According to mineralogy textbooks, a mineral is a solid INORGANIC substance of natural occurrence. That definition rules out human-made simulants (e.g., cubic zirconia and nearly all mossanite --- see one of my recent previous posts). That definition also rules out organic substances secreted by organisms, even though these organic compounds have the same (or nearly same) chemical composition as “true” minerals. What to do? The answer is to use the terms biomineral, organic mineral, or biogenic mineral when dealing with organically secreted shells, teeth, bones, kidney stones, etc.
All five kingdoms and 55 phyla of organisms contain members that create biominerals. At last count, there are at least 60 known biominerals, but new ones are being discovered all the time. Most (80%) are crystalline, and some (20%) are amorphous. Many of these biominerals can be in the following groups (with examples given below): carbonates (calcite, aragonite), phosphates (dahllite, francolite), sulfates (gypsum, barite), silica (opal), iron-oxides (magnetite, goethite, ferrihydrite), iron sulfides (pyrite), halides (fluorite), and oxalates (weddelite).
Three comparisons of a biomineral versus its mineral counterpart are given below:
FIRST COMPARISON: The biomineral that makes up vertebrate (including humans) bones and teeth is called dahllite [= hydroxylapatite (or also spelled hydroxyapatite)] consisting of Ca5(PO4)3(OH), which is = 45% calcium-rich phosphate, 33% organic matrix material (mainly collagen), and 22% water.
The following image is of an Eocene mammal tooth (5.3 cm length) consisting of generally well preserved dahllite:
The mineral apatite Ca5(PO4)3(F, Cl, OH) is an inorganic substance consisting of calcium phosphate combined, in varying amounts, with fluorine, chlorine, and hydroxyl ions. The mineral apatite, furthermore, is an end member of the complex apatite group, or series, of minerals with varying amounts of F, Cl, and OH. The mineral apatite has been long been known as a calcium-phosphate series of minerals found in igneous rocks (especially in hydrothermal veins) and in phosphate-rich sedimentary rocks. In summary, the biomineral dahllite [= hydroxylapatite] and the mineral apatite are chemically quite similar, and the difference is based on the content of the elements fluorine and/or chlorine, which are both subject to variation depending on the surrounding environment.
The following image is of a crystal (2 cm height) of inorganic apatite, with a hardness of 5 on the “Moh’s Scale of Hardness” (0 to 10, with 10 being the hardest = diamond).
SECOND COMPARSION: The biomineral aragonite (calcium carbonate [CaCO3 + organic matrix material]) versus the mineral aragonite (calcium carbonate [CaCO3]). Biomineral aragonite, common in some invertebrates, especially certain gastropods, bivalves, and, cephalopods (nautiloids and ammonites) can have iridescent “mother-of-pearl” luster, which I have mentioned in several of my previous posts. This luster indicates that there has been no alteration of the original composition of the shell material. The mineral aragonite never has iridescent luster, as is obvious in the image below of inorganic aragonite.
The following image is of the inside of a modern-day abalone shell (18.7 cm long) showing its “mother-of-pearl” biomineral aragonite luster. See my previous post July 28, 2020 for more details.
The next image is of the interior of a modern-day Nautilus shell (16 cm wide) showing its “mother-of-pearl” biomineral aragonite luster. See my previous post August 1, 2016 for more details.
The next image is of radiating crystal clusters of two specimens of aragonite of inorganic origin. Small cluster 2 cm wide, larger cluster 4 cm long. Impurities cause the reddish color.
THIRD COMPARISON: The biomineral opal (SiO2-xH2O) versus the mineral opal (SiO2-xH2O). Biomineral opal (transparent) is present in the test [microscopic shell] of single-cell, micro-organisms like diatoms [plant kingdom] and radiolarians [animal kingdom], as well as in glass sponges. The mineral opal, which can have an amazing array of colors but not transparency, is present in some sedimentary deposits.
The following two images are of the biomineral (opal), which forms the tests of a modern-day, microscopic radiolarians from deep-ocean sediment.
The following image is of the biomineral (opal), which forms the exoskeleton (21 cm long) of Euplectella, a modern-day glass sponge. See my March 15, 2020 post for more details.
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