Investigations of the phase relations among e 1, e 2 and C {\bar {\bf 1}} structures of Na-rich plagioclase feldspars: a single-crystal X-ray diffraction study

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Vol. 73; no. 5; pp. 992 - 1006
• Main Authors: Jin, Shiyun, Xu, Huifang
• Format: Journal Article
• Language: English
• Published: 01-10-2017
• ISSN: 2052-5206; 2052-5206
• DOI: 10.1107/S2052520617010976

The subsolidus phase relations of plagioclase feldspar solid solution have been puzzling mineralogists and petrologists for decades, mainly due to the complicated structures of intermediate plagioclase at low temperature. The crystal structures of 12 Na-rich plagioclase samples are investigated by single-crystal X-ray diffraction analyses. The samples studied cover a compositional range from An
21
to An
49
(An is anorthite, CaAl
2
Si
2
O
8
), as well as a wide variety of origins, from extremely slow-cooled gabbroic rocks to pegmatite and metamorphic rocks. The structures fall into three different types:
C
{\bar 1},
e
2 and
e
1, with an obviously increasing trend in the ordering states of the structures. The phase transitions from C{\bar 1} to
e
2 and
e
2 to
e
1 are both continuous in nature, as no abrupt structure change is required for the transformation. However, the structural difference between
C
\bar 1 and
e
1 is large enough to create a miscibility gap causing the Bøggild intergrowth. As the plagioclase structure becomes more and more ordered, Al–Si reorganization in the framework would occur before the ordering of Ca and Na in
M
sites. Dramatic variations of Na occupancy would only appear in
e
1 structure with density modulation. This result confirms that Al–Si ordering is the major driving force of the formation of
e
-plagioclase structure. The composition of the lower end of the Bøggild intergrowth is precisely constrained to An
44
–An
45
, based on the structural differences between two samples from the same pegmatite crystal. The modulation periods and directions of
e
-plagioclase are dependent on the conditions at which
e
-ordering starts to happen, other than the composition of the plagioclase. However, the three components (δ
h
, δ
k
and δ
l
) of the
q
vector show strong linear correlations among one another, indicating some crystallographic constraint on the modulation direction which might be independent from the composition. The detailed subsolidus phase relations among
e
1,
e
2 and C{\bar 1} are illustrated with a local phase diagram, and schematic free energy curves at different temperatures are provided.