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Oftedalite

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Heftetjern pegmatite, Tørdal, Drangedal, Telemark, Norway
© R.Kristiansen




Formula:
(Sc,Ca,Mn
2+
 
)
 
2
K[(Be,Al)
 
3
Si
 
12
O
 
30
]
System:HexagonalColour:Greyish white
Hardness:6


Osumilite Group.

Classification of Oftedalite

IMA status:Approved 2004
Strunz 8th edition ID:8/E.22-
Nickel-Strunz 10th (pending) edition ID:9.CM.05

9 : SILICATES (Germanates)
C : Cyclosilicates
M : [Si6O18]12- 6-membered double rings (sechser-Doppelringe)
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Type Occurrence of Oftedalite

Type Locality:Heftetjern pegmatite, Tørdal, Drangedal, Telemark, Norway
General Appearance of Type Material:short prismatic crystals up to 100 µm in diameter, or as a product of replacement together with other Sc-bearing minerals
Geological Setting of type material:Granitic pegmatite

Physical Properties of Oftedalite

Lustre:Vitreous
Colour:Greyish white
Streak:White
Hardness (Mohs):6
Tenacity:Brittle
Cleavage:Poor/Indistinct
parallel to {001}
Fracture:Conchoidal
Density (calculated):2.614 g/cm3

Crystallography of Oftedalite

Crystal System:Hexagonal
Class (H-M):6/mmm (6/m 2/m 2/m) - Dihexagonal Dipyramidal
Space Group:P6/mcc {P6/m 2/c 2/c}
Cell Parameters:a = 10.097Å, c = 13.991Å
Ratio:a:c = 1 : 1.386
Unit Cell Volume:V 1235.3 ų
Z:2

Epitaxial Relationships of Oftedalite

Epitaxial Minerals:
Milarite
K
 
2
Ca
 
4
Al
 
2
Be
 
4
Si
 
24
O
 
60
 · H
 
2
O
Epitaxi Comments:frequently with very thin crystals of Sc-free milarite up to ~10–20 µm across in parallel orientation on the (001) faces
X-Ray Powder Diffraction:
d-spacingIntensity
3.229 (10)
4.097 (7)
5.044 (5)
3.504 (5)
7.012 (4)
1.836 (4)
1.751 (4)
2.735 (3)
Comments:Recorded on type material; Format of data: d/Å (I/%) (hkl)

Optical Data of Oftedalite

Type:Uniaxial (-)
RI values: nω = 1.554 - 1.558 nε = 1.551 - 1.555
Maximum Birefringence:δ = 0.003

Chart shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:Low
Pleochroism:Non-pleochroic

Chemical Properties of Oftedalite

Formula:
(Sc,Ca,Mn
2+
 
)
 
2
K[(Be,Al)
 
3
Si
 
12
O
 
30
]
All elements listed in formula:Al, Be, Ca, K, Mn, O, Sc, Si
Analytical Data:Results from electron microprobe analysis, Be content calculated per formula unit as n(Be) = 3 – n(Al);
Empirical formula based on 30 oxygen atoms per formula unit
SiO2 (73.32)
Al2O3 (0.47)
Sc2O3 (6.77)
Y2O3 (0.36)
FeO (0.26)
MnO (1.32)
CaO (4.49)
K2O (4.71)
BeO (7.41)

sum 99.11 wt.-%
Empirical Formula:
(Sc
 
0.96
Ca
 
0.79
Mn
2+
0.18
Fe
2+
0.04
)
 
=2.00
K
 
0.98
(Be
 
2.91
Al
 
0.09
)
 
=3.00
Si
 
11.98
O
 
30

Relationship of Oftedalite to other Species

Related Minerals - Nickel-Strunz Grouping):

- +
9.CM.05Armenite
BaCa
 
2
Al
 
6
Si
 
9
O
 
30
 · 2H
 
2
O
9.CM.05Brannockite
(K,Na)□
 
2
Li
 
3
(Sn,Zr,Ti)
 
2
[Si
 
12
O
 
30
]
9.CM.05Chayesite
K(Mg,Fe)
 
4
FeSi
 
12
O
 
30
9.CM.05Darapiosite
KNa
 
2
(Zn,Li)
 
