May 7 2013
Robertsite is a secondary phosphate mineral that belongs to the mineral group of mitridatite. It was discovered in 1974 from the Tip Top mine, Custer County, South Dakota, USA. It was named after Willard Lincoln Roberts, mineralogist, from the South Dakota School of Mines.
Properties of Robertsite
The following are the key properties of Robertsite:
- Cell Data
- Space Group: A2/a
- a = 17.36(2)
- b = 19.53(5)
- c = 11.30(3)
- β = 96°
- Z = 12
- Crystal Data
- Monoclinic, pseudohexagonal
- Point Group: 2/m
- Platy to wedge-shaped, pseudorhombohedral to barrel-shaped crystals, to 5 mm, showing {100}, {001}, {031}; fibrous, in botryoidal to feathery aggregates
- Twinning: By rotation ⊥ {100}, common
- X-ray Powder Pattern: 8.63(10), 2.749 (6), 5.61 (5), 1.623 (5), 3.27 (4), 2.590 (4), 2.876 (3)
- Chemical Composition
| Elements |
Content 1 |
Content 2 |
| Mn2O3 |
38.1 |
38.45 |
| P2O5 |
35.6 |
34.57 |
| CaO |
17.9 |
18.21 |
| H2O |
8.1 |
8.77 |
| Fe2O3 |
0.9 |
- |
| Total |
100.6 |
100.00 |
- Optical Properties
- Optical Class: Biaxial (–)
- Pleochroism: X = pale reddish pink; Y = Z = deep reddish brown.
- Orientation: X≈ ⊥ {100}
- Absorption: Y = Z »X
- α = 1.775(5)
- β = 1.82(1)
- Z = 1.82(1)
- 2V(meas.) = ∼8°
- Estimated Properties
| Electron density |
Bulk density (electron density)=3.08 g/cm3
note: Specific gravity of Robertsite =3.17 g/cm3 |
| Photoelectric |
PERobertsite = 9.75 barns/electron
U= PERobertsite x ρElectron density= 30.07 barns/cm3 |
| Fermion index |
Fermion index = 0.0003
Boson index = 0.9996 |
| Radioactivity |
Robertsite is not radioactive |
How to Identify Robertsite
Robertsite occurs in black, reddish brown, dark red, brownish black or blood red in color. It has good cleavage, translucent to opaque appearance, brown streak and adamantine luster. It can be formed as numerous individual crystals or clusters. The density of robersite is 3.17 g/cm3, and its relative hardness is 3.5.
Global Distribution
Robertsite is distributed in the following places:
- Tip Top mine, 8.5 km southwest of Custer, USA
- White Elephant mine, near Pringle, Custer Co, USA
- Gap Lode pegmatite, Pennington Co., South Dakota, USA
- White Picacho district, Yavapai Co., Arizona
- Stewart mine, Pala, San Diego Co., California
- Emmons quarry, Greenwood, Oxford Co., Maine
- Hagendorf, Bavaria, Germany
- Jebilet, Morocco
- Rubicon pegmatite, south of Karibib, Namibia
- Khoa Rang Kai phosphate deposit, near Chiang Mai, northwestern Thailand.
Occurrence of Robertsite and Useful Mineral Association
Robertsite occurs in phosphate deposit in limestone and complex zoned granite pegmatites. It is closely associated with clay minerals, quartz, dolomite, calcite, carbonate-fluorapatite, hureaulite, collinsite, montgomeryite, jahnsite, leucophosphite, ferrisicklerite and rockbridgeite.
References