Mineralized fossil bone

Fossil bone can be mineralized in several ways. Permineralized fossils have their original pore space infilled with minerals. Permineralization is commonly confused with petrification, in which the original material of an organism is replaced with minerals, and the pore space is infilled with minerals. In other words, petrification is a combination of permineralization and replacement.

By far, permineralization is the most common type of preservation for most fossil bone, and even when petrification has occurred, there is almost invariably evidence that permineralization occurred first (otherwise, there would be no preservation of the original cavities in the bone!). So, if you are wondering what petrified bone looks like, imagine the bone material being replaced by other minerals, sometimes preserving the fine structure of the bone, sometimes not, and the open pore spaces infilled as seen here. I plan to eventually present some truly petrified bone eventually.

In either case, the boundary between the original, open pore space and the replaced material is quite obvious, because of variations in the shape and orientation of the crystals infilling the pores. In the case of the Haversian canals of bone, this is usually indicated by concentric growth of crystals from the inner surface of the canal towards the interior, often with clear radially-arranged crystals and/or layers of different minerals at early infilling versus later stages.

For more information on bone fossilization processes, including illustrations, see Reid (1996) and Hubert et al. (1996).

Acknowledgements

My thanks to Jun Resultay for loaning some thin sections of permineralized fossil dinosaur bone.

Optical petrography trivia

All the images below are transmitted light microscope images of a thin section of rock about 20 microns thick. Two types of illumination are presented:

Plane polarized light (PPL) -- light is polarized in one orientation. This sometimes shows variations in the colour and relief of minerals depending upon their orientation, but is superficially similar to what would be seen in normal light. (Relief is the appearance of "roughness" that depends upon the index of refraction of the mineral compared to the mounting media.)
Crossed nichols (XN) -- a second polarizer is placed into the optical path at 90 degrees to the first polarizer, and after the polarized light from the first polarizer has passed through the specimen. Theoretically, crossed nichols with no specimen would result in no light reaching the camera, but many types of crystals effectively rotate and slow the light depending upon the relative orientation of the crystal lattice. Depending upon orientation, this results in interference colours, as the light travelling along different directions in the lattice interferes upon exiting the crystal.

The interference colours, relief, and other optical properties are specific to the crystal lattice parameters, and, therefore, the type of mineral and the orientation of its crystals. These two illumination modes enable the precise identification of minerals by transmitted light optical properties. Similar properties are also useful in reflected light. Terminology and explanations have been simplified for this presentation. Look up an optical mineralogy text if you are interested in more detail.

Imaging

Images were digitized using a video camera attached to a Leitz Orthoplan petrographic microscope. Images were captured at full size, reduced 60%, sharpened, and scale bars were added in Adobe Photoshop. This is the same procedure as used for the examination of specimen EC96-001. Images at the same magnification are directly comparable.

Magnification

Magnification as indicated by the lenses on the microscope is provided. Effective magnification depends upon your display, and can be determined by measuring the scale bars on screen and calculating. Scale bars were determined by a physical scale digitized using identical procedure (see above) at each magnification.

Specimen 1

This specimen is a large limb bone from a dinosaur from the Cretaceous of Alberta, Canada. It has compact (i.e. haversian) bone near its margins, and trabeculae (i.e. "marrow") towards its core. Somewhat laminar haversian bone occurs towards the outside surface (see below). The bone has been permineralized with pyrite (FeS2), calcite (CaCO3) and the chalcedony variety of quartz (SiO2). The bone itself is still preserved as calcium phosphate (probably hydroxyapatite).

compact (Haversian) bone

PPL

XN -- compact (Haversian) bone, transverse section. 20x magnification. Note the more laminar arrangement of the Haversian canals towards the margin (arrowed). The phosphatic bone material is generally dark in crossed nichols. Most of the canals are infilled with blocky calcite crystals, but a few are still open.

