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Lacroixite
A valid IMA mineral species - grandfathered
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About Lacroixite
Alfred Lacroix at his desk
Formula:
NaAl(PO4)F
Colour:
Colourless, light yellow, light green, almost white; colourless in transmitted light.
Lustre:
Sub-Vitreous, Resinous, Dull
Hardness:
4½
Specific Gravity:
3.126 - 3.29
Crystal System:
Monoclinic
Member of:
Name:
Named in 1914 by František Slavik in honor of Antoine François Alfred Lacroix [February 4, 1863, Macon, Saône-et-Loire, France - March 12, 1948, Paris, France], a famous French mineralogist.
Dimorph of:
Frequently occurs as grains and rods in the altered margins of blocky crystals and grains of amblygonite or montebrasite embedded in granite pegmatite matrix. Not usually found in montebrasite crystals extending into cavities. Pure grains of lacroixite are uncommon.
Unique Identifiers
Mindat ID:
2310
Long-form identifier:
mindat:1:1:2310:4
IMA Classification of Lacroixite
Approved, 'Grandfathered' (first described prior to 1959)
First published:
1914
Classification of Lacroixite
8.BH.10
8 : PHOSPHATES, ARSENATES, VANADATES
B : Phosphates, etc., with additional anions, without H2O
H : With medium-sized and large cations, (OH,etc.):RO4 = 1:1
8 : PHOSPHATES, ARSENATES, VANADATES
B : Phosphates, etc., with additional anions, without H2O
H : With medium-sized and large cations, (OH,etc.):RO4 = 1:1
41.5.5.1
41 : ANHYDROUS PHOSPHATES, ETC.CONTAINING HYDROXYL OR HALOGEN
5 : (AB)2(XO4)Zq
41 : ANHYDROUS PHOSPHATES, ETC.CONTAINING HYDROXYL OR HALOGEN
5 : (AB)2(XO4)Zq
22.1.2
22 : Phosphates, Arsenates or Vanadates with other Anions
1 : Phosphates, arsenates or vanadates with fluoride
22 : Phosphates, Arsenates or Vanadates with other Anions
1 : Phosphates, arsenates or vanadates with fluoride
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 |
|---|---|---|
| Lac | 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 Lacroixite
Sub-Vitreous, Resinous, Dull
Transparency:
Translucent
Colour:
Colourless, light yellow, light green, almost white; colourless in transmitted light.
Streak:
White
Hardness:
4½ on Mohs scale
Tenacity:
Brittle
Cleavage:
Poor/Indistinct
Parallel to (111) and 111) only.
Parallel to (111) and 111) only.
Fracture:
Irregular/Uneven
Density:
3.126 - 3.29 g/cm3 (Measured) 3.29 g/cm3 (Calculated)
Optical Data of Lacroixite
Type:
Biaxial (-)
RI values:
nα = 1.546 nβ = 1.563 nγ = 1.580
2V:
Measured: 89° , Calculated: 88°
Birefringence:
0.034
Max. Birefringence:
δ = 0.034
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:
Low (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:
Slight r
Optical Extinction:
Inclined
Chemistry of Lacroixite
Mindat Formula:
NaAl(PO4)F
Element Weights:
Crystallography of Lacroixite
Crystal System:
Monoclinic
Class (H-M):
2/m - Prismatic
Space Group:
B2/b
Setting:
C2/c
Cell Parameters:
a = 6.414 Å, b = 8.207 Å, c = 6.885 Å
β = 115.47°
β = 115.47°
Ratio:
a:b:c = 0.782 : 1 : 0.839
Unit Cell V:
327.20 ų (Calculated from Unit Cell)
Morphology:
Usually massive or intergrown with other minerals. Rarely observed as fragmentary crystals.
Comment:
Pseudo-orthorhombic
Crystal Structure
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Unit Cell | Unit Cell Packed
2x2x2 | 3x3x3 | 4x4x4
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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) |
|---|---|---|---|---|---|---|---|
| 0000996 | Lacroixite | Lahti S I, Pajunen A (1985) New data on lacroixite, NaAlFPO4 American Mineralogist 70 849-855 |  | 1985 | 0 | 293 |
CIF Raw Data - click here to close
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 4.73 Å | (51) |
| 4.627 Å | (32) |
| 3.155 Å | (100) |
| 2.895 Å | (98) |
| 2.476 Å | (57) |
| 2.166 Å | (43) |
| 1.578 Å | (27) |
Comments:
ICDD 39-325
Geological Environment
Paragenetic Mode(s):
| Paragenetic Mode | Earliest Age (Ga) |
|---|---|
| Stage 4b: Highly evolved igneous rocks | >3.0 |
| 34 : Complex granite pegmatites | |
| 36 : Carbonatites, kimberlites, and related igneous rocks |
Type Occurrence of Lacroixite
General Appearance of Type Material:
intergrowths, alteration rims, druses
Place of Conservation of Type Material:
Charles University, Prague, Czech Republic, 7400;
Harvard University, Cambridge, Massachusetts, USA, 86746.
Harvard University, Cambridge, Massachusetts, USA, 86746.
Geological Setting of Type Material:
Granite pegmatites
Associated Minerals at Type Locality:
Other Language Names for Lacroixite
Dutch:Lacroixiet
German:Lacroixit
Russian:Лакруаит
Simplified Chinese:锥晶石
Spanish:Lacroixita
Traditional Chinese:錐晶石
Relationship of Lacroixite to other Species
Member of:
Other Members of Tilasite Group:
| Arsenatrotitanite | NaTi(AsO4)O | Mon. 2/m : B2/b |
| Durangite | NaAl(AsO4)F | Mon. 2/m : B2/b |
| Isokite | CaMg(PO4)F | Mon. 2/m : B2/b |
| Kononovite | NaMg(SO4)F | Mon. 2/m : B2/b |
| Maxwellite | NaFe3+(AsO4)F | Mon. 2/m : P2/c |
| Panasqueiraite | CaMg(PO4)(OH) | Mon. |
| Reznitskyite | CaMg(VO4)F | Mon. 2/m : B2/b |
| Tilasite | CaMg(AsO4)F | Mon. |
Common Associates
Associations Based on Photo Data:
| 2 photos of Lacroixite associated with Amblygonite | LiAl(PO4)F |
| 1 photo of Lacroixite associated with Berlinite | AlPO4 |
| 1 photo of Lacroixite associated with Scorzalite | Fe2+Al2(PO4)2(OH)2 |
| 1 photo of Lacroixite associated with Montebrasite | LiAl(PO4)(OH) |
| 1 photo of Lacroixite associated with 'Apatite' | Ca5(PO4)3A |
| 1 photo of Lacroixite associated with Wodginite | Mn2+Sn4+Ta2O8 |
Related Minerals - Strunz-mindat Grouping
| 8.BH. | Peterchinite | Zn3Zn2(OH)6As[O3(OH)3] |
| 8.BH. | Reznitskyite | CaMg(VO4)F |
| 8.BH. | Plumbogottlobite | PbMg(VO4)(OH) |
| 8.BH. | Cuprozheshengite | Pb4CuZn2(AsO4)2(PO4)2(OH)2 |
| 8.BH. |
Rožná Hradisko, Rožná pegmatite, Rožná, Žďár nad Sázavou District, Vysočina Region, Czech Republic