Ellenbergerite occurs as purple millimetre-size grains associated with talc, kyanite, clinochlore, rutile, and zircon in composite inclusions within decimetre-large pyrope crystals (90–98 mole percent end-member) in the quartzite layer of the Dora Maira massif, Western Alps, from which coesite has been recently reported (Chopin 1984). It is hexagonal, a=12.255(8), c=4.932(4) Å, Z=1, space group P6
3. Mohs hardness 6.5; D
mes 3.15, D
cal 3.10; no cleavage. Uniaxial negative and vividly pleochroic,
colourless,
colourless to deep lilac with colour zoning. The intensely coloured variety has
1.6789(5),
1.670(1); microprobe analysis yields SiO
2 39.1, P
2O
5 0.45, Al
2O
3 25.1, TiO
2 4.0, MgO 22.2, FeO 0.20, sum 99.05 wt.% including H
2O 8.0 (coulometrically). The formula calculated on a O
28(OH)
10 basis (implying 7.5 wt.% H
2O) is Mg
6.71 Fe
0.03 Ti
0.61 Al
6.00 Si
7.92 P
0.08 O
28(OH)
10 The colour zoning is due to nearly complete Ti
Zr substitution. In addition ellenbergerite may contain more than 8 wt.% P
2O
5 with strictly correlated changes of Si, Mg, Al and Ti+Zr contents, over 80% of which represent the SiAl
PMg substitution.
The structure has been determined from 1049 observed independent reflections and refined to R=0.034, R
w=0.031, including six of ten protons. It consists of single chains of face-sharing octahedra with one third vacancies extending along the six-fold screw axes, and of pairs of fully occupied face-sharing octahedra linked by edge-sharing to form octahedral double chains parallel to the twofold screw axes, all interconnected by SiO
4 tetrahedra. It may be compared with the dumortierite polymorph with space group P6
3mc derived hypothetically by Moore and Araki (1978). The structural formula is (Mg,Ti,Zr,
)
2 Mg
6(Al,Mg)
6 (Si,P)
2 Si
6 O
28(OH)
10 Face-sharing octahedra are an unusual feature in silicates which results in a dense structure and reflects, considering the common bulk composition, the uncommon high-pressure formation conditions (about 25–30 kbar, 700–800° C). Ti
4+-Fe
2+ charge transfer between face-sharing octahedra on the six-fold screw axes most likely accounts for the absorption scheme.