"Rock of the Month #196, posted for October 2017" ---

Galena

is one of the most familiar ore minerals, noted for its cubic crystal forms, sub-metallic grey lustre, and serious heft (density: specific gravity 7.5). It is soft (Mohs 2.5) with a grey-black streak. This month's specimen is from the Madan lead-zinc ore field in the Rhodope Mountains of Bulgaria, just north of the frontier with Greece. The history of mining in the Madan region extends back at least as far as the Thracians in the fifth century B.C. (Petrussenko, 1991). The recrystallized, part-dissolved or (apparently) "melted" crystal habit of galena has been recovered from this region, including the Deveti Septemvri (9th of September) mine. I first became aware of this unusual form of galena in a mineral specimen of the John and Jim Reimer collection at the University of Waterloo Earth Sciences Museum, southwest Ontario. That piece shows a particularly striking crystal habit of modified cm-size cubes that seem almost molten and rounded, with 5% fine-grained, dark green, granular sphalerite, from the Petrovitsa mine, Madan district.

The Madan ore field is indeed noted for some unusual crystal habits of galena (Bonev, 1993). The lead sulphide is often found with cleophane, a transparent, low-Fe, olive-hued variety of sphalerite (Bonev and Kouzmanov, 2002). Other noted minerals from Madan include pale Mn-rich calcite from the Androvo mine (Moore, 2012), helvite (a silicate of manganese and beryllium, Larson, 2009), chalcopyrite, pyrite and quartz. The mines were developed on a series of major veins and associated skarns. The deposit types are reviewed by Vassileva et al. (2009). The main veins are simple structures following the host faults, while, to depth, stockworks with sulphide veinlets occur in volumes of intense hydrothermal alteration. Various forms of replacement bodies (skarns) occur where faults intersect marbles.

In all, some 50 mineral species have been reported from 35 mines in the Madan district (Petrussenko, 1991). The minerals include a variety of sulphides and sulphosalts as well as skarn-hosted silicates such as diopside- hedenbergite pyroxenes, johannsenite, rhodonite and manganilvaite (Bonev et al., 2005). The Mn-rich pyroxenes and pyroxenoids are considered typical of skarns distal from their intrusive heat sources. Madan sphalerite and galena are also noted in Moore (2016). Macroscopic fluid inclusions have been found in Madan galena. Study of these has suggested that meteoric water may be a major source of the hydrothermal fluid (Piperov and Penchev, 1982).

Experimental, nanometer-scale dissolution has been observed on cleaved galena faces, relevant to the gradual breakdown of galena in surficial environments, as in mine wastes (Higgins and Hamers, 1996). Presumably the "melted" (recrystallized, or dissolved?) faces on Madan crystals may have developed at greater rates, at higher temperatures. Since the melting point of PbS at ambient pressure is 1118°C, the "molten" aspect of these samples is taken to reflect appearance, but not the mode of formation of these curious samples. Cubic sphalerite transforms to its hexagonal polymorph, wurtzite, at 1020°C, and melts at 1850°C. The rear of the sample displays some pale blue-grey euhedral faces of galena. This suggests that the sulphide has not been subject to recrystallization, let alone melting. Rather, the curious habit may be due to gradual partial dissolution of the crystals in open space (cavities known as vugs, many of which would have been exposed to late-stage hydrothermal fluids) in the Madan veins. Material within the solid mass of the vein would be shielded from the fluids that brought about the partial dissolution of exposed faces. See also Bonev (1992, 1993).

Madan galena may display a "cavernous" (nearly "melted away" / dissolved) crystal habit (Wilson, 2009, p.46), an extreme development of the more typical appearance of the mineral (from, e.g., the Deveti Septemvri and South Petrovitsa mines: Polityka, 2016, pp.100,133). David K. Joyce showed me an excellent "molten galena" from the Nanisivik Mississippi-Valley lead-zinc deposit on Baffin Island. See also diverse crystal habits of galena as displayed in the DeBruin collection, from the Millikan mine, Missouri; Huanzala, Peru; the Elmwood mine, Tennessee; Joplin, Missouri; the Buick/Magmont mine, Missouri; Tsumeb, Namibia; Dalnegorsk, Russia; and Madan (Huizing and Wilson, 2015, pp.51-53)

Galena may also exhibit spinel-law twinning, on (111) [octahedral] faces as seen, for example, at the Silvermines district, Ireland (Cooper, 2000) and at the Borieva Reka mine in Madan (Polityka, 2016). Galena also forms pseudomorphs after pre-existing minerals such as pyromorphite, a noted locality for this being the Kautenbach mine of the Mosel Valley, Germany (see, e.g., Polityka, 2016, p.110). Such pseudomorphs from this locality also figure in the early 19th century collecton of Robert Gilmor, Jr. (Wilson, 2015, p.738) and the modern Barker collection (Moore, 2017, p.22).

Closer to (Turnstone's) home, galena may also be found in the Proterozoic Grenville province (see, e.g., this galena-bearing vein from southeast Ontario). The Rossie lead mines in New York state produced large calcite and galena specimens, collected in the 1830s to 1860s. Lead isotope data can be interpreted as supporting a Jurassic age date of 186 Ma for the Rossie-type veins, with derivation of Pb from host Grenville metasediments (model age 1040 Ma: Robinson et al., 2001).

References (n=19)

Bonev,IK (1992) Dissolution and regeneration of galena crystals from the Rhodope Mountains. Abs. 29th International Geological Congress, 681, Kyoto.

Bonev,IK (1993) Non-equilibrium highly anisometric crystals and whiskers of galena. Mineral.Mag. 57, 231-240.

Bonev,IK and Kouzmanov,K (2002) Fluid inclusions in sphalerite as negative crystals: a case study. Eur.J.Mineral. 14, 607-620.

Bonev,IK, Vassileva,RD, Zotov,N and Kouzmanov,K (2005) Manganilvaite, CaFe²⁺³⁺(Mn,Fe²⁺)(Si₂O₇)O(OH), a new mineral of the ilvaite group from Pb-Zn skarn deposits in the Rhodope Mountains, Bulgaria. Can.Mineral. 43, 1027-1042.

Cooper,MP (2000) Mogul mine. Mineral.Record 31 no.6, 519.

Higgins,SR and Hamers,RJ (1996) Chemical dissolution of the galena (001) surface observed using electrochemical scanning tunneling microscopy. Geochim.Cosmochim.Acta 60, 3067-3073.

Huizing,T and Wilson,WE (2015) Mineral Collections in the American Midwest. Mineral.Record 46 no.4, supplement, 240pp.

Larson,B (2009) Sainte Marie-aux-Mines 2008. Mineral.Record 40, 65-70.

Moore,TP (2012) Ste.Marie-aux-Mines show 2012. Mineral.Record 43, 616-624.

Moore,TP (2016) Moore's Compendium of Mineral Discoveries, 1960-2015. Mineralogical Record, Inc., Tucson, 2 volumes, 809+813pp.

Moore,TP (editor) (2017) Mineral Collections in California. Mineral.Record 48 no.4, supplement, 296pp.

Petrussenko,S (1991) Minerals of the Madan orefield, Bulgaria. Mineral.Record 22, 439-445.

Piperov,NB and Penchev,NP (1982) Deuterium content of the inclusion water from hydrothermal galenas, Madan, Bulgaria: preliminary investigation. Econ.Geol. 77, 195-197.

Polityka,J (organizer) (2016) Mineral Collections in the American Northeast. Mineral.Record 47 no.4, supplement, 460pp.

Robinson,GW, Dix,GR, Chamberlain,SC and Hall,C (2001) Famous mineral localities: Rossie, New York. Mineral.Record 32, 273-293.

University of Waterloo (2015) Earth Sciences Museum. Centre for Environmental and Information Technology (EIT), University of Waterloo, museum visit, 26 November.

Vassileva,RD, Atanassova,R and Bonev,IK (2009) A review of the morphological varieties of ore bodies in the Madan Pb-Zn deposits, Central Rhodopes, Bulgaria. Geochemistry, Mineralogy and Petrology Sofia 47, 31-49.

Wilson,WE (editor) (2009) Private Mineral Collections in Texas. Mineral.Record 40 no.1, supplement, 180pp.

Wilson,WE (2015) Collector profile: Robert Gilmor, Jr. of Baltimore (1774-1848). Mineral.Record 46, 735-748.