PPL XN -- compact (Haversian) bone, transverse section, 50.4x. The laminar structure of the bone around the Haversian canals is visible. Black dots in this area are the lacuna formerly occupied by bone cells (osteocytes). In crossed nichols, Haversian canals infilled by calcite have a bright, mottled "rainbow" of colours, wheras unfilled canals are a uniform dark blue grey.

PPL XN -- compact (Haversian) bone, transverse section, 128x. At this scale, the type and shape of the crystals infilling the Haversian canals can be clearly seen. In PPL, the crystals are clear and colourless, with a "rough", high-contrast appearance ("high relief") compared to the surrounding hydroxyapatite of the bone. Arrows point to the infilled Haversian canals.

In XN, the bone material is dark, with some concentric lighter zones arranged on either side of each canal in an "X" pattern. This is caused by the radial orientation of very tiny hydroxyapatite crystals. When parallel to the polarizers, they are dark (i.e. left and right, above and below the Haversian canal). When at an intermediate orientation, the crystals cause some light to get through, hence the light zones. This can be confirmed by rotating the specimen -- the "light" and "dark" zones keep the same orientation while the specimen rotates, because they are fixed by the optics of the microscope. Eventhough the individual hydroxyapatite crystals are too small to observe (SEM can resolve them), their presence and preferred orientation is indicated by this "extinction" pattern. Better examples of this effect are shown below for quartz.

The calcite shows up well in XN with bright "rainbows" of interference colours known as interference fringes. Although not resolvable in the digitized images of this specimen (see below for better), several "orders" of "rainbows" are visible on many of the crystals, indicating a large difference between the refractive indices at different orientations in calcite's crystal lattice. This difference is known as "birefringence", and calcite has one of the highest of the common rock forming minerals, making it easy to recognize. Note the blocky shape to the crystals, and slightly smaller grains along the inner surface of the Haversian canal. This is due to the initial random nucleation of many crystals on the open surface, and dominance of only some crystals as crystal growth continued.

XN -- compact (Haversian) bone, transverse section, 200x. At this scale, the interference fringes of the calcite crystals are clearly visible (lower arrow), although the calcite is fine enough at the upper arrow that it looks like a blur of random colour.

trabeculae (marrow)

Towards the interior of many bones, the Haversian canals of compact bone expand into large cavities, with the bone left as laminated "trabeculae". This part of the bone is commonly called "marrow". This leaves larger cavities (up to millimetres) for minerals to infill during permineralization. The larger scale of the cavities makes them easily visible with the naked eye.

PPL XN -- trabeculae, transverse section, 20x. Trabecular pores completly infilled by minerals. The bone material (arrowed in the XN image) is similar to that seen in the compact bone, but it is laminated around much larger cavities. An initial, irregular layer of opaque (black) pyrite is present, followed by concentric growths of chalcedonic quartz. There are only faint outlines of these in PPL because of the low relief of quartz. Also visible in PPL are minor amounts of high relief calcite (grey, rough appearance, arrowed in the PPL image). In XN, the radial nature of the quartz is much more obvious. See below for more detail.

XN -- trabeculae infill, transverse section, 50.4. Spectacular "cross extinction" is visible in the quartz crystals, where the needle-like elongated crystals are parallel (dark) or not parallel (light grey) one or the other microscope polarizer. Also note the concentric light and dark variations, caused by very slight changes in the orientation and size of the quartz crystals as they grew. Note that these patterns match between each of the radial quartz growths. This occurs because the variations in growth were likely caused by chemical changes in the solutions precipitating the quartz, which would simultaneously affect the growth of the crystals around the entire cavity. Arrows point to the black, opaque pyrite rim. Note the way the quartz growths preferentially nucleated on projections in the pyrite rim.

XN -- trabeculae, transverse section, 128x. Closer view of one of the radial growths of quartz, resolving individual needle-like quartz crystals.

