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Ameghinite mineral

Ameghinite is the approved sodium-borate mineral NaB3O3(OH)4, IMA1966-034, in the September 2026 IMA-CNMNC list. The original Tincalayu, Salta, Argentina specimens contained ameghinite nodules separated from surrounding borax by tincalconite. The 1967 chemical aliquot retained an optically estimated two percent tincalconite; its analysis is not a pure end-member composition. Dal Negro, Martin Pozas and Ungaretti's 1975 single-crystal study established a C2/c structure with isolated triborate units, each containing one boron tetrahedron and two triangles, linked through sodium coordination and hydrogen bonds. Hydrogen positions were found in difference-Fourier maps; the study did not supply a new bulk chemical assay of its fragment. The distinct sodium hexaborate tetrahydrate produced in Xiong et al.'s 2017 borax experiments was described as an ameghinite polymorph and possible precursor, not as experimentally demonstrated formation of ameghinite. This mineral species is distinct from elemental boron and the borates commodity; criticality and microbial activity are not inferred.

Identifiers & Classification

System Identifier Label
IMA IMA:1966-034 Ameghinite

Microbial & Biochemical Mechanisms (1)

Laboratory thermal alteration of natural ameghinite

other

Heating natural ameghinite produced an amorphous material, followed by a crystalline phase and melting. The original authors interpreted the initial major endotherm as water loss and the later exotherm as crystallization. These are specimen-specific thermal observations with partly unresolved product identities, not a measured boron recovery yield.

Experimental Context & Analytical Limits:
Aristarain and Hurlbut studied natural Tincalayu material. Differential thermal analysis ran in air at one atmosphere from 40 to 1000 degrees C. Figure 5 specifies a heating rate of 25 degrees C per minute, a Pt/Pt-13%Rh thermocouple and Al2O3 reference. Thermal sample mass, replicate count and quantified purity are not reported. The approximately two percent tincalconite estimate pertains to the chemical-analysis sample and is not assigned as a measured purity of every thermal aliquot. The separate 850-degree, 12-hour heating and air-quench experiment is not the same temperature program. No microorganisms, processing pilot or isolated boron product were tested.
Curated Evidence Points:
Figure 5 and pp. 942-943 report a major endothermic peak at 245 degrees C, interpreted as water loss, after which the material was amorphous. An exotherm at 635 degrees C accompanied a crystalline phase characterized by optical observations and listed powder-diffraction peaks. The reported melting endotherm peaked at 780 degrees C, beginning at 740 and ending at 790 degrees C. These temperatures describe this heating experiment, not demonstrated equilibrium boundaries or quantitative fractions of transformed material.
Minor endotherms at 165 and 190 degrees C were suggested to represent intermediate water-loss steps, but powder patterns showed only minor intensity changes. The authors assigned the 635-degree product to Na2O·3B2O3 while noting its refractive indices differed from an earlier synthetic compound; that assignment is not a new product-composition assay. A proposed phase associated with the 700-degree event was not found. The separate 850-degree, 12-hour experiment left two small crystalline phases in glass, too scarce for X-ray photographs or complete optical identification. Their proposed formulas remain tentative. Earlier melting values cited by this paper differ; those underlying historical experiments were not independently verified here. No mass balance or extraction efficiency is supplied.

Sources & References

  1. ima_2026_09: IMA-CNMNC, The New IMA List of Minerals, September 2026: PDF page 9, Ameghinite row, NaB3O3(OH)4, status A, approval 1966-034; page 1, definitions. Official current landing page and linked dated PDF checked 16 October 2026.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-16)
  2. aristarain_hurlbut1967: Aristarain and Hurlbut (1967), Ameghinite, Na2O·3B2O3·4H2O, a new borate from Argentina, American Mineralogist 52, 935-945. Complete eleven-page original: occurrence and Figure 1 pp. 935-936; chemistry/purification pp. 939-942, Table 5 p. 941; thermal methods and Figure 5 p. 942, results and separate 850-degree hold experiment pp. 943-944. The article ends on p. 945, despite a 935-946 range in some derivative metadata.
    https://msaweb.org/AmMin/AM52/AM52_935.pdf (accessed 2026-10-16)
  3. dal_negro1975: Dal Negro, Martin Pozas and Ungaretti (1975), The crystal structure of ameghinite, American Mineralogist 60, 879-883. Complete five-page original: methods pp. 879-880, Tables 1-2 p. 880, structure Figures 1-3 pp. 880-881, hydrogen distances Table 6 p. 882 and discussion pp. 882-883. The refinement used 1024 selected reflections from 1636 collected and reported R=0.038. The publisher deposit AM-75-007 was acquired separately as a scanned supplement.
    https://msaweb.org/AmMin/AM60/AM60_879.pdf (accessed 2026-10-16)
  4. dal_negro1975_deposit: Publisher deposit AM-75-007 accompanying Dal Negro et al. (1975), eighteen-page scanned file including the article and observed/calculated structure-factor Table 3. The table header and representative rows were visually inspected; no independent least-squares re-refinement or exhaustive numerical transcription of the deposited factors was performed.
    https://msaweb.org/MSA/AmMin/Supplemental_Data/Vol%2060%20p0879%20AM-75-007.pdf (accessed 2026-10-16)
  5. xiong2017_boundary: Xiong, Kirkes, Knox and Marrs (2017), Experimental determination of solubilities of sodium polyborates in MgCl2 solutions: solubility constant of di-sodium hexaborate tetrahydrate, and implications for the diagenetic formation of ameghinite, Canadian Mineralogist 55, 1001-1008, DOI 10.3749/canmin.1700026. Complete 27-page OSTI manuscript SAND2017-12226J inspected: methods pp. 5-6, Table 1 pp. 18-19, Figure 1 p. 25, identity/proposed conversion pp. 11-13. The distinct-polymorph experiment and model-derived constants are not measurements of ameghinite dissolution or conversion.
    https://www.osti.gov/servlets/purl/1421624 (accessed 2026-10-16)

Curation History

Timestamp Curator Action Summary
2026-10-16T08:05:01.853472Z codex (GPT-6; codex) CREATE Created ameghinite species identity and qualified 1967 thermal evidence; excluded a distinct 2017 polymorph from species mechanisms.
2026-10-16T08:10:49.759455Z codex (GPT-6; codex) EDIT Added the official scanned structure-factor deposit after acquisition; replaced the earlier access limitation without changing scientific claims.

Record source: data/records/ameghinite.yaml