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Falcondoite mineral

Hydrous nickel-magnesium phyllosilicate mineral of the sepiolite group with current September 2026 IMA-CNMNC approved formula Ni4Si6O15(OH)2(H2O)2·4H2O (the Ni-dominant analogue of sepiolite, written [Ni8(H2O)4(OH)4Si12O30]·(H2O)8 for the 8-octahedral half-cell), status A, and approval number IMA 1976-018. Springer (1976) described the holotype specimen (ROM M 34324, Royal Ontario Museum, Toronto) from a 2 cm-wide whitish-green garnierite vein at 3 m depth crosscutting decomposed serpentinized harzburgite in the Loma Peguera lateritic nickel deposit worked by Falconbridge Dominicana C. por A. near Bonao, Dominican Republic, where it is intergrown with sepiolite, Ni-bearing talc, and serpentine. Bulk wet-chemical/XRF analysis after drying at 60 degrees C (Table 2, p. 408: SiO2 45.93, NiO 26.7, MgO 8.46, FeO 1.99, Cr2O3 0.45, Al2O3 0.39, MnO <0.01, CoO <0.01, LOI 15.0 wt%, plus 7 wt% moisture lost below 60 degrees C) and four electron-microprobe point analyses of pure falcondoite fibers (Analyses A-D, Table 2, average: SiO2 49.2, NiO 23.6 [range 20.1-27.6], MgO 9.26 [6.89-12.5], FeO <0.03, Al2O3 and Cr2O3 <0.05, H2O/LOI 18.0 wt%) gave the average probe formula {(Ni0.58Mg0.42)8(H2O)4(OH)4[Si12O30]}·(H2O)8. Powder X-ray diffraction (Table 1, p. 407) indexed on an orthorhombic sepiolite-type unit cell with a = 13.5, b = 26.9 A (printed as 26.9 A on p. 408 and misprinted as 29.9 A in the p. 407 abstract; both the reported cell volume V = 1903 A3 = 13.5 x 26.9 x 5.24 A3 and d060 = 4.48 A in Table 1 confirm b = 26.9 A), c = 5.24 A, Z = 2 (for the 8-octahedral formula) or Z = 4 (for Ni4Si6O15(OH)2(H2O)2·4H2O), Dmeas = 1.9(1) g/cm3 (low due to microporous fibrous aggregation), and Dcalc = 2.54 g/cm3 (biaxial with mean refractive index <1.55, Mohs hardness 2-3). Differential thermal and thermogravimetric analysis up to 1000 degrees C demonstrated four-stage dehydration (15.1 wt% total mass loss) followed by exothermic crystallization of orthopyroxene above 850 degrees C. Distinct from sepiolite, willemseite, pimelite, nepouite, and elemental nickel or magnesium; no criticality designation, microbial mechanism, or industrial recovery yield is asserted.

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Identifiers & Classification

System Identifier Label
IMA IMA:1976-018 Falcondoite

Microbial & Biochemical Mechanisms (1)

Falcondoite other Four-stage thermal dehydration to orth

Four-stage thermal dehydration to orthopyroxene and weak MgCl2 ion-exchange leaching of falcondoite

other

Heating natural falcondoite at 8 degrees C/min from 25 to 1000 degrees C in DTA/TGA produced four endothermic dehydration steps totaling 15.1 wt% mass loss—7.0 wt% zeolitic water lost between 25 and 200 degrees C (DTA endotherm at 180 degrees C), 3.1 wt% and 2.3 wt% structurally bound water lost across 200-370 degrees C (360 degrees C peak) and 370-600 degrees C (540 degrees C peak), and 2.3 wt% hydroxyl water lost across 600-850 degrees C (840 degrees C peak)—followed by an exothermic reaction at ~850 degrees C and solid-state recrystallization to orthopyroxene by 1000 degrees C; separately, reacting 100 mg falcondoite in 50 mL of 1 M MgCl2 solution at 60-70 degrees C for 3 weeks leached only 0.24% Ni (1.14% of total mineral Ni).

Experimental Context & Analytical Limits:
Laboratory DTA/TGA and ion-exchange experiments reported by Springer (1976), Table 3 and pp. 408-409, on Loma Peguera falcondoite pre-dried at 60 degrees C (which already removed 7 wt% loosely held moisture). The low Ni extraction in hot 1 M MgCl2 confirms that Ni2+ is octahedrally coordinated within the ribbon structure rather than weakly adsorbed on external channel surfaces.
Curated Evidence Points:
Table 3 and pp. 408-409 document the four DTA endothermic peaks (180, 360, 540, and 840 degrees C) and corresponding TGA mass losses (7.0% zeolitic H2O at 25-200 degrees C, 3.1% and 2.3% bound H2O at 200-370 and 370-600 degrees C, and 2.3% hydroxyl H2O at 600-850 degrees C; 15.1% total loss at 25-1000 degrees C) and state that heating to 1000 degrees C converts falcondoite to orthopyroxene.
Page 409 reports that soaking 100 mg falcondoite in 50 mL of 1 M MgCl2 at 60-70 degrees C for 3 weeks extracted only 0.24% Ni (1.14% of total contained Ni), showing that abiotic salt exchange alone does not substantially leach structural Ni.

Sources & References

  1. ima_2026_09: IMA-CNMNC, The New IMA List of Minerals, September 2026, PDF page 66, Falcondoite row: formula Ni4Si6O15(OH)2(H2O)2·4H2O, status A, IMA No. 1976-018, country Dominican Republic, reference Canadian Mineralogist 14 (1976), 407; column definitions on PDF page 1.
    https://cnmnc.units.it/files/editor/IMA_Master_List_(2026-09).pdf (accessed 2026-10-23)
  2. springer_1976_falcondoite: Springer G (1976), Falcondoite, nickel analogue of sepiolite, Canadian Mineralogist 14:407-409. Complete 3-page primary PDF visually inspected: Loma Peguera garnierite laterite occurrence and holotype ROM M 34324 p. 407; powder XRD Table 1 p. 407; bulk and 4-point electron-microprobe compositions Table 2, DTA/TGA 4-step dehydration and orthopyroxene transformation Table 3, IR spectroscopy, and 1 M MgCl2 leaching experiments pp. 408-409.
    https://www.rruff.net/doclib/cm/vol14/CM14_407.pdf (accessed 2026-10-23)

Curation History

Timestamp Curator Action Summary
2026-10-23T07:15:00Z antigravity CREATE Created falcondoite mineral record with verified IMA:1976-018 identity, Loma Peguera holotype ROM M 34324 chemistry, orthorhombic sepiolite-group cell, and bounded laboratory DTA/TGA dehydration and orthopyroxene crystallization mechanism from the complete 1976 primary study.

Record source: data/records/falcondoite.yaml