Robertsite

Robertsite, Ca3(Mn3+)4[(OH)3| (PO4)2]2·3(H2O) (alternatively formulated Ca2(Mn3(PO4)3O2)(H2O)3), is a secondary phosphate mineral named for Willard Lincoln Roberts (1923–1987), mineralogist and professor at South Dakota School of Mines in Rapid City, South Dakota.

Robertsite
Robertsite-Beraunite-220952.jpg
Robertsite spheres (reddish brown) on beraunite
General
CategoryPhosphate minerals
Formula
(repeating unit)		Ca3(Mn3+)4[(OH)3| (PO4)2]2·3H2O
Strunz classification8.DH.30
Dana classification42.08.04.02
Mitridatite group
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupA2/a
Unit cella = 17.36 Å, b = 19.53 Å
c = 11.30 Å; β = 96.0°; Z = 12
Identification
ColorRed, red-brown, deep red, bronzy brown, black
Crystal habitlaty to wedge-shaped, pseudorhombohedral crystals; fibrous, in botryoidal to feathery aggregates
TwinningCommon perpendicular to {100}
CleavageVery good {100}
FractureMicaceous
Mohs scale hardness​3 1⁄2
LusterVitreous to resinous or waxy
StreakChocolate-brown
DiaphaneityTranslucent
Specific gravity3.13 - 3.17
Optical propertiesBiaxial (-)
Refractive indexnα = 1.775 nβ = 1.820 nγ = 1.820
Birefringenceδ = 0.045
PleochroismStrong, X pale red to pink Y,Z deep red brown
2V angleMeasured: 8°
References[1][2][3]

The type locality for Robertsite is the Tip Top mine, Custer County, South Dakota, US. Robertsite occurs at the Tip Top Mine as minute crystal aggregates and crusts found on quartz associated with triphylite. It is a dark reddish brown to black monoclinic mineral.

It occurs as a secondary mineral in pegmatite. It is also reported from the Khoa Rang Kai phosphate deposit, Chiang Mai, Thailand in a limestone guano deposit.[2] It is associated with rockbridgeite, ferrisicklerite, leucophosphite, jahnsite, montgomeryite, collinsite and hureaulite in the type locality. In the guano deposit it occurs with carbonate-fluorapatite, calcite, dolomite, quartz and clay minerals.[2] In the Omo Valley of Ethiopia it occurs with mitridatite associated with fossil fish in Pliocene/Pleistocene lake sediments.[4]

Recently, in an exploration conducted by the Italian La Venta Geographical Association, confirmed the existence of Robertsite in the Puerto Princesa Subterranean River National Park, located about 50 kilometres (31 mi) north of the city center of Puerto Princesa, Palawan, Philippines.[5][6]

  • Mitridatite group:
    • Arseniosiderite-mitridatite series:[7]
      • Ca2(Fe3+)3[(O)2|(AsO4)3]·3H2O
      • to
      • Ca2(Fe3+)3[(O)2|(PO4)3]·3H2O
    • Arseniosiderite-robertsite series:[8]
      • Ca2(Fe3+)3[(O)2|(AsO4)3]·3H2O
      • to
      • Ca3(Mn3+)4[(OH)3|(PO4)2]2·3H2O

 


This article uses material from the Wikipedia article
 Metasyntactic variable, which is released under the 
Creative Commons
Attribution-ShareAlike 3.0 Unported License.