CHERRY BLOSSOM STONES
One of the most famous & visually intriguing
geologic materials collected in Japan is the cherry blossom stone.
These interesting structures have a complex geologic history.
What are they now?
Cherry blossom stones are relatively small,
subhexagonal-shaped masses of fine-grained muscovite mica that show a
flower-like pattern in transverse cross-section.
ÒCherry blossom stoneÓ (9 mm across) - pinite
(= muscovite mica replacing intergrown cordierite-indialite) from Kameoka, west
of Kyoto, Kyoto Prefecture, southwestern Honshu Island, southern Japan.
Specimen donated by Nicole Anderson.
What did they used to be?
The muscovite mica is not the original material
making up these structures. Before the growth of muscovite mica, these
were complex intergrowths of six cordierite crystals and one indialite
crystal. So, cherry blossom stones represent muscovite mica replacing
cordierite-indialite (muscovite pseudomorphs after cordierite-indialite).
Such complex pseudomorphs have been referred to as pinite.
ÒCherry blossom stonesÓ - pinite (=
muscovite mica replacing intergrown cordierite-indialite) from Kameoka, west of
Kyoto, Kyoto Prefecture, southwestern Honshu Island, southern Japan.
Limper Geology Museum specimens (Miami University, Oxford, Ohio, USA).
ÒCherry blossom stonesÓ - pinite (=
muscovite mica replacing intergrown cordierite-indialite) from Kameoka, west of
Kyoto, Kyoto Prefecture, southwestern Honshu Island, southern Japan.
Limper Geology Museum specimens (Miami University, Oxford, Ohio, USA).
ÒCherry blossom stonesÓ - pinite (=
muscovite mica replacing intergrown cordierite-indialite) from Kameoka, west of
Kyoto, Kyoto Prefecture, southwestern Honshu Island, southern Japan.
Limper Geology Museum specimens (Miami University, Oxford, Ohio, USA).
What is the host rock?
Cherry blossom stones are hosted in a matrix of hornfels,
a fine-grained, contact metamorphic rock. Hornfels form by intense
alteration (heating & chemical alteration) of shales by nearby lava or
magma.
Hornfels with pinite (= muscovite mica replacing intergrown
cordierite-indialite; Òcherry blossom stonesÓ), Tamba Group, Mesozoic (Triassic
to lowermost Cretaceous), ~98 m.y. contact metamorphic date.
Locality:
at or near Mikata, Mikata District, Fukui Prefecture, Honshu Island, southern
Japan.
Limper Geology Museum specimen (Miami University, Oxford,
Ohio, USA).
Hornfels with pinite (= muscovite mica replacing intergrown cordierite-indialite;
Òcherry blossom stonesÓ), Tamba Group, Mesozoic (Triassic to lowermost
Cretaceous), ~98 m.y. contact metamorphic date.
Locality:
at or near Ashio, Tochigi Prefecture, Honshu Island, central Japan
Limper Geology Museum specimen (Miami University,
Oxford, Ohio, USA).
Hornfels with pinite (= muscovite mica replacing intergrown
cordierite-indialite; Òcherry blossom stonesÓ), Tamba Group, Mesozoic (Triassic
to lowermost Cretaceous), ~98 m.y. contact metamorphic date. Limper
Geology Museum specimen (Miami University, Oxford, Ohio, USA).
How did they form?
The hornfels host rocks were originally fine-grained
siliciclastic sedimentary rocks (shales) of the Tamba Group
(Triassic-Jurassic-lowermost Cretaceous). In the mid-Cretaceous (early
Cenomanian Stage, ~98 m.y.), underground igneous activity resulted in granites
and granodiorite intrusions altering the shales into hornfels by contact
metamorphism. These hornfels had decent-sized masses of intergrown
cordierite-indialite.
Indialite is a magnesium aluminosilicate mineral (Mg2Al4Si5O18).
Cordierite is an iron magnesium aluminosilicate mineral ((Fe,Mg)2Al4Si5O18).
The subhexagonal-shaped masses of cordierite-indialite in the hornfels consist
of seven individual crystals. At the center of each mass is a
dumbbell-shaped indialite crystal - very narrow at the center, and relatively
wide at the ends (look at the varying sizes of the center hexagon in the
cherry blossom stones shown above). Surrounding the indialite crystal are
six prism-shaped cordierite crystals. They are widest at the center of
each cherry blossom stone and narrowest at the ends.
A second metamorphic event altered the
cordierite-indialite masses. Hydrothermal metamorphism resulted in
fine-grained muscovite mica replacing the original minerals.
ÒCherry blossom stoneÓ growth history.
Diagrams composed by John Rakovan & provided by the Limper Geology Museum
(Miami University, Oxford, Ohio, USA).
ÒCherry blossom stoneÓ (9 mm across) - pinite
(= muscovite mica replacing intergrown cordierite-indialite) from Kameoka, west
of Kyoto, Kyoto Prefecture, southwestern Honshu Island, southern Japan.
The small, central, hexagonal structure represents where indialite used
to be. The ÒpetalsÓ of the cherry blossom flower (= the 6 wedge-shaped
masses surrounding the center hexagon) represent where cordierite used
to be.
Specimen donated by Nicole Anderson.
Much info. from:
Rakovan et al. (2006) - Sakura Ishi (cherry
blossom stones): mica pseudomorphs of complex cordierite-indialite intergrowths
from Kameoka, Kyoto Prefecture, Japan. in Minerals from
Japan. Rocks & Minerals Reprint 2006: 31-39.