The origin of rhyolitic spherulites at Rockhound State Park

Nelia W. DunbarVirginia T. McLemore

New Mexico Bureau of Geology and Mineral ResourcesNew Mexico Tech

Diameter = 14 cm

Schematic cross section of a Rockhound Spherulite

Questions

• 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 meanatomic 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 76.6 wt.%

• Al2O3 12.8 wt.%

• K2O 4.9 wt.%

• Na2O 3.8 wt.%

• FeO 1.2 wt.%• CaO 0.5 wt.%• TiO2 0.1 wt.%

• H2O 0.1 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)

Backscattered electron imageSilica X-ray map

200 microns

Backscattered electron imagePotassium X-ray map

200 microns

Backscattered electron imageSodium X-ray map

200 microns

Backscattered electron image Potassium X-ray map

20 microns

Backscattered electron image Sodium X-ray map

20 microns

Sanidine

Anorthoclase

Albite Oligoclase Andesine Labradorite Bytownite Anorthite

Ab An

Or

0 10 202

3

4

5

6

7

K2O

Na2

O

Compositions of morphologically-different feldspar in RHP spherulite

May represent crystallization in 2-feldspar field (T<~660oC)

Schematic cross section of a Rockhound Spherulite

Corecomposed of many small, fine-grained spherulites (quartz and Na-rich alkali feldspar)

Intermediate part.Feathery quench crystalsof quartz and alkali, K-rich feldspar

Outer layered part.Rhythmic intergrowth of quartz and two feldspars

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 H2O-dominated 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