In:
European Journal of Mineralogy, Copernicus GmbH, Vol. 32, No. 2 ( 2020-03-11), p. 235-249
Abstract:
Abstract. A single crystal (∼ 20 µm × 20 µm × 330 µm) of mullite-2c, a natural
polytype of mullite, was separated from a radially grown cluster of acicular
crystals from Ettringer Bellerberg (Quarternary Eifel volcanic fields,
Germany). The chemical composition determined from electron microprobe
analysis (EMPA) is
Na0.01Mg0.05Al8.52Fe0.293+Si3.13Ti0.02O19.55,
corresponding to x=0.22(8) in the generalised mineral formula
My+Mgz2+M8+4x+y-2z3+M4-4x-y+z4+O20-2x.
Only Fe3+ as foreign cation was considered in the refined structure
model, partially replacing Al3+ in the octahedral chains. A crystal of
a similar type, though exhibiting a significantly different composition with
x=0.02, was first described in 2015, tentatively named “sillimullite”
by Fischer et al. (2015). This crystal and our new
sample have similar structural properties, now classified as a polytype of
mullite, designated mullite-2c. Single-crystal X-ray diffraction showed that the mullite-2c crystal investigated here exhibits partial Si ∕ Al ordering in
the double chains of (Si,Al)O4 tetrahedra in contrast to the sample
described in 2015 as being completely ordered. The ordering in mullite-2c
results in a doubled c lattice parameter with respect to mullite. It
crystallises in space group Pnam, with cell parameters for the new sample of
a=7.5432(5) Å, b=7.7048(5) Å, c=5.7965(3) Å,
V=336.89(6) Å3 and Z=1. X-ray powder diffraction data are
presented with a detailed discussion of the differences between the
diffraction patterns of sillimanite, mullite and mullite-2c. Crystals of
mullite-2c are translucent to lightly violet, they possess a vitreous lustre
and the calculated density is 3.199 g cm−3. The optical
character is biaxial (+), with refractive indices determined by
spindle-stage microscopy of nx=1.6673, ny=1.6687 and
nz=1.680(4) (adjusted to conform to 2VZ=39(4)∘).
Applying the Gladstone–Dale approach, the compatibility index is 0.007,
representing superior compatibility. In terms of chemical composition and
structural features mullite-2c is an outstanding example of mullite-type
compounds falling into the postulated miscibility gap between sillimanite
and mullite. Its crystal structure combines characteristics from both
mullite (oxygen vacancies, triclusters of tetrahedral building units) and
sillimanite (high degree of Si ∕ Al ordering in the tetrahedral building units, causing the doubled c parameter). The lattice parameters (normalised to 1c) of the new sample lie between those of sillimanite and 3 / 2 mullite; the chemical composition is close to 3 / 2 mullite and thus differs significantly from the silica-rich composition of the species previously
determined by Fischer et al. (2015), indicating a relatively large
compositional variation.
Type of Medium:
Online Resource
ISSN:
1617-4011
DOI:
10.5194/ejm-32-235-2020
DOI:
10.5194/ejm-32-235-2020-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2020
detail.hit.zdb_id:
1000286-8
detail.hit.zdb_id:
2039451-2
Permalink