3
(Mn,Zr)
 
2
[Si
 
12
O
 
30
]
9.CM.05Eifelite
KNa
 
3
Mg
 
4
Si
 
12
O
 
30
9.CM.05Merrihueite
(K,Na)
 
2
(Fe,Mg)
 
5
Si
 
12
O
 
30
9.CM.05Milarite
K
 
2
Ca
 
4
Al
 
2
Be
 
4
Si
 
24
O
 
60
 · H
 
2
O
9.CM.05Osumilite-(Mg)
KMg
 
2
Al
 
3
(Al
 
2
Si
 
10
)O
 
30
9.CM.05Osumilite
(K,Na)(Fe
2+
 
,Mg)
 
2
(Al,Fe
3+
 
)
 
3
(Si,Al)
 
12
O
 
30
9.CM.05Poudretteite
KNa
 
2
B
 
3
Si
 
12
O
 
30
9.CM.05Roedderite
(Na,K)
 
2
(Mg,Fe)
 
5
Si
 
12
O
 
30
9.CM.05Sogdianite
K(□,Na)
 
2
Li
 
3
(Zr,Fe,Ti)
 
2
[Si
 
12
O
 
30
]
9.CM.05Sugilite
K(Na,□)
 
2
Li
 
3
(Fe,Mn,Al,Zr)
 
2
[Si
 
12
O
 
30
]
9.CM.05Yagiite
(Na,K)
 
3
Mg
 
4
(Al,Mg)
 
6
(Si,Al)
 
24
O
 
60
9.CM.05Berezanskite
K□
 
2
Li
 
3
(Ti,Zr,Sn)
 
2
[Si
 
12
O
 
30
]
9.CM.05Dusmatovite
K(Na,□)
 
2
(Zn,Li)
 
3
(Mn
2+
 
,Y,Zr)
 
2
[Si
 
12
O
 
30
]
9.CM.05Shibkovite
K(□,K,Na)
 
2
Zn
 
3
Ca
 
2
[Si
 
12
O
 
30
]
9.CM.05Almarudite
K(□,Na)
 
2
(Mn
2+
 
,Fe
2+
 
,Mg)
 
2
(Be,Al)
 
3
[Si
 
12
O
 
30
]
9.CM.05Trattnerite
□(Fe
3+
 
,Mg,Fe
2+
 
)
 
2
(Mg,Fe
2+
 
,Fe
3+
 
)
 
3
(Si
 
12
O
 
30
)
9.CM.10Faizievite
K
 
2
Na(Ca
 
6
Na)Ti
 
4
Li
 
6
[F
 
2
|Si
 
24
O
 
66
]

Other Names for Oftedalite

Synonyms:
IMA2003-045aScandiomilarite
Other Languages:
German:Oftedalit

Other Information

Fluorescence in UV light:None observed
Health Warning:No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Oftedalite

Reference List:

- +
Kristiansen, R. (2005): "Milarittgruppens mineraler i Norge" ["The milarite group minerals in Norway]. Norsk Bergverksmuseums Skrift, 30, 21-29.

Cooper, M.A., Hawthorne, F.C., Ball, N.A., Černý, P., Kristiansen, R. (2006): Oftedalite, (Sc,Ca,Mn2+)2K(Be,Al)3Si12O30, a new member of the milarite group from the Heftetjern pegmatite, Tørdal, Norway: description and crystal structure. Canadian Mineralogist, 44, 943-949.

Internet Links for Oftedalite

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    • Localities for Oftedalite

    Hide Locality List
    (TL) indicates type locality. ? indicates mineral may be doubtful at this locality. All other localities listed without reference should be considered as uncertain and unproven until references can be found.
    Norway
     
    • Telemark
      • Drangedal
        • Tørdal
    		Cooper, M.A., Hawthorne, F.C., Ball, N.A., Černý, P., Kristiansen, R. (2006): Oftedalite, (ScCa)KBe3Si12O30, a new mineral species of the milarite group from the Heftetjern pegmatite, Tørdal, Norway: description and crystal structure. Canadian Mineralogist, 44, 943-949; Kristiansen, R. (2005): Milarittgruppens mineraler i Norge. Norsk Bergverksmuseum Skrifter, 30, 21-29
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