During my microscopic studies, I have observed numerous skeletal form magnetite grains in andesite porphyry rocks. I wonder under what conditions they have formed.
I already know that such skeletal forms of magnetite occur in some types of (ancient copper) slags, but in natural rocks they were rather new to me!
Dear Dr. Pourkhorsandi,
Many thanks for your quick reply. Could you possibly provide me with a published reference? I would be very grateful.
Actually it has nothing to do with T and P, and only circumstantially with the cooling rate - essential being the high rate of crystallization (triggered by oversaturation) as compared to the ease of nucleation. Literature is vast, as a first step a very good basic explanation is here (and refs. therein): http://www.springerreference.com/docs/html/chapterdbid/3593.html
Many thanks Dr. Gavril Sabau! Very helpful comment.
I have also found some very interesting explanations in "A Practical Guide to Rock Microstructure, Vernon, 2004".
I would call it dendritic crystals. Their appearance is not that unusual on natural rocks (i.e. also known for pyrolusite). This type of crystal growth occurs for crystallisation from oversaturated gas or molten phase and is determined by the kinetics of growth (high number of initial nuclei, diffusion limited rate of crystal growth at insular centres of growth) and by the anisotropy of the substrate.
I agree with Gavril Săbău that it is a question of cooling and crystallization rate. You could have dendritic ore minerals in the chilled margin of pillow lavas.
Dendritic crystals form as far as I know, in the following ways:
1. as a result of strong temperature gradients
2. as a result of strong concentration gradients
3. as a combined effect of 1 + 2
This is a case of hopper growth than dendritic growth. Hopper development of octahedron is not uncommon in magnetite. Hopper growth is quite common especially in halite, bismuth and ice crystals. Rapidly growing magnetite can develop into hopper crystals
Rapidly chilled margins of basalt flows typically exhibit spectacular skeletal growth of titanomagnetite with corresponding behavior in ilmenite. Examples are given and growth patterns traced in Min. Soc. Amer. series: "Reviews in Mineralogy & Geochemistry" Vol. 25. OXIDE MINERALS.
Dear Professor Dr. Haggerty,
Many thanks for your answer and the helpful reference.
With best regards,
Those are dendritic crystals, which form when the growth is diffusion-controlled rather than surface-controlled. With diffusion-controlled growth, a concentration gradient exists, with lower concentration next to the growing surface than farther out. Rapid surface reaction depletes the nutrients faster than they can be replaced by diffusion from farther away. Consequently, any projection from the crystal enters a zone of higher concentration and supersaturation, resulting in growth. That robs nutrient from growth points farther from the tip, starving them and producing the reentrants.
With surface-controlled growth, nutrients can diffuse to the growing surface faster than they are incorporated into the surface by the surface reaction. Consequently, there is no appreciable concentration gradient from the surface outward, and no preferential growth of projections. Surface-controlled growth tends to produce blocky, not dendritic crystals. Also, do not confuse dendritic crystals, a growth form, with skeletal crystals produced by corrosion, a dissolution form.
The increase in viscosity of a silicate melt during cooling leads to slower diffusion rates of components moving toward crystallization sites. This can lead to diffusion-controlled growth, especially when cooling is rapid as in volcanic rocks. Magnetite in volcanics is often dendritic.
Chilled margin of MORBs are typically associated with skeletal olivine, Ti-magnetite and plagioclase crystals. It is a matter of fast undercooling.
In terms of conditions producing dendrites, rapid colling certainly favors dendritic growth. But Degree of undercooling is not the whole story, nor is it the mechanism by which the dendrites form. As I explained above, cooling increases melt viscosity and decreases diffucion rates, setting the stage for formation of concentration gradients near the growing surface if the surface reaction (incorporation of nutrient into the growing crystal) is faster than the supply of components via diffusion. The effects of diffusion- vs surface-control can be seen in crystal growth from hydtothermal fluids and vapors as well as from melts. The degree of undercooling affects the degree of supersaturation of the melt with respect to the phase that is crystallizing. The degree of supersaturation then affects the nucleation rate, which ultimately affects the number and size of the crystals that form, given a fixed amount of reactants. But whether those crystals grow as blocky crystals or dendrites does not directly depend on the undercooling--it depends on the diffusion rate vs surface reaction rate, which can differ from phase to phase. Undercooling plays an indirect role through decreasing the diffusion rate more than the surface reaction rate, favoring dendritic growth.