CALCSILICATES Chapter 12. Mineral assemblages

The calcium-silicate minerals

General relations and mineralogy

The geologic features of the calcium-silicate units are discussed in the section entitled “The geology and structure of the zinc deposits.” This subject is complicated by the long history of strong biases in preservation and collecting. The colorful minerals and those which fluoresce in ultraviolet were likely selectively overcollected, and the less-attractive amphiboles, pyroxenes, and feldspars were likely grossly undercollected, relative to their natural abundance. Other numerous collection and preservation biases are noted in the section entitled “The specimen base.” Much of this unattractive material was used for mine-fill and road-metal, and much was discarded.




	Figure 12-19. Franklin calcium-silicate assemblage showing diopside (black), andradite (gray), and calcite (white). Specimen is 11 cm in maximum dimension. Smithsonian Institution, #147230. Photo by the author.		Figure 12-20. Calcium-silicate assemblage from Franklin showing bustamite (light-colored lower part, bearing Ca, Mn, and Si), diopside (black, bearing Ca, Mg, Fe, and Si), andradite (top-left, gray-mottled, bearing Ca, Fe, and Si), microcline (light-gray, right center, bearing K, Al, and Si), and white calcite (top left, bearing Ca, and C). Note the segregated occurrences of these minerals. Specimen is 11 cm in maximum dimension. Smithsonian Institution, #147233. Photo by the author.

The calcium-bearing silicates occur in much greater species diversity at Franklin (Figures 12-19 through 12-23) than at Sterling Hill (Figures 12-24 and 12-25). The observations in this section are largely from such biased hand-specimens (Figures 12-19 through 12-23). The sparse descriptive literature on calcium silicates from these deposits is largely but not wholly restricted to Franklin material.




	Figure 12-21. Silicate assemblage from Franklin, consisting of microcline (white), franklinite (black), and andradite (gray). Specimen is 14 cm in maximum dimension. Smithsonian Institution, #R18302. Photo by Vic Krantz.		Figure 12-22. Calcium-silicate assemblage from Franklin consisting of vesuvianite (abundant gray), bustamite (lath-like at left), and andradite (dark-gray patches). Platy small black “crystals” are vesuvianite which has replaced a pre-existing mica. Specimen is 13 cm in maximum dimension. Smithsonian Institution, #R3636-1. Photo by Vic Krantz.

The average chemical composition of the calcium silicate units at Franklin is given earlier in this section. Frondel and Baum (1974) noted that the silicates in the ore units (tephroite, manganese-humites, willemite, and esperite) all have cation-to-silicon ratios greater than 1 or 2; these ratios are higher than those for silicates in the calcium silicate units. Frondel and Baum remarked that “the presence of [such] minerals in the ore units and their absence in the calcsilicate units may mark an increasing content of siliceous material in a passage from the purely oxidic or carbonate-containing precipitates of the original deposit, that yielded the massive franklinite-willemite-zincite ore, to the relatively siliceous units...”




	Figure 12-23. Five Franklin specimens, four from the Smithsonian Institution, showing calcium-silicate minerals. Top left (#R3927) is pearly prehnite (white) with andradite (dark gray) and franklinite (black). Top right (#C6152-2) is wollastonite (white) with andradite (gray). Bottom left (#C4080) is willemite (white), franklinite (black), hardystonite (gray at top), andradite (dark gray, bottom left). Bottom center is slightly altered platy mica (caswellite) and granular willemite-franklinite ore at lower right; andradite occurs at contact between mica and ore. Bottom right (privately owned) is willemite with calcite (both white), and andradite mixed with platy, altered mica (caswellite) (gray). Large specimen (bottom right) is 10 cm in maximum dimension. Photo by Vic Krantz.

The calcium silicate units at Franklin contain ubiquitous calcite; many of them also contain hydroxyl and some contain water. At the Franklin deposit, the feldspars are predominantly potassian and barian; microcline is dominant, and hyalophane is common. Micas, pyroxenes, and amphiboles host much Mg. Most of the silicates, in particular the micas, pyroxenes, amphiboles, and pyroxenoids, are highly manganoan and zincian. Manganese and/or zinc are occasionally the dominant octahedral cation, giving rise to uncommon species such as hendricksite, johannsenite, petedunnite, tirodite, and others. Rhodonite is commonly present, is nearly always calcium-bearing, and is widely distributed; it is also present with andradite at the large-scale pegmatite- orebody contacts and at some marble-orebody contacts. In general, iron minerals are quite limited in occurrence; iron is almost wholly ferric and is hosted by andradite and franklinite. There is a great number of accessory minerals.

Franklinite is widely but sporadically distributed in the silicate units; willemite is less so volumetrically, yet occurs quite extensively in thin films and masses which pervasively penetrate many assemblages. In these silicate-dominant units, zincite is uncommon and sporadic in occurrence.

The phase equilibria for systems containing zinc were discussed in detail by Essene and Peacor (1987), using in part the atypical petedunnite type specimen and other assemblages as reported in the literature.

Much less is known of the silicate assemblages at Sterling Hill (Figures 12-24 and 12-25). There are markedly less calcium silicate species here and less feldspar and rhodonite in general, but pyroxenes are abundant. The micas are more magnesium-rich and iron-rich, and manganese-poor, than those at Franklin. At Sterling Hill, the two principal calcium-silicate units are spatially distinct from the economic-grade ore (Metsger et al., 1958) and were therefore not explored in detail as were the ore-bearing areas. These units are composed in part of pyroxene, amphibole, franklinite, garnet, micas, and calcite. The franklinite associated with the calcium silicates is in general more iron-rich than ore-grade franklinite. Reilly (1983) proposed that the Sterling Hill calcium silicates be considered in three groups: Mg-rich (Mn- and Zn-poor), Fe-rich (Mn- and Zn-poor), and Mn-Zn-rich (with Mg about equal to Fe); she provided discussions of these and discussed linkages of some these assemblages to ones in the Franklin Marble.




	Figure 12-24. Four specimens of calcium-silicate minerals from Sterling Hill, all from the Smithsonian Institution. Top left (#147568) is grossular (pale gray), diopside (darker gray), and calcite (white). Top right (#147558) is franklinite (black), andradite (gray), and calcite (white). Bottom left (#148373) is bustamite (gray), diopside (black), and calcite (white). Bottom right (#149018) is actinolite