets.

Place of Conservation of Type Material:

The Natural History Museum, London, England, 1947,295-296.
Harvard University, Cambridge, Massachusetts, USA, 104705.
National Museum of Natural History, Washington, D.C., USA, 105855.

Geological Setting of Type Material:

Hydrothermal alteration of manganese deposits.

Associated Minerals at Type Locality:

Reference:

Smith W C, Bannister F A, Hey M H (1946) Pennantite, a new manganese-rich chlorite from Benallt mine, Rhiw, Carnarvonshire. Mineralogical Magazine 27, 217-220

- This locality has map coordinates listed. - This locality has estimated coordinates. ⓘ - Click for references and further information on this occurrence. ? - Indicates mineral may be doubtful at this locality. - Good crystals or important locality for species. - World class for species or very significant. (TL) - Type Locality for a valid mineral species. (FRL) - First Recorded Locality for everything else (eg varieties). Struck out - Mineral was erroneously reported from this locality. Faded * - Never found at this locality but inferred to have existed at some point in the past (e.g. from pseudomorphs).

All localities listed without proper references should be considered as questionable.