Bobfergusonite

Bobfergusonite is a mineral with formula Na2Mn5FeAl(PO4)6. The mineral varies in color from green-brown to red-brown. It was discovered in 1986 in Manitoba, Canada, and named for Robert Bury Ferguson (born 1920). As of 2012, the mineral has only been found in Canada and Argentina.

Bobfergusonite
General
CategoryPhosphate mineral
Formula
(repeating unit)		Na2Mn5FeAl(PO4)6
Strunz classification8.AC.15
Dana classification38.2.4.5
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/n[1]
Unit cella = 12.773 Å,
b = 12.486 Å,
c = 11.038 Å;
β = 114.35(13)°; Z = 4
Identification
ColorGreen-brown to red-brown
CleavagePerfect on {010}
Parting on {100}[2]
FractureIrregular/Uneven
TenacityBrittle
Mohs scale hardness4
LusterSub-vitreous, resinous, greasy
StreakYellow-brown
DiaphaneityTranslucent[3] to transparent[2]
Density3.54 g/cm3
Optical propertiesBiaxial (+)
Refractive indexnα = 1.694(1) nβ = 1.698(1) nγ = 1.715(2)
Birefringenceδ = 0.021
PleochroismVisible: X=Y= yellow-orange Z= orange
2V angle46°
DispersionRelatively strong
Ultraviolet fluorescenceNot fluorescent
References[3]

Description

Bobfergusonite occurs as equant anhedral single crystals up to 1 cm (0.39 in) in size or as nodular aggregates of few crystals.[2][4] The mineral varies in color from green-brown to red-brown.[2] Thin fragments of bobfergusonite are transparent.[5] Bobfergusonite has been found in association with alluaudite, apatite, beusite, fillowite, and triplite.[2]

Bobfergusonite is a primary mineral that occurs in the intermediate zone of manganese and fluorine enriched granitic pegmatites.[3]

Structure

Bobfergusonite has a layered crystal structure topologically identical to that of alluaudite and wyllieite but with differences in the ordering of metal cations. The two types of layer alternate along Y. One layer consists of chains of metal cation octahedra cross-linked by phosphate tetrahedra. Within the chains metal cations are ordered M3+–M2+ in a similar fashion to wyllieite. However, the structure of bobfergusonite is distinct by the presence of Al and Fe3+ ordering between chains.[6]

The other layer, identical to its wyllieite counterpart, consists of chains running parallel to X: one consisting of alternating, face-sharing sodium and manganese polyhedra and the other edge-sharing sodium polyhedra. These chains are not cross-linked but bind the other layers together.[6]

History

Alan J. Anderson discovered large brown crystals in a granitic pegmatite at Cross Lake in Manitoba. Study by electron microprobe and X-ray diffraction identified it as a new mineral related to the wyllieite and alluaudite groups.[7]

The mineral was named for professor Robert Bury Ferguson to celebrate his 65th birthday and retirement from the University of Manitoba.[7][8] The mineral and the name bobfergusonite were approved by the IMA Commission on New Minerals and Mineral Names.[7]

Distribution

As of 2012, bobfergusonite is known from the Nancy pegmatite in Argentina and the Gottcha Claim in Manitoba, Canada.[3] The type material is held at the University of Manitoba and the Royal Ontario Museum in Toronto.[2]

References
  1. "Bobfergusonite". Webmineral. Retrieved June 11, 2012.
  2. "Bobfergusonite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved June 8, 2012.
  3. "Bobfergusonite". Mindat. Retrieved June 8, 2012.
  4. Ercit, p. 600.
  5. Roberts, Andrew C. (1988). "New Mineral Names" (PDF). American Mineralogist. 73: 189–199.
  6. Ercit, p. 605.
  7. Ercit, p. 599.
  8. Hawthorne, Frank C. (December 1986). "Special Issue Dedicated to Robert Bury Ferguson" (PDF). Canadian Mineralogist. Mineralogical Association of Canada. 24: 597–598.

Bibliography

Further reading
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