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Agate in Thin Section

Generally, the first examination of a new piece of rock will be the hand specimen; this is easy with a hand lens but often a weathered sample will reveal only minimal information. Thin sections are frequently used for a secondary examination of a rock or mineral. There are few minerals that remain opaque when ground down to 0.03 mm : the size of  standard thin section. However, it is not so much the transparency that is important but the structure that is revealed when the thin section is examined in polarised light.

Thin sections of bones and teeth had been made for several years before Henry Sorby, the “father of petrology” made the first of several hundred rock thin sections in 1849. The basic process is quite simple although the skill of the professional thin section technician is high. Nevertheless, rock thin sections can be made at home and the softer rocks can be ground down by hand on a glass plate. A standard rock thin section is produced when the section is ground down to 30 µm ; at this thickness most rock-forming minerals become transparent. Unfortunately, the hardness of agate makes hand grinding nonviable. The lapidary will have a diamond saw, lap and perhaps a simple biological microscope. A cheap biological microscope can be readily converted for petrological use where a new world will be revealed.

Converting a biological microscope and examining rock sections My own home choices for the conversion are a second hand Beck microscope (probably made in the 1960’s) and a cheap Chinese microscope that I use for examining thin sections during the messy preparatory and grinding work. Petrological microscopes have a number of extra features. The main difference between a biological and petrological microscope is the ability to examine transparent rocks in polarised light. Polaroid is available from photographic suppliers and is needed to convert a biological microscope. Two small pieces of polaroid are required with one piece cut and placed in the filter carrier or held by Blue-Tack below the substage; this behaves as the polarizer. Any rock section that is now examined is being viewed in plane polarised light. The appearance of many minerals including agate will be little different whether being examined in plane polarised light or ordinary light.
A second piece of polaroid (analyzer) should be fitted to an old 35 mm film case (or other suitable tube) that has been centrally drilled with a 15 mm hole. The film case is placed over the eyepiece and twisted until darkness is produced. These conditions are required to examine agate in thin section: the polars are now in the crossed position. Effectively, the passage of plane polarised light through an agate can illustrate a wide variety of textures. When the polars are crossed, every agate will show new textural features that cannot be observed in the hand specimen or a thin section in ordinary light.  Even the most bland agate can often reveal a very complex microstructure that is worthy of further study.

Rock thin sections The basics can be simply described as grinding a flat face on a rock and finishing with silicon carbide grit (600). The rock is given a flat face starting with 100, then 200 and finishing with 600 silicon carbide.  The whole sample must be clean and dry before being stuck to a microscope slide; the adhesive is important and I use “2 Ton Epoxy resin”. Clamp and leave for 24 hrs and then trim the surplus rock to approximately 1 to 2 mm.  The slab is then ground down to 30 µm.  In the case of agate or quartz, the 30 µm is reached when the polarisation colours are grey. However, the necessary frequent checking under the microscope leaves a wet and gritty microscope stage and I will often use a digital  micrometer to check the thickness. It will be obvious that you do not use a quality microscope for making thin sections.
Two examples demonstrate some of the microstructures that are revealed by agate in thin section

1) Mexican agate
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Fig. 1 This Mexican agate has an initial ~ 1 cm of alternating red and yellow banding followed by ~ 1cm of clear chalcedony. The central part of the agate is empty. a)  a 2 mm slab of the agate before grinding. b)  the same agate in thin section and viewed with the polaroids in the crossed position. (Scale: bottom edge ~ 4 cm)
The thin section reveals a number of features:

  • There is an initial growth of ~ 2 mm that has a different structure compared to the rest of the agate. This is shown at its best at the bottom right hand corner of each micrograph.
  • The iron oxide staining has persisted even at 30 µm.
  • After the initial growth, further development is via a number of sheaf-like growths that only cease when eliminated by a more dominant growth: best shown in the upper section b).
  • Although the red and clear sections in Fig. 1a) suggest that this is the result of two distinct growths, the thin section shows that growth is continuous and only distinguished by the iron oxide staining.

2) Botswana agate
The agate in Fig.2 a) shows the strong white banding that has formed in a Botswana agate.  This mushroom-shaped agate will have growth development in the direction of the arrows. For some reason, the growth of banding is severely limited at the base of agate and any separation of the white banding here seems to be non existent. An interesting question would be the nature of the texture along d-d1 compared to the banding around C.
Normal agate growth is fibrous and the fibres twist very much like the twists in a rope. The darkness that is observed in the typical fibrous chalcedony is due to the optical axis twisting away from the viewer. Hence, it could be expected that the thin section at C would be similar to the observations made in the Mexican agate in Fig.1. However, the observations made on a thin section cut along d-d1 would similar to viewing the cross section of a cut in a piece of rope.
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Fig. 2 The arrows show the direction of growth in this Botswana agate. Two thin sections have been made a) Fig. 3 is a section made from a cut along d-d1 and Fig. 4 is normal to d-d1 and taking in area C. Scale: maximum height of agate is 6 cm.
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Fig. 3 Region C from Fig. 2. Scale Bar = 1cm; polars crossed.
The image in as expected with the initial growth different from the rest of the agate and then growth continues until band a) is reached when fresh sheaf-like growth starts again. The darker grey bands are the white bands in Fig. 2. However, the cut along line d-d1 shows a different image (Fig.3). If the eye scans from right to left, then the region to the right of the tramlines does show fibrosity while the area to the left of the tramlines appears more granular.
The right-hand section includes part of the curve of the agate just before the flat base in Fig. 2: showing fibrosity. The granular-looking area is from the base and the view shows the fibres end on. 

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image010Fig. 4 A thin section made from the face that is cut along d-d1. The area to the right of the tramlines is from the upward curved section shown  in Fig. 2. The area to the left of the tramlines is from the base shown in Fig.2. Scale bar =0.3 mm.
Thin sections of agate have implications in any discussion of agate genesis.
More details on making thin sections and photomicroscopy are discussed  in Chapter 2 of  the new book: The Science of Agate – due to be published mid 2009.