Wikipedia

Zigrasite

Zigrasite is a phosphate mineral with the chemical formula of MgZr(PO4)2(H2O)4. Zigrasite was discovered and is only known to occur in the Dunton Quarry[4] at Oxford County, Maine. Zigrasite was specifically found in the giant 1972 gem tourmaline-bearing pocket at the Dunton Quarry. Zigrasite is named after James Zigras[5] who originally discovered and brought the mineral to attention.

Zigrasite
General
CategoryPhosphate minerals
Formula
(repeating unit)		MgZr(PO4)2(H2O)4
IMA symbolZig[1]
Strunz classification8.CE.75
Dana classification40.1.5.2
Crystal systemTriclinic
Crystal class1 – Pinacoidal
Space groupP1
Unit cella = 5.3049(2) Å, b = 9.3372(4) Å, c = 9.6282(4) Å α = 97.348(1)°, β = 91.534(1)°, γ = 90.512(1)°
Identification
ColorOff-white to pale yellow or light tan
Crystal habitSubhedral blocky
CleavageImperfect cleavage in two directions, parallel to (010) and (001)
FractureHackly
TenacityBrittle
Mohs scale hardness3
LusterVitreous
Streakwhite
DiaphaneityOpaque
Specific gravity2.76
Density2.76–2.66
Optical propertiesUniaxial (−)
Refractive indexα 1.597(1), β 1.622 (1), γ 1.635 (1)
References[2][3]

Occurrence

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Zigrasite is found in association with tourmaline, microcline, quartz, albite, beryl, amblygonite-montebrasite, childrenite-eosphorite, and apatite. It is crystallized as one of the last minerals during pocket formation. The crystals themselves were found perched on a crystal of tourmaline from the giant 1972 pocket at the Dunton Quarry.[4] The quarry itself is located in a complex rare-element granitic pegmatite that has produced large quantities of gem tourmaline as well as many other rare phosphate species.[2]

Physical properties

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Zigrasite is an offwhite to pale yellow or light tan in color. Zigrasite exhibits a vitreous luster, and a white streak. The Mohs hardness is 3. The measured (Berman Balance) and calculated densities are 2.76(4) and 2.66 g/cm3 respectively. Zigrasite exhibits imperfect cleavage in two directions, parallel to (010) and (001), it shows no parting, is brittle and has hackly fracture. In transmitted light zigrasite is colorless and non-pleochroic, biaxial negative with indices of refraction α 1.597(1) β 1.622 (1), γ 1.635 (1). Zigrasite is translucent and shows a light blue to pale yellow cathodoluminescence.[2]

Crystal structure

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Zigrasite's atomic coordination consists of two phosphorus sites which are solely occupied by phosphorus and tetrahedrally coordinated by four oxygen anions with <P-O> distances of 1.532 and 1.533 Å. There are also two magnesium, both of which are occupied solely by Mg and are octahedrally coordinated by two oxygen anions and four (H2O) groups with <Mg-O> distances of 2.064 and 2.075 Å. There is a single zircon site solely occupied by zircon and octahedrally coordinated by six oxygen anions with a <Zr-O> distance of 2.065 Å.

The zircon octahedron shares corners with six (PO4) tetrahedra to form a pinwheel cluster. This cluster forms a fragment of a Zr(PO4)2 sheet which is parallel to the (001) direction. Alternating tetrahedra point up and down relative to the plane of the sheet. Isolated Mg(H2O)6) octahedra are arranged in layers parallel to (001). These layers are intercalated between the [Zr(PO4)2] sheets linking through the apical vertices of the (PO4) tetrahedra. This structure is formally a heteropolyhedral framework with linkage weaker in the c direction accounting for cleavage.[3]

Chemical properties

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Zigrasite is one of three compositional variations in zircon phosphate grains from the giant 1972 tourmaline pocket, representing the magnesium phase with malhmoodite representing the iron phase and an as of yet unnamed calcium-analogue. Zigrasite can be considered to be the magnesium end member for the zircon phosphate grains found in the giant 1972 pocket.[2] Zigrasite exhibits hydrogen bonding which increases linkage within the layer of magnesium octahedra.[3]

Chemical composition

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Oxide wt%
P2O5 37.59
ZrO2 32.27
HfO2 0.34
FeO 0.20
MgO 10.37
ZnO 0.17
F 0.13
LOI 18.60
Sub Total 99.67
Less OΞF2 0.05
Total 99.62

X-ray crystallography

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The powder-diffraction pattern was recorded with Cu-Ka X-radiation on a DebyeScherrer camera with a diameter of114.6 mm and a Gandolfi attachment. Refinement of the unit-cell parameters gave the following values: a = 5.321(7) Å , b = 9.360(10) Å , c = 9.660(8)Å , a = 97.38(10)º, b = 91.29(9)º, g = 90.58(9)º, V = 477.0(5) Å 3 . Unit-cell dimensions were also determined on a Bruker single-crystal diffractometer using graphite-monochromated Mo-Ka Xradiation and the resulting values (a = 5.3049(2) Å , b = 9.3372(4) Å, c = 9.6282(5) Å, a = 97.348(1)º, b = 91.534(1)º, g = 90.512(4)º) are in close agreement with those determined by powder diffraction.[3]

Intensity d-spacing hkl
100 9.550 001
10 7.489 002
40 4.589 110
50 4.411 021
70 4.108 111
50 4.008 111
30 3.569 022
20 3.273 112
40 3.177 112
10 2.805 023
10 2.688 130
30b 2.660 200
10b 2.198 042
10 2.052 222
10b 2.006 222

See also

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List of Minerals
List of minerals named after people

References

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  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c d F. C. Hawthorne, N. A. Ball, J. W. Nizamoff, W. B. Simmons; Zigrasite, MgZr(PO4)2(H2O)4, a new phosphate mineral from the Dunton Quarry, Newry, Oxford County, Maine, USA. Mineralogical Magazine ; 73 (3): 415–420. doi: https://doi.org/10.1180/minmag.2009.073.3.415
  3. ^ a b c d Hawthorne, F., & Simmons, W. (2010). The crystal structure of zigrasite, MgZr(PO4)2(H2O)4, a heteropolyhedral framework structure. Mineralogical Magazine, 74(3), 567–575. doi:10.1180/minmag.2010.074.3.567
  4. ^ a b "Dunton Gem Quarry, Newry, Oxford County, Maine, USA".
  5. ^ http://www.jameszigras.com/about.html
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