PPL

XN -- trabeculae transitional to Haversian canal, transverse section, 50.4x. This cavity is completely infilled with blocky calcite crystals. In PPL, the high relief of calcite makes the crystal boundaries obvious. In XN, the crystals are brighter or darker depending upon their orientation with respect to the polarizers. Note the way the intial nucleation of calcite growth began with small crystals (arrowed), which were replaced by fewer, larger crystals as infilling progressed.

The colours of this image are a bit off. Most of the calcite crystals have a very light pink or light brown colour in XN, but the brightness of the image has partially saturated the camera and muted the colours.

XN -- detail of calcite infilling, 128x, showing the interference fringes (arrowed) characteristic of calcite. At least five orders of interference spectra are visible, versus the one order of interference colours typical of quartz (see quartz examples above and below).

PPL

XN -- trabeculae, transverse section, 20x. The open space of the trabeculae has been infilled with an initial layer of opaque (black in PPL) pyrite, calcite (grey and high relief in PPL), and concentric growth of the chalcedony variety of quartz (low relief, clear in PPL, radial crystals of first-order interference colours in XN). In XN, the formation of a concentric layer of chalcedonic quartz around the inner surface of most of the pores is obvious (arrows). The lower arrowed example has the remaining space infilled by a large calcite crystal, whereas the upper arrowed cavity is still partially open.

PPL XN -- trabeculae, transverse section, 50.4x. The laminar bone structure and lacunae are visible in the bone in PPL, as are the fine, radial growths of quartz on the inner surface of the pore space. In XN, the fine needle-like radial growths of the quartz are more obvious, and the interference colours range from first-order grey, through white, to a maximum of yellow (this section is a bit thick, so the colours reach slightly higher level than normal). This type of radial, elongated, concentric growth of quartz is known as the variety chalcedony. This is the type of quartz that forms many types of agate.

The smaller cavities (arrowed) have been completely infilled by quartz, whereas the the larger ones have only developed a layer, with remaining pore spaces open or infilled with calcite.

XN -- trabeculae, transverse section, 128x. A closer view of the smaller infilled pore, similar in size and structure to the Haversian canals of compact bone.

XN -- trabecalae, transverse section, 128x. A closer view of the incomplete infilling of radial crystals of chalcedonic quartz on the interior of a larger pore.

Specimen 2

Another specimen, also from the Cretaceous of Alberta, but with larger Haversian canals in its compact bone, and without preservation of the fine hydroxyapatite crystal details.

XN -- compact (Haversian) bone, transverse sectin, 20x. The Haversian canals of this specimen have been infilled with a layer of opaque pyrite, followed by blocky calcite crystals.

Implications

From these specimens, the following observations can be made:

Bone mineralization textures are very distinct from most other types of mineralization, because they concentrically infill tubular cavities (Haversian canals). This would preserve indication of the former presence of the canals, even if surrounding material were completely replaced, with none of the structure of the bone itself preserved.
The textures of the typical minerals involved (calcite and quartz) are drastically different from that observed in clastic sedimentary rocks like sandstone in thin section. even if similar minerals (e.g., quartz) are major components.
In general, it is not possible to confuse the infilling of Haversian canals with other types of structure, except perhaps other types of fossils that also produce tubular cavities at this scale (e.g., the vascular systems of plants -- e.g., permineralized or petrified wood), because the infilling of former cavities is so distinctive.
Haversian canals must be infilled prior to replacement of fossil bone in order for the canals to be preserved. (This is common sense, but it seemed worth mentioning anyway :-))

References

Reid, R.E.H., 1996. Bone histology of the Cleveland-Lloyd dinosaurs and of dinosaurs in general, Part I: Introduction: Introduction to bone tissues. Brigham Young University, Geology Studies, v.41, p.25-71.

Hubert, J.F.; Panish, P.T.; Chure, D.J.; and Prostak, K.S., 1996. Chemisty, microstructure, petrology, and diagenetic model of Jurassic dinosaur bones, Dinosaur National Monument, Utah. Journal of Sedimentary Research, v.66, no.3, p.531-547.

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Andrew MacRae macrae@geo.ucalgary.ca