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Neptunite
A valid IMA mineral species - grandfathered
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About Neptunite
Neptune
Formula:
KNa2Li(Fe2+)2Ti2[Si4O12]2
Colour:
Black to dark red
Lustre:
Vitreous
Hardness:
5 - 6
Specific Gravity:
3.19 - 3.23
Crystal System:
Monoclinic
Member of:
Name:
For Neptune, Roman god of the sea, for its close association at its type locality with aegirine (named after Aegir, a Scandinavian sea god).
The Fe(II) analogue of manganoneptunite.
Rarely a minor rock forming mineral in alkaline pegmatites. Best known in association with benitoite in natrolite veins from California.
Visit gemdat.org for gemological information about Neptunite.
Rarely a minor rock forming mineral in alkaline pegmatites. Best known in association with benitoite in natrolite veins from California.
Visit gemdat.org for gemological information about Neptunite.Unique Identifiers
Mindat ID:
2883
Long-form identifier:
mindat:1:1:2883:3
IMA Classification of Neptunite
Approved, 'Grandfathered' (first described prior to 1959)
IMA Formula:
KNa2LiFe2+2Ti4+2Si8O24
First published:
1893
Classification of Neptunite
9.EH.05
9 : SILICATES (Germanates)
E : Phyllosilicates
H : Transitional structures between phyllosilicate and other silicate units
9 : SILICATES (Germanates)
E : Phyllosilicates
H : Transitional structures between phyllosilicate and other silicate units
70.4.1.1
70 : INOSILICATES Column or Tube Structures
4 : Column or Tube Structures with linked chains forming cages
70 : INOSILICATES Column or Tube Structures
4 : Column or Tube Structures with linked chains forming cages
14.9.26
14 : Silicates not Containing Aluminum
9 : Silicates of Ti
14 : Silicates not Containing Aluminum
9 : Silicates of Ti
Mineral Symbols
As of 2021 there are now IMA–CNMNC approved mineral symbols (abbreviations) for each mineral species, useful for tables and diagrams.
| Symbol | Source | Reference for Standard |
|---|---|---|
| Npt | IMA–CNMNC | Warr, L.N. (2021). IMA–CNMNC approved mineral symbols. Mineralogical Magazine, 85(3), 291-320. doi:10.1180/mgm.2021.43 |
Physical Properties of Neptunite
Vitreous
Transparency:
Opaque
Colour:
Black to dark red
Comment:
Deep blood-red in splinters
Streak:
Cinnamon-brown
Hardness:
5 - 6 on Mohs scale
Tenacity:
Brittle
Cleavage:
Perfect
on {110}
on {110}
Fracture:
Conchoidal
Density:
3.19 - 3.23 g/cm3 (Measured) 3.24 g/cm3 (Calculated)
Optical Data of Neptunite
Type:
Biaxial (+)
RI values:
nα = 1.69 - 1.6908 nβ = 1.6927 - 1.7 nγ = 1.7194 - 1.736
2V:
Measured: 36° to 49°, Calculated: 36° to 56°
Max. Birefringence:
δ = 0.029 - 0.045
Based on recorded range of RI values above.
Based on recorded range of RI values above.
Interference Colours:
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
Surface Relief:
Very High (positive)
Relative to Canada balsam mounting medium (n ≈ 1.537).
Relative to Canada balsam mounting medium (n ≈ 1.537).
This shows the grain boundary and Becke line effect under plane-polarised
light, based on the contrast between this mineral's average refractive
index and the mounting medium. It does not take into account mineral
colouration.
In focus
Interference Figure:
This shows the idealized biaxial acute bisectrix (Bxa) interference figure
- the conoscopic view for a grain cut perpendicular to the acute bisectrix, using
this mineral's 2V. The two small white dots mark the melatopes - the points
where the two optic axes emerge - and are shown only when they fall within the
field of view. The coloured bands are isochromatics, and the dark bands are
isogyres.
Rotate the stage: at 0°/90° the isogyres form a cross through the melatopes; at 45° they pull apart into curved hyperbolas. That splitting on rotation - absent in a uniaxial figure - is the standard diagnostic test for telling biaxial minerals from uniaxial ones. If 2V is large, the melatopes may fall outside the field of view, as they often do at the microscope too.
Rotate the stage: at 0°/90° the isogyres form a cross through the melatopes; at 45° they pull apart into curved hyperbolas. That splitting on rotation - absent in a uniaxial figure - is the standard diagnostic test for telling biaxial minerals from uniaxial ones. If 2V is large, the melatopes may fall outside the field of view, as they often do at the microscope too.
Dispersion:
r < v strong
Pleochroism:
Visible
Comments:
X= pale yellow
Y= yellow-orange
Z= red-orange to red-brown
Y= yellow-orange
Z= red-orange to red-brown
Chemistry of Neptunite
Mindat Formula:
KNa2Li(Fe2+)2Ti2[Si4O12]2
Element Weights:
Common Impurities:
Ca
Chemical Analysis
Oxide wt%:
| 1 | 2 | |
|---|---|---|
| SiO2 | 51.53 % | 52.29 % |
| TiO2 | 18.13 % | 17.38 % |
| FeO | 10.91 % | 11.92 % |
| MnO | 4.97 % | 2.27 % |
| MgO | 0.49 % | 1.55 % |
| K2O | 4.88 % | 5.58 % |
| Na2O | 9.26 % | 6.81 % |
| CaO | 0.62 % | |
| Li2O | 1.63 % | |
| Total: | 100.17 % | 100.05 % |
Sample references:
| ID | Type | Locality | Reference | Notes |
|---|---|---|---|---|
| 1 | Type Specimen | Narssârssuk pegmatite, Narsaarsuk Plateau, Igaliku, Kujalleq, Greenland | Wet-chemical analysis (Flink), analysis incomplete, Li not analyzed | |
| 2 | California State Gem Mine, Santa Rita Peak, San Benito County, California, USA | Microprobe |
Crystallography of Neptunite
Crystal System:
Monoclinic
Class (H-M):
m - Domatic
Space Group:
Bb
Cell Parameters:
a = 16.427(2) Å, b = 12.478(2) Å, c = 9.975(1) Å
β = 115.56(1)°
β = 115.56(1)°
Ratio:
a:b:c = 1.316 : 1 : 0.799
Unit Cell V:
1,844.54 ų (Calculated from Unit Cell)
Z:
4
Morphology:
Prismatic crystals
Twinning:
Interpenetrant on {301}
Crystallographic forms of Neptunite
Crystal Atlas:
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Neptunite no.1 - {110} - Goldschmidt (1913-1926)
Neptunite no.2 - {110} - Goldschmidt (1913-1926)
Neptunite no.3 - {001} - Goldschmidt (1913-1926)
Neptunite no.16 - {110} - Goldschmidt (1913-1926)
3d models and HTML5 code kindly provided by
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Crystal Structure
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Data courtesy of the American Mineralogist Crystal Structure Database. Click on an AMCSD ID to view structure
| ID | Species | Reference | Link | Year | Locality | Pressure (GPa) | Temp (K) |
|---|---|---|---|---|---|---|---|
| 0018847 | Neptunite | Cannillo E, Mazzi F, Rossi G (1966) The crystal structure of neptunite Acta Crystallographica 21 200-208 |  | 1966 | San Benito, California, USA | 0 | 293 |
CIF Raw Data - click here to close
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 3.186 Å | (100) |
| 9.6 Å | (60) |
| 3.517 Å | (45) |
| 3.308 Å | (35) |
| 2.942 Å | (32) |
| 2.837 Å | (32) |
| 2.480 Å | (32) |
Geological Environment
Paragenetic Mode(s):
| Paragenetic Mode | Earliest Age (Ga) |
|---|---|
| Stage 4b: Highly evolved igneous rocks | >3.0 |
| 35 : Ultra-alkali and agpaitic igneous rocks |
Geological Setting:
alkaline rocks
Type Occurrence of Neptunite
General Appearance of Type Material:
Crystals from mm-sized up to 3-4 cm large, on aegirine or embedded in feldspar
Place of Conservation of Type Material:
University of Copenhagen, Copenhagen, Denmark.
Geological Setting of Type Material:
Nepheline syenite pegmatite
Associated Minerals at Type Locality:
Synonyms of Neptunite
Other Language Names for Neptunite
Relationship of Neptunite to other Species
Member of:
Other Members of Neptunite Group:
| Magnesioneptunite | KNa2Li(Mg)2Ti2[Si4O12]2 | Mon. 2/m : B2/b |
| Manganoneptunite | KNa2Li(Mn2+)2Ti2[Si4O12]2 | Mon. m : Bb |
| Rotherkopfite | KNa2Fe2+(Fe2+)2(Ti1.5Fe2+0.5)[Si4O12]2 | Mon. 2/m : B2/b |
| Watatsumiite | KNa2Li(Mn2+)2V4+2[Si4O12]2 | Mon. m : Bb |
Forms a series with:
Common Associates
Associations Based on Photo Data:
| 568 photos of Neptunite associated with Benitoite | BaTi(Si3O9) |
| 536 photos of Neptunite associated with Natrolite | Na2Al2Si3O10 · 2H2O |
| 184 photos of Neptunite associated with Joaquinite-(Ce) | NaBa2Ce2FeTi2[Si4O12]2O2(OH,F) · H2O |
| 71 photos of Neptunite associated with 'Crossite' | |
| 17 photos of Neptunite associated with Albite | Na(AlSi3O8) |
| 16 photos of Neptunite associated with Djurleite | Cu31S16 |
| 15 photos of Neptunite associated with Aegirine | NaFe3+Si2O6 |
| 10 photos of Neptunite associated with Microcline | K(AlSi3O8) |
| 8 photos of Neptunite associated with Zektzerite | LiNaZrSi6O15 |
| 6 photos of Neptunite associated with Quartz | SiO2 |
Related Minerals - Strunz-mindat Grouping
| 9.EH.05 | Manganoneptunite | KNa2Li(Mn2+)2Ti2[Si4O12]2 |
| 9.EH.05 | Magnesioneptunite | KNa2Li(Mg)2Ti2[Si4O12]2 |
| 9.EH.05 | Watatsumiite | KNa2Li(Mn2+)2V4+2[Si4O12]2 |
| 9.EH.05 | Rotherkopfite | KNa2Fe2+(Fe2+)2(Ti1.5Fe2+0.5)[Si4O12]2 |
| 9.EH.10 | Grumantite | Na(HSi2O5) · H2O |
| 9.EH.15 | Sarcolite | Na4Ca12Al8Si12O46(SiO4,PO4)(OH,H2O)4(CO3,Cl) |
| 9.EH.20 | Ussingite | Na2AlSi3O8OH |
| 9.EH.25 | Telyushenkoite | (Cs,Na,K)Na6[Be2Al3Si15O39]F2 |
| 9.EH.25 | Leifite | (Na,H2O)Na6[Be2Al2(Al,Si)Si15O39]F2 |
| 9.EH.25 | Eirikite | KNa6Be2(Si15Al3)O39F2 |
| 9.EH.30 | Nafertisite | Na3Fe2+10Ti2(Si6O17)2O2(OH)6F(H2O)2 |
| 9.EH.35 | Veblenite | KNa(Fe2+5Fe3+4Mn7)Nb4(Si2O7)2(Si8O22)2O6(OH)10(H2O)3 |
Radioactivity
Radioactivity:
| Element | % Content | Activity (Bq/kg) | Radiation Type |
|---|---|---|---|
| Uranium (U) | 0.0000% | 0 | α, β, γ |
| Thorium (Th) | 0.0000% | 0 | α, β, γ |
| Potassium (K) | 4.3054% | 1,335 | β, γ |
For comparison:
- Banana: ~15 Bq per fruit
- Granite: 1,000–3,000 Bq/kg
- EU exemption limit: 10,000 Bq/kg
Note: Risk is shown relative to daily recommended maximum exposure to non-background radiation of 1000 µSv/year. Note that natural background radiation averages around 2400 µSv/year so in reality these risks are probably extremely overstated! With infrequent handling and safe storage natural radioactive minerals do not usually pose much risk.
