Micro Visions Ribbons

We’ve seen curtains of microscopic bubbles before in meteorite thin sections. Readers might remember some in Greg Hupé’s NWA 6704 ungrouped achondrite, third photo. Here are others in another ungrouped achondrite that are intriguing for a couple of reasons. They often occur in multiple parallel sheets and they are sometimes quite wavy like heavy drapery.

When viewing sections we should keep in mind that we are inspecting samples of a distinct thickness taken through material at unknown orientations to features therein. For example, a glancing slice through a barred olivine chondrule with a thick igneous rim (or through a hardboiled egg) might miss the bars (or yoke). Likewise, a section through the bars might show many thin or a few apparently wide bars depending on the angle of the cut.

A section cut from drapery will produce a ribbon. Here we see multiple ribbons side by side. In NWA 7835 the angle of cut through the bubble drapery combined with the transparency of the resultant ribbon sometimes gives the appearance of helixes, augers or screws. But this is just an illusion.

Still, there are questions. What’s in the “bubbles?” When and how do they form? Elsewhere in the mineral kingdom there are superficially similar features that are probably not related but they are fun to consider.

Actual helical inclusions occur in several minerals and from a number of locations. Some of these are beryl including aquamarine from Pakistan, heliodor from Tajikistan and emerald. Also, there is spodumene from Afghanistan, natrolite from Namibia and Canada, tourmaline from Madagascar and topaz. Their mode of formation is debated but a new, perhaps definitive study of the phenomenon is underway.

Sheets of fluid filled inclusions make up the milky thread that runs through faden quartz. These are usually said to be break-and-heal formation features under conditions unlike those an achondrite would encounter.

Are our curtains of bubbles partially healed defects? Why are they wavy? What forces were at work? Is there a clue in beryl? Most materials expand as they are heated and contract as they cool. Some do the reverse. Some do both – water contracts as it cools UNTIL it reaches about 4°C and its maximum density. Further cooling expands water. Some materials have a different coefficient of expansion in each crystal axis direction (anisotropic thermal expansion). Some materials are highly anisotropic in thermal expansion. When beryl is heated from 0°C to 300°C it expands in the a-axes directions but the c-axis direction contracts. Above 300°C it expands in all directions

Our rippled ribbons probably don’t deserve a dedicated study but they are interesting to contemplate.

Ribbon-like inclusions in NWA 7835 ungrouped achondrite. Thin section in cross-polarized light.

Ribbon-like inclusions in NWA 7835 ungrouped achondrite. Thin section in oblique incident light.

Curtains of microscopic bubbles in Greg Hupé’s NWA 6704 ungrouped achondrite. Thin section in oblique incident light. Field of view is 0.3mm wide.

Ribbons, sections of curtains, side by side. Only the top of each ribbon is in focus to better illustrate the depth, i.e. thickness in the thin section. NWA 7835 Ung achondrite. Thin sections in cross-polarized light. Field of view is 0.3mm wide.

Same as above with the full thickness of the thin section in focus.

Ribbon-like inclusions in NWA 7835 ungrouped achondrite. Thin section in cross-polarized light. Field of view is 0.3 mm wide.

Ribbon-like inclusions in NWA 7835 ungrouped achondrite. Thin section in cross-polarized light. Field of view is 0.2 mm wide.

Ribbon in which the size of the individual bubbles vary, increasing toward the upper right. NWA 7835 ungrouped achondrite. Thin section in cross-polarized light. Field of view is 0.3 mm wide.

Part slice of NWA 7835 harzburgitic ungrouped achondrite from which thin sections were made. 1.037 grams.

Thin section made from part of above part slice.

Ribbon-like inclusions can be seen in the large grain on the left. NWA 7835 ungrouped achondrite. Thin section in cross-polarized light. Field of view is 3 mm wide.

Beryl var. aquamarine – Marambaia, Minas Gerais, Brazil. The crystal is traversed by a helical dislocation sheet, typical but rare. Height: 25 cm. Photo Munich 2015, Roger Warin.

Beryl var. heliodor – Yellow Water Mine, Tajikistan. Crystal contains spectacular helical inclusion. Height: 4.9 cm. Advertising for Westward Look Mineral Show 2016. Photo by Joe Budd.

Faden quartz section taken at right angle to the length of the faden viewing several curtains of defects face-on through the depth of the section. Thick section (T~100um) in oblique incident light. Field of view is 0.53mm wide

Faden quartz section with faden length running left-right, curtains of defects viewed edge-on. Thick section (T~100um) in oblique incident light. Field of view is 0.44mm wide

Faden quartz, Afghanistan. 51 mm long.

About the Author

John Kashuba
John is a natural history enthusiast living in Oregon.
Top