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Cairncrossite

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Professor Bruce Cairncross
Formula:
Sr2Ca7(Si4O10)4(OH)2·15H2O
idealized structural formula: Sr2Ca7–xNa2x(Si4O10)4(OH)2(H2O)15–x (x ~ 0-1);
simplified formula of Na-rich crystals: SrCa3Na(Si4O10)2(OH)(H2O)7 (Z = 2).
System:
Triclinic
Colour:
Colourless
Hardness:
3
Member of:
Name:
Named after Professor Bruce Cairncross (born 13 August 1953), Head of the Department of Geology 2003-2015 at the University of Johannesburg, Johannesburg, South Africa, in recognition of his work on the mineralogy of South Africa and especially its Mn deposits.


Belongs to both the Gyrolite and Reyerite groups. The combination of elements in the mineral is unique.

The structure is based on sheets of edge-sharing CaO6 octahedra. The sheets link by corners, on both sides, to silicate layers. Layers of paired edge-sharing SrO8 polyhedra are intercalated between the former units. The Sr polyhedra are further bound to disordered NaO6 polyhedra. Linkage to adjacent silicate layers is strenghtened by a complex system of hydrogen bonds.

Classification of Cairncrossite

Approved
Approval Year:
2013
mindat.org URL:
http://www.mindat.org/min-43802.html
Please feel free to link to this page.

Type Occurrence of Cairncrossite

General Appearance of Type Material:
radiating platy-micaceous aggregates, up to 1 cm
Place of Conservation of Type Material:
In the collections of the Institute of Mineralogy and Crystallography, University of Vienna, Vienna, Austria, registration number 13079
Geological Setting of Type Material:
dump material (within Mn ore)
Associated Minerals at Type Locality:

Physical Properties of Cairncrossite

Vitreous, Pearly
Diaphaneity (Transparency):
Transparent, Translucent
Colour:
Colourless
Comment:
Appears white
Streak:
White
Hardness (Mohs):
3
Tenacity:
Sectile
Cleavage:
Perfect
|| (001)
Comment:
"sectile before brittle fracture"
Density:
2.846 g/cm3 (Calculated)

Crystallography of Cairncrossite

Crystal System:
Triclinic
Class (H-M):
1 - Pinacoidal
Space Group:
P1
Cell Parameters:
a = 9.6265(5) Å, b = 9.6391(5) Å, c = 15.6534(10) Å
α = 100.89(1)°, β = 91.27(1)°, γ = 119.73(1)°
Ratio:
a:b:c = 0.999 : 1 : 1.624
Unit Cell Volume:
V 1227.08 ų
Z:
1
X-Ray Powder Diffraction Data:
d-spacingIntensity
15.27 (or 15.23)
5.091 (or 5.080)
4.198 (11)
4.136 (8)
3.818 (or 3.807)
3.164 (10)
3.071 (10)
3.061 (or 3.045)

Optical Data of Cairncrossite

Type:
Biaxial (+)
RI values:
nα = 1.518(2) nβ = 1.522(2) nγ = 1.546(2)
2V:
Measured: 33.9° (6), Calculated: 44.97°
Max Birefringence:
δ = 0.028
Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:
Low
Dispersion:
weak (r < v)
Pleochroism:
Non-pleochroic
Comments:
indicatrix orientation: Z ∧ c* = 10°

Chemical Properties of Cairncrossite

Formula:
Sr2Ca7(Si4O10)4(OH)2·15H2O

idealized structural formula: Sr2Ca7–xNa2x(Si4O10)4(OH)2(H2O)15–x (x ~ 0-1);
simplified formula of Na-rich crystals: SrCa3Na(Si4O10)2(OH)(H2O)7 (Z = 2).
All elements listed in formula:
Analytical Data:
Electron microprobe, wt.%; empirical formula basis: 16 Si apfu & TGA analysis
Na2O 3.06
K2O 0.11
CaO 18.61
SiO2 54.91
SrO 11.75

total 88.44 
Empirical Formula:
Sr1.99K0.02Ca5.81Na1.73Si16O55.84H30.33
Common Impurities:
Na

Relationship of Cairncrossite to other Species

Member of:
Other Members of Group:
GyroliteNaCa16Si23AlO60(OH)8 · 14H2O
OrlymaniteCa4Mn3Si8O20(OH)6 · 2H2O
TungusiteCa4Fe2Si6O15(OH)6

Other Names for Cairncrossite

Name in Other Languages:

Other Information

light-blue, intense (SWUV)
Thermal Behaviour:
25–400oC: two-phase endothermic weight loss of water vs nearly complete rehydration capability up to 400oC
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Cairncrossite

Reference List:
Giester, G., Lengauer, C.L., Pristacz, H., Rieck, B. and von Bezing, K.-L. (2013) Cairncrossite, IMA 2013-012. CNMNC Newsletter No. 16, August 2013, page 2703; Mineralogical Magazine, 77, 2695-2709.

Pristacz, H., Giester, G., Rieck, B., Lengauer, C.L. (2013): Thermal behaviour of the new mineral Cairncrossite, Sr2Ca7(Si4O10)4(OH)2·15H2O. Poster, MinPet 2013, September 19-23; abstract in Mitt. Österr. Mineral. Ges. 159 (in press) [http://erdwissenschaften.uni-graz.at/aktuelles/veranstaltungen/minpet2013/downloads/ThirdCircular2013_Final.pdf]

Giester, G., Lengauer, C.L., Pristacz, H., Rieck, B., Topa, D., Von Bezing, K.L.(2016): Cairncrossite, a new Ca-Sr (-Na) phyllosilicate from the Wessels Mine, Kalahari Manganese Field, South Africa. European Journal of Mineralogy 28, 495-505.

Internet Links for Cairncrossite

Localities for Cairncrossite

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
(TL) indicates type locality for a valid mineral species. (FRL) indicates first recorded locality for everything else. ? 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.
South Africa (TL)
 
  • Northern Cape Province
    • Kalahari manganese field
      • Hotazel
Giester, G., Lengauer, C.L., Pristacz, H., Rieck, B. and von Bezing, K.-L. (2013) Cairncrossite, IMA 2013-012. CNMNC Newsletter No. 16, August 2013, page 2703; Mineralogical Magazine, 77, 2695-2709.; Giester, G., Lengauer, C.L., Pristacz, H., Rieck, B., Topa, D., von Bezing, K.-L. (2016): Cairncrossite, a new phyllosilicate from the Wessels Mine, Kalahari Manganese Field, South Africa. European Journal of Mineralogy, 28, 495-505
Mineral and/or Locality  
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