Julgoldite

Julgoldite
Black spheres of Julgoldite-(Fe2+)
General
Category	Sorosilicates
Formula (repeating unit)	(Ca,Mn)2(Fe2+,Fe3+,Mg)(Fe3+,Al)2(SiO4)(Si2O7)(OH)2·(H2O)
IMA symbol	Jul [1]
Strunz classification	9.BG.20
Crystal system	Monoclinic (Julgoldit-(Mg) unknown)
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	A2/m
Identification
Color	Black to greenish olive
Crystal habit	blades or prisms.
Cleavage	[100] Good
Mohs scale hardness	4.5
Luster	sub-metallic
Streak	greenish olive
Specific gravity	3.6
Optical properties	Biaxial (−)
Refractive index	nα=1.776 nβ=1.814 nγ=1.836
Birefringence	δ = 0.0600
Fusibility	loses water
References	[2] [3]

Julgoldite is a member of the pumpellyite mineral series, a series of minerals characterized by the chemical bonding of silica tetrahedra with alkali and transition metal cations. Julgoldites, along with more common minerals like epidote and vesuvianite, belong to the subclass of sorosilicates, the rock-forming minerals that contain SiO₄ tetrahedra that share a common oxygen to form Si₂O₇ ions with a charge of 6− (Deer et al., 1996). Julgoldite has been recognized for its importance in low grade metamorphism, forming under shear stress accompanied by relatively low temperatures (Coombs, 1953). Julgoldite was named in honor of Professor Julian Royce Goldsmith (1918–1999) of the University of Chicago.

Composition

The chemical formula of julgoldite is (Ca,Mn)₂(Fe²⁺,Fe³⁺,Mg)(Fe³⁺,Al)₂(SiO₄)(Si₂O₇)(OH)₂·(H₂O) (Moore, 1971). Pumpellyites are classified according to the prevailing metals in the X and Y sites (Moore, 1971). When Mg in the X position and Al in the Y position and both occupy greater than 50% molarities of their positions, then the mineral is identified as a pumpellyite (Moore, 1971). Julgoldites are identified when Fe²⁺ in the X and Fe³⁺ in the Y each occupy greater than 50% molarity of their positions (Moore, 1971).

Geologic occurrence

Julgoldites were first collected as samples entrenched in large plates of apophyllite and barite, comprising a fissure inside granular hematite-magnetite ore in Långban, Sweden (Moore, 1971). Julgoldite has since been discovered in other parts of the world: Edinburgh, Scotland (in quartz dolerite) (Livingstone, 1976) and Norilsk, Taymyr Peninsula, Russia, one of the largest nickel deposits in the world, in metamorphosed basalts and diabases associated with prehnite and laumontite (Zolotukin et al., 1965). Julgoldite has also been found exposed in basalt cavities in the Khondivili Quarry near Bombay, India along with other silicates, including pumpellyite-Fe²⁺, ilvaite, babingtonite, hydroandradite, prehnite, and chlorite (Wise and Moller, 1990). These minerals crystallized in the same basaltic cavities, which were primarily formed from gas bubbles in the compound lava flows; all of these Ca-Fe silicates formed in different phases of a low temperature environment (Wise and Moller, 1990).

Atomic structure

The atomic structures of pumpellyites and julgoldites consist of chains of edge-sharing octahedra linked by SiO₄, Si₂O₇, and CaO₇ polyhedra: in the julgoldite atomic structure, the Ca site, the W site, is a seven-coordinated site with oxygen; the X and Y are two crystallographically independent octahedral sites; and the SiO₄ site is tetrahedral (Passaglia and Gottardi, 1973). Like epidote, for which the chemical formula is Ca₂(Fe³⁺,Al)Al₂(SiO₄)(Si₂O₇)(OH), julgoldite contains additional SiO₄ tetrahedra that are independent of the Si₂O₇ structural units (Deer et al., 1996). The octahedral sites form chains along the b axis by sharing opposite edges (Allman and Donnay, 1973). The octahedral chains are joined in the ac plane by SiO₄ and Si₂O₆ (OH) groups, forming five-member rings of two octahedra and three tetrahedral (Allman and Donnay, 1973). Half of the rings are open ended and have a Ca²⁺ ion in their center; the other rings are closed, and they surround a Ca2+ ion (Allman and Donnay, 1973). The X and Y chains are parallel to the crystallographic direction [010]; therefore, the two edge sharing polyhedra cause variations in the b cell parameter (Artioli et al., 2003). Two layers of X chains and one layer of Y chains occur along the [100] direction, whereas two layers of both X and Y chains occur along the [001] direction (Artioli et al., 2003).

Physical properties

Moore (1971) sampled flat prismatic or bladed crystals, with the greatest dimension of each mineral to be no more than 2mm. The julgoldites found in the basalt cavities in India were almost 10 mm in length (Wise and Moller, 1990). Julgoldite is elongated parallel b [010] and flattened parallel a {100} (Moore, 1971). Allman and Donnay (1973) calculated the cell dimensions to be a 8.922(4), b 6.081(3), c 19.432 (9) Å. The color of julgoldite is usually a deep lustrous black, and it has a hardness of 4.5 and cleavage on the a-axis {100} (Moore, 1971). It has a greenish-olive powdery streak with a blue tinge (Moore, 1971). Under the petrographic microscope, a thin section of the mineral will display brilliant interference figures in greens or blues (Moore, 1971). The mineral is classified under the space group A2/m (Moore, 1971). A monoclinic mineral, julgoldite is isostructural to pumpellyite and epidote (Allman and Donnay, 1973).

References

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. Webmineral.com
3. Mindat.org

• Allmann, R. and Donnay, G. (1973) The crystal structure of julgoldite. Mineralogical Magazine 39, 271–281.
• Artioli, G., Geiger, C.A., and Dapiaggi, M. (2003) The crystal chemistry of julgoldite-Fe3+ from Bombay, India, studied using synchrotron X-ray powder diffraction and 57Fe Mössbauer spectroscopy. American Mineralogist 88, 1084–1090.
• Coombs, D.S. (1953) The pumpellyite mineral series. Mineralogical Magazine 30, 221, 113–133.
• Deer, W.A., Howie, R.A., and Zussman, J. (1996) An Introduction to the Rock-Forming Minerals. Prentice Hall, NY.
• Livingstone, A. (1976) Julgoldite, new data and occurrences; a second recording. Mineralogical Magazine 40, 761–763.
• Moore, P.B. (1971) Julgoldite, the Fe +2-Fe+3 dominant pumpellyite. A new mineral from Långban, Sweden. Lithos 4, 93–99.
• Passaglia, E. and Gottardi, G. (1973) Crystal chemistry and nomenclature of pumpellyites and julgoldites. Canadian Mineralogist 12, 219–223.
• Wise, W. and Moller, W. (1990) Occurrence of Ca-Fe silicate minerals with zeolites in basalt cavities at Bombay, India. European Journal of Mineralogy. 2, 875–883.
• Zolotukin, V.V., Vasilyev, Y., Zyuzin, N. (1965) Babingtonite-prehnite-pumpellyite paragenetic association in Norilsk metasomatites. Dok. Akad. Nauk. SSSR, 161, 138–141.