Interactive Simulator:
Note: The mass selector refers to the mass of radioactive mineral present, not the full specimen, also be aware that the matrix may also be radioactive, possibly more radioactive than this mineral!
Activity: –
| Distance | Dose rate | Risk |
|---|---|---|
| 1 cm | ||
| 10 cm | ||
| 1 m |
The external dose rate (D) from a radioactive mineral is estimated by summing the gamma radiation contributions from its Uranium, Thorium, and Potassium content, disregarding daughter-product which may have a significant effect in some cases (eg 'pitchblende'). This involves multiplying the activity (A, in Bq) of each element by its specific gamma ray constant (Γ), which accounts for its unique gamma emissions. The total unshielded dose at 1 cm is then scaled by the square of the distance (r, in cm) and multiplied by a shielding factor (μshield). This calculation provides a 'worst-case' or 'maximum risk' estimate because it assumes the sample is a point source and entirely neglects any self-shielding where radiation is absorbed within the mineral itself, meaning actual doses will typically be lower. The resulting dose rate (D) is expressed in microsieverts per hour (μSv/h).
D = ((AU × ΓU) + (ATh × ΓTh) + (AK × ΓK)) / r2 × μshield
Other Information
Electrical:
Piezoelectric
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Internet Links for Neptunite
mindat.org URL:
https://www.mindat.org/min-2883.html
Please feel free to link to this page.
Please feel free to link to this page.
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References for Neptunite
Reference List:
Flink, Gust. (1893) Om några mineral från Grönland. Geologiska Föreningen i Stockholm Förhandlingar, 15 (4) 195-208 doi:10.1080/11035899309442185
Ford, W. E. (1909) Neptunitkrystalle von San Benito County, Californien. Zeitschrift für Kristallographie, 46 (1-6). 321-325 doi:10.1524/zkri.1909.46.1.321
Bradley, W. M. (1909) Analyse des Neptunits von San Benito County, Californien. Zeitschrift für Kristallographie, 46 (1-6). 516-517 doi:10.1524/zkri.1909.46.1.516
Schaller, W. T. (1911) Mineralogical notes, series 1. Bulletin 490. US Geological Survey 109 pp. doi:10.3133/b490 pp.55-57 - (Notes on neptunite)
Schaller, W. T. (1911) Krystallographische Notizen über Albit, Phenakit und Neptunit. Zeitschrift für Krystallographie, 48 (1-6). 552-560 doi:10.1524/zkri.1911.48.1.552
Berry, L. G. (1963) Neptunite; unit cell and X-ray powder data. The Canadian Mineralogist, 7 (4). 679-681
Cannilo, E., Mazzi, F., Rossi, G. (1966) The crystal structure of neptunite. Acta Crystallographica, 21 (2) 200-208 doi:10.1107/s0365110x66002627
Bancroft, G. M., Burns, R. G., Maddock, A. G. (1967) Oxidation state of iron in neptunite from Mössbauer spectroscopy. Acta Crystallographica, 22 (6) 934-935 doi:10.1107/s0365110x67001884
Laird, Jo, Albee, Arden L. (1972) Chemical composition and physical, optical, and structural properties of benitoite, neptunite, and joaquinite. American Mineralogist, 57 (1-2) 85-102
Slansky, E., Glen, R. A. (1982) Neptunite from the Woodsreef Serpentinite, New South Wales: A new occurrence. TMPM Tschermaks Mineralogische und Petrographische Mitteilungen, 30 (4). 237-247 doi:10.1007/bf01087170
Kunz, Martin, Armbruster, Thomas, Lager, GeorgeA., Schultz, ArthurJ., Goyette, RickJ., Lottermoser, Werner, Amthauer, Georg (1991) Fe, Ti ordering and octahedral distortions in acentric neptunite: Temperature dependent X-ray and neutron structure refinements and Mössbauer spectroscopy. Physics and Chemistry of Minerals, 18 (3). 199-213 doi:10.1007/bf00234004
Localities for Neptunite
Showing 63 localities.
symbol to view information about a locality.
The
Narssârssuk pegmatite, Narsaarsuk Plateau, Igaliku, Kujalleq, Greenland