Crystal growth can be studied:

• ... by documenting the orientation of striations on the faces of natural crystals. Striations are often produced by steps, i.e. the front of layers that spread over crystal faces in the last stage of growths. Many gem-quality mineral specimens retain quasi-pristine faces which can be examined for evidence of spiral growth and two-dimensional nucleation.
• ... by precipitating crystals in laboratory under controlled conditions to see how the presence of impurities and the concentration of major ions in the growth medium influences their morphology.
• ... by documenting evidence of the growth process left behind in minerals and their synthetic equivalents, either in the form of sector zoning or as microstructures, i.e. compositional and structural heterogeneities "buried" within single crystals because of specific growth mechanisms.

 

Techniques applied include:

atomic force microscopy

DIC microscopy

scanning electron microscopy

compositional mappping by electron probe microanalysis

cathodoluminescence (to image compositional zoning)

TEM on Pt-C replicas (with Hojatollah Vali)

computer modelling of the geometry of zoning and crystal morphology using SHAPE

 

  Hydrothermal diopside from the Orford Nickel Mine, near Brompton (Quebec), is known for its unusual flattened morphology. Some natural crystals have surfaces well enough preserved to show arrays of growth steps on the (100) face.  Kristy Thornton (kiki@eps.mcgill.ca) worked with me on an undergraduate research project focussing on this mineral, and presented her first results at GeoCanada 2000, (Calgary, June 2000).
 

This AFM topographic image shows part of a growth hillock on the diopside face (100). Straight steps parallel to the [001]- direction meet with wavy steps along the [010] direction. The smallest steps detected are about 30 angstroms high.
 

The 2 plots above show the small but systematic differences in FeO and MnO concentrations detected by electron probe microanalysis among the  different portions of one growth hillock on the pyroxene surface.  The Fe and Mn content are always slightly higher on the wavy steps [010]⁺ than elsewhere on any given growth hillock.  The difference is not large (about 0.5 wt. % FeO) but it indicates that the configuration of the cation sites at the wavy steps enhances the incoporation of impurities.

The vesuvianite project

  I recently picked up some vesuvianite crystals from the Jeffrey Mine (Asbestos, Quebec) and noticed striking V-shaped arrays of growth steps on the larger prismatic faces. Vesuvianite offers a special challenge for a student with a keen interest in some of the finer points of mineral structure and chemistry. A few papers have discussed the lowering of symmetry in some vesuvianite specimens but no one, to my knowledge, has examined this from the point of view of its growth mechanism. Dissymetrization is a phenomenon which had been related in some minerals to the relative orientation of growth steps and face symmetry. This M.Sc. project would involve a description of the surface microtopography by AFM, compositional mapping of oriented sections through some crystals and an optical microscopy study to look for evidence of a correlation between domains of lower symmetry and the arrays of surface steps.
 

The calcite project

  Calcite is my first love among minerals partly because of the stunning diversity of its morphology. Alfonso Mucci and I keep an eye for students willing to grow calcite crystals from aqueous solutions in the presence of all kinds of impurities. Mounir Temmam completed a Ph.D. thesis on the effect of Mn, Zn and Cd and of NaCl concentration on calcite morphology (Temmam et al., 2000, and two more manuscripts in preparation and review). Kristy Thornton will now tackle the effect of Pb on calcite growth for her M.Sc. project, and pursue work initiated by Jody Vaillancourt (undergraduate project, 1995).

  Al and I also team up with Hojatollah Vali who trains some of our students in the preparation of Pt-C replicas of their surfaces. This approach can be used on microcrystalline powders to image by transmission electron microscopy (TEM) the changes in crystal morphology induced by changes in the growth medium on crystallites that are barely 5-20 µm across.

  Trace elements in natural calcite are often intricately zoned, but the interpretation of zoning patterns is a frustrating exercise without a basic understanding of the relationship of crystal morphology. Students interested in carbonate diagenesis can get quite an education in the interpretation of 2-D zoning patterns by sectioning natural calcite crystals and reproducing the zoning patterns visible in cathodoluminescence using the software SHAPE.
 

The barite project

 Natural barite shows fairly diverse habits, commonly with sector zoning. Some gem-quality specimens preserve interesting arrays of growth steps, particularly ontheir {102} and {001} faces.  Luc Rock (1997, undergraduate project) documented step-specific incorporation of Sr on (102) as well as sector zoning of Sr among non-equivalent faces on a specimen donated by Dr. Peter Richards. M.Sc. student Peggy Varga extended this work to a suite of barite crystals from 4 other localities (obtained from the Canadian Museum of Nature) and mapped in each one the distribution of Sr on sections polished parallel to the (001) face.
  One of the most intriguing observations so far has been the patchy zoning of Sr within {102} and {001} sectors, and whose geometry clearly correlates with the different orientations of steps on the as-grown surface. This type of intrasectoral zoning suggests that layers nucleated at and spread out from specific surface sites, in the typical fashion of spiral growth.  Yet, Pina et al. (1998: see Nature, 395, 483-486) dismissed the possibility of spiral growth on barite on the basis of their experimental observations and theoretical calculations. Are these natural barite crystals trying to tell us that the spiral growth mechanism is possible in barite, but under conditions not yet reproduced in laboratory?