Pilanesbergite (original) (raw)

The Ti analogue of madeiraite.

Unique combination of elements among the approved species (at the upload time); compare UM2004-51-SiO:CaClFFeHNaNbTi.

Relation to the isomorphic species (that are also chemically related): Mn2+ ↔ Fe2+ & Zr4+ ↔ Ti4+.

Genesis:
*relatively reducing conditions
*formed from an agpaitic nepheline syenite magma, evolved via fractional crystallization of (mainly) aegirine.
A further change of mineral composition from pilanesbergite towards normandite was due to crystallization of arfvedsonite that resulted ain an increase in f(O2) and thus Mn enrichment of the magma.

RI values:

_n_α = 1.743(3) _n_β = 1.768(3) _n_γ = 1.795(5)

2V:

Measured: ~90° , Calculated: 88°

δ = 0.052

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

Comments:

straw yellow to orange red, or - if oil immersion is used - light yellow (α) / yellowish orange (γ)

Crystal System:

Monoclinic

Class (H-M):

2/m - Prismatic

Cell Parameters:

a = 10.7811(2) Å, b = 9.7836(1) Å, c = 7.0348(1) Å
β = 108.072(2)°

Ratio:

a:b:c = 1.102 : 1 : 0.719

Unit Cell V:

705.41 ų (Calculated from Unit Cell)

General Appearance of Type Material:

partly intergrown, with aegirine

Place of Conservation of Type Material:

collections of (1) NHM Økern, Natural History Museum, University of Oslo, Kabelgata 38–40, 0580 Oslo, Norway, catalogue number KNR 44406, and (2) the Laboratoire de Minéralogie et de Cristallochimie, Université de Liège, Bâtiment B18, Sart Tilman, 4000 Liège, Belgium, catalogue number 21980

Geological Setting of Type Material:

green foyaite. Type material has been sampled from a natural exposure in a river gulley near the southern margin of the complex at 25.3463∘ S, 27.0658∘ E.

Associated Minerals at Type Locality: