The origin of rhyolitic spherulites at
Rockhound State Park
Nelia W. Dunbar
Virginia T. McLemore
New Mexico Bureau of Geology and Mineral Resources
New Mexico Tech
Diameter = 14 cm
Schematic cross section of a Rockhound Spherulite
Questions
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•
•
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What are the spherulites made of?
What does the visible structure represent?
How do spherulites form?
Why are some hollow?
Electron microprobe used to produce:
-Backscattered electron images (map of mean
atomic number)
-X-ray maps to show element distribution
-Quantitative chemical analyses of spots as
small as 1 micron
Minerals found in spherulites
• Quartz (SiO2)
• Two alkali feldspars (K, Na)[AlSi3O8]
• Magnetite (Fe3O4)
Composition of rhyolitic magma
• SiO2
• Al2O3
• K2O
• Na2O
• FeO
• CaO
• TiO2
• H2O
76.6
12.8
4.9
3.8
1.2
0.5
0.1
0.1
wt.%
wt.%
wt.%
wt.%
wt.%
wt.%
wt.%
wt.%
Backscattered Electron image of spherulite core (field of view 1.2 mm)
BSE image of interior spherulite and medial “rapid-growth” crystals FOV 2.5 mm
BSE image of transition between “rapid-growth” and banded zone FOV 2.5 mm
Backscattered Electron image of spherulite (field of view 1.2 mm)
Silica X-ray map
Backscattered electron image
200 microns
Potassium X-ray map
Backscattered electron image
200 microns
Sodium X-ray map
Backscattered electron image
200 microns
Backscattered electron image
20 microns
Potassium X-ray map
Backscattered electron image
20 microns
Sodium X-ray map
Or
S an i d i n e
An o rt h o cl ase
Al b i t e Ol i g o cl ase An d es i n e L ab rad o ri tBe y t o w n i t eAn o rt h i t e
Ab
An
Compositions of morphologically-different feldspar in RHP spherulite
May represent crystallization in 2-feldspar field (T<~660oC)
7
6
Na2O
5
4
3
2
0
10
K2O
20
Schematic cross section of a Rockhound Spherulite
Outer layered part.
Core
Rhythmic intergrowth of
composed of
quartz and two feldspars
many small, fine-grained
spherulites
(quartz and Na-rich alkali feldspar)
Intermediate part.
Feathery quench crystals
of quartz and alkali, K-rich feldspar
Why are some spherulite hollow?
Observations:
• Within a single lava flow, some spherulites may be hollow
whereas others are solid. In some cases, there appears to
be some stratigraphic control on location of hollow vs.
solid spherulites
• In some hollow spherulites, the original solid form appears
to have been expanded from within to form the void space.
• Some “solid” spherulites contain many small, finely
dispersed void spaces, which appear to be small bubbles.
Why are some spherulite hollow?
Speculation:
• Rhyolite magma contains 0.1 wt% H2O at atmospheric
pressure, whereas quartz and feldspar are anhydrous.
Crystallization would cause water to come out of solution
and form bubbles.
• From a simple ideal gas law calculation, at atmospheric
pressure, 0.1 wt.% H2O would generate void space equal
to 10 times the initial volume of crystallizing melt, ample
to create the void space found in spherulites.
• Creation of a void space requires coalescence of this H2Odominated vapor phase. This may depend on some critical
combination of crystal growth rate and pressure at which
the spherulite forms.
Conclusions
• Spherulites are composed of quartz, feldspar and magnetite
• Spherulites grew at high temperatures from a rhyolitic
magma, and the internal structure is controlled by
crystallization dynamics.
• Crystals near the core of the spherulite show texture typical of
rapid crystal growth
• Diffusion of elements at the crystal-melt interface may be
responsible for banding
• By analogy to experimental systems, spherulites may have
grown in periods of days to weeks
• The cavities could have been formed by H2O vapor generated
during crystallization