edited by: RON DELORENZO Middle Georgia College Cochran, Georgia 31014
Was It Glass or Was ISmnrita in Me Canned Tuna?
because, although thedatashow concluliively that theslivers are not struvite, they do not show, unambiguously, that they are elass. The ouestion nosed in the titlehas not heen answered conclusively from a good scientific point of view, although - the conclusion mav have been useful for legal ourposes. Further data would need to he gathered to establish the identity of the slivers. Instructors might wish to point out to their students that this is an illustration of the continuous cycle of data and hypothesis in the scientific method. T h e rejecting of one hypothesis (the slivers are struvite) has led t o another hypothisis (the slivers are glass). his second hypothesis now needs investigation. Students could he asked to suggest ways of confirming or rejecting the new hypothesis.
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H. I. Feinstein 10411 Forest Avenue Fairfax, VA 22030
In this age of big instrumentation and high technology, often simple tests with relatively inexpensive equipment can answer interesting practical questions. Several yearsago a student showed me a number of slivers whichsheand her h u s h ~ n dhad found in a can oftuna. As far as they could tell, the pieces were glass fragments. When they reported this to the various individuals and agencies responsible for the control of the quality of the tuna, they were assured that the slivers were "com~letelvharmless struvite crystals".' Struvite is a mineralogical name for M g N H ~ P 0 4 . 6 H z 0( I , 2). I was well acquaintid with the compound, having analyzed many silicate and carbonate rocks and portland cements. Struvite is insoluble in water and will form under proper condition^.^ It is therefore not too uncommon to find struvite in canned seafood. Without testing, however, it is not wise to assume that unexpected matter in seafood is struvite. I therefore made some relatively simple tests and suhmitted the following report: ~
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Literature Cited 1. Ps1ache.C.; BerrnwH.: Frondel,C.Dano'sSystsmofMimology, 7th ed.: Wiley: New York, 1951:Vol. 2. p 715. 2. Lsnen,E.S.; Beman. H.TheMicraseopicDelerminoiionofLhrNonopngu~Miner& 2nd ed.; Bulletin 848, US.Gealogiesl Survey, G.P.O.: Washington, DC, 1934:p 99. 3. Chemot, E. M.: Maaon. C. W. Handbook of Chemical Mieroreopy, 3rd ed.: Wiley: New York. 1958; Vol. 1. p 314. 4. Corning Glass Works. Properties of S&el@d Commerriol Olosaos; B-83IRev.1, Corning: New York, 1960; p9. 5. Borgstmm, Georg. Principlrsaffood Science; MacMillan: New York. 1968:Vol. 11.
(1) Three discolored slivers were submitted with a request to deter-
mine whether the sample was struvite ( M ~ N H I P O ~ ~ H ~ O ) . According to p 70 of ref 5. "Struvite. Small crystals, a harmless chemlcal substance, magnesium ammonium phosphate, also reand, if it was not, to identify it. ferred to as struvite, often gradually form in fish and shellfish packs. (2) The discoloration varied from gray to black. These pieces had The constituents of these crystals are necessary for normal health; the fallowing masses: 19 mg, 82 mg, and 167 mg. they cause no harm if swallowed." (3) The slivers appeared vitreous, had a hardness comparable to The K,, for struvite is 2.5 X 10-13at 25 OC. If the constituent ions that of glass, and had conchoidal fracture. Of this compound are present in sufficient concentration and in the (4) The following tests were made and compared to the properties proper environment such as pH and ratio, struvite may precipitate. of struvite as given in the literature: From my experience, it does have a tendency to supersaturate. See 1, p 716, under Occur., p 717, # I 3 references to Palache. Also, ref Test Result Strwite seep 70 in ref 5. References 1-4 are those i actually used. Since then a third Solubility in 6 M HNOs insoluble soluble edition of ref 2 has been published under the authorship of Fleischer, Density. g cm+ 2.7 1.7 Michael; Wilcox, R. E.; J. J. Matzko. J. J. Bull. 1627,U.S.G.S., G.P.O.,: LOSS on ignition (meker).% 0.6 54.7 Washington. DC. 1984. Extensive references are collected on p 402(5) The loss on ignition was apparently due to a small amount of 406. A 4th ed. of ref 3 has been published as Handbook of Chemical organic matter, which probably accounted for the discoloration. Microscopy, by Clyde Walter Mason; Wiley: New York, 1982: Vol. 1. Thousands of references are scattered throughout the 448 pages of The residue from loss on ignition appeared t o be clear fused glass. this edition. While Corning may have published more recent editions (6) Examinationwith a polarizing microscope showed that the samthe tables are usually the same for the standard of B E 3 (Rev.) (4, ole was aoticallv isotrooie with a refractive index between 1.510 olasses. " and 1.520 (I41' using Cargille Index of Reframon Liquids. The terms Isotropic and anisotropic used by chemical microStrurire i repurted t~ be oprically anisotropic (hiaxial. orthoscopists and optical mineralogists refer to optical propenies such as rhamhic).' refractive index of nonopaque suostances. The refractive index of an (7) The results of the above tests indicate that the sample suhmitisotropic substance has one value independent of the direction ofthe ted is not struvite but is in all probability a soft (sodalime)glass. incident light with respect to the orientation of the substance. An anisotropic substance exhibits one or more indices depending upon After copies of this report were transmitted to the conthe direction of the incident liaht and the orientation of the substance. cerned parties, denials that the slivers were glass abruptly A cnemical or a petrographi&icroscope (with polarizer an0 analyrstopped. Note, however, that the investigation is not yet over er) is usea. NaCl is isotrop c, str-vite is anisotrop c See refs 2ana 3. ~
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Volume 65 Number 10 October 1988
887
