Near-liquidus growth of feldspar spherulites in trachytic melts: 3D morphologies and implications in crystallization mechanisms

Highlights

• First 3D analysis of spherulites in trachytic melts.

• The nucleation and growth mechanism of spherulites can be investigated through X-ray microtomography and EBSD data.

• Heterogeneous nucleation dominates the crystallization of feldspar spherulites in trachytic melts.

• Spherulites can grow in few hours at subliquidus conditions.

Abstract

The nucleation and growth processes of spherulitic alkali feldspar have been investigated in this study through X-ray microtomography and electron backscatter diffraction (EBSD) data. Here we present the first data on Shape Preferred Orientation (SPO) and Crystal Preferred Orientation (CPO) of alkali feldspar within spherulites. The analysis of synchrotron X-ray microtomography and EBSD datasets allowed us to study the morphometric characteristics of spherulites in trachytic melts in quantitative fashion, highlighting the three-dimensional shape, preferred orientation, branching of lamellae and crystal twinning, providing insights about the nucleation mechanism involved in the crystallization of the spherulites. The nucleation starts with a heterogeneous nucleus (pre-existing crystal or bubble) and subsequently it evolves forming “bow tie” morphologies, reaching radially spherulitic shapes in few hours. Since each lamella within spherulite is also twinned, these synthetic spherulites cannot be considered as single nuclei but crystal aggregates originated by heterogeneous nucleation. A twin boundary may have a lower energy than general crystal–crystal boundaries and many of the twinned grains show evidence of strong local bending which, combined with twin plane, creates local sites for heterogeneous nucleation.

This study shows that the growth rates of the lamellae (10^− 6–10^− 7 cm/s) in spherulites are either similar or slightly higher than that for single crystals by up to one order of magnitude. Furthermore, the highest volumetric growth rates (10^− 11–10^− 12 cm³/s) show that the alkali feldspar within spherulites can grow fast reaching a volumetric size of ~ 10 μm³ in 1 s.

Introduction

Spherulites are confocal radial polycrystalline aggregates that commonly occur in a wide variety of materials crystallized under highly non-equilibrium conditions (Gránásy et al., 2005, Watkins et al., 2009). In geology, spherulitic textures observed in volcanic rocks (Baker and Freda, 2001, Breitkreuz, 2013, Castro et al., 2008, Clay et al., 2012, Keith and Padden, 1963, Lofgren, 1971a, Monecke et al., 2004, Seaman, 2013, Smith et al., 2001, Watkins et al., 2009), typically consist of radiating structure that can be formed by alkali feldspar, plagioclase, cristobalite and pyroxene (Lofgren, 1971a). Polymineralic spherulitic aggregates, such as intergrowths of quartz, feldspar and magnetite (Castro et al., 2008, Seaman, 2013), or feldspar, pyroxene and biotite (Kesler and Weiblen, 1968) are quite common in silicate melts. The formation conditions of spherulitic textures in natural silicate materials are still much debated, with some studies suggesting subsolidus formation (Lofgren, 1971a) and others suggesting formation from strongly undercooled liquids (Fenn, 1977, Swanson, 1977).

Understanding the growth of spherulites as a function of temperature (T), undercooling (ΔT = T_liquidus − T_experimental), pressure (P_H2O) and superheating (− ΔT = T_{above liquidus} − T_liquidus) is critical to investigations of the physical–chemical conditions required for spherulite growth. Previous studies have shown that spherulitic shapes are strongly dependent on ∆T and cooling rate (Fenn, 1977, Lofgren, 1974). Spherulitic growth as a function of cooling could include a primary crystallization at high undercooling (∆T > 200 °C), resulting in a rapid crystallization above the glass transition temperature (T_g) (Baker and Freda, 2001, Castro et al., 2008, Clay et al., 2012, Dunbar et al., 1995, Fenn, 1977, Monecke et al., 2004, Smith et al., 2001, Swanson, 1977), or hydration and devitrification below T_g (Castro et al., 2008, Lofgren, 1971a, Lofgren, 1971b, Stasiuk et al., 1996, Swanson et al., 1989, Watkins et al., 2009).

To study the crystallization of spherulitic alkali feldspar in trachytic melts a dual approach was employed. The first one was to study three-dimensional features of spherulitic textures from a previous experimental study (Arzilli and Carroll, 2013) in order to obtain information about their shapes, morphologies of lamellae and the nucleation mechanisms. The second one was to obtain the Crystal Preferred Orientation (CPO) through electron backscatter diffraction (EBSD) technique. The latter approach employs EBSD analysis to examine the incipient stages of alkali feldspar crystallization within spherulites. The complementary nature of a technique able to provide 3D morphometric information (synchrotron X-ray computed microtomography) with a technique focused at getting crystallographical information (EBSD) through 2D images, allowed us to obtain crucial information on the nucleation mechanism (homogeneous vs heterogeneous) at the scale of 1 μm and its influence on the growth and twinning.

3D textural analysis is a powerful tool to derive crystal shapes and preferred orientations based on the morphology of the objects: a Shape Preferred Orientation (SPO). Depending on the relationship between shape and crystallographic orientation SPO and CPO might or might not be related, and the two approaches for texture analysis are complementary (Zucali et al., 2014). In this work a novel approach for the microtomographic data analysis has been employed: the synchrotron X-ray microtomography data were collected taking advantage of the coherence of synchrotron X-rays to obtain a phase contrast effect (in “near field” conditions) due to free space propagation to highlight the interfaces between feldspars and glass. This effect provides an edge enhancement that aids the visualization, compared to more “pure absorption” experimental setups (Baker et al., 2012). On the other hand, phase-contrast artifacts are a significant problem when trying to obtain volumes with binary data that commonly are the starting point for morphometric analysis. The density contrast between feldspars and glass is too weak to provide a good separation of the two materials. Single distance phase-retrieval algorithms can be employed on this kind of dataset to obtain two main results: i) the effect of the phase-contrast artifacts is canceled (in ideal cases) or at least reduced; and ii) the phase information retrieved provides a better contrast in the reconstructed images. The consequence of this processing is generally a slight blurring of images, since acquiring conditions can be quite far from the ideal ones (~ homogeneous monophase “phase objects” and perfectly monochromatic X-ray beam). These algorithms can be employed, with some caution, even on dense materials and with polychromatic light (e.g., Meyers et al., 2007). Our results show for the first time the application of such algorithms on rocks in a case where the application is crucial in providing a segmentable dataset for quantitative analysis.