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MacFallite and orientite: calcium manganese (III) silicates from upper Michigan

Published online by Cambridge University Press:  05 July 2018

Paul B. Moore ,
Jun Ito  and
Ian M. Steele
Paul B. Moore
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, Chicago Illinois 60637, USA
Jun Ito
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, Chicago Illinois 60637, USA
Ian M. Steele
Affiliation:
Department of the Geophysical Sciences, The University of Chicago, Chicago Illinois 60637, USA
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Summary

MacFallite, Ca2(Mn3+,Al)3(OH)3[SiO4] [Si2O7], is a new species occurring with manganite, braunite, orientite and pyrolusite blebs, stringers, and framboidal aggregates, which replace calcite in fissures and lenses in Keweenaw basalt near Manganese Lake, Copper Harbor, Keweenaw County, Michigan. It is rich reddish brown to maroon in coarse aggregates; compact massive material is brown to dull pink. The streak and powder are brown with a reddish tint. Lustre silky to subadamantine, specific gravity 3.43(2), hardness 5+, cleavage {001} perfect; twinning by reflection on {100} is universal. The mineral is monoclinic, space group P21, or P21/m, Z = 2, a 8.929(6), b 6.045(5), c 10.905(7) Å, β 119.10(3)°, α 1.773(5), β 1.795(5), γ 1.815(5), sign + or −, pleochroism α yellow, β light brown, γ dark brown, γ‖b. Orientite, Ca2Mn 2+ (OH)4[Si3O10]-Ca2 (OH)2[Si3O10] ·2H2O, orthorhombic, space group Cc2m, Ccm21or Ccmm, a 9.042(4), b 6.090(2), c 18.990(7) Å, α 1.765(5), β 1.79(1), γ 1.81(1), sign + or −, α brownish yellow, β reddish brown, γ deep brownish red, α‖a, γ‖b, also occurs in moderate abundance. It is turbid chocolate brown in platy masses but crystals are transparent rich reddish brown. Streak and powder brown. Observed forms are m{110}, c{001}, and poorly developed to absent α{100}, thin to thick tabular parallel to {001}, cleavage (or parting) {001} perfect, specific gravity 3.33.

A fumarolic origin is proposed for the assemblage, which in many respects is similar to the great manganese oxide deposits in Oriente Province, Cuba. Macfallite appears to be structurally related to pumpellyite while orientite is apparently related to ardennite.

Type
Research Article
Information
Mineralogical Magazine , Volume 43 , Issue 327 , September 1979 , pp. 325 - 331
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1979

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Footnotes

1

Died 6 June 1978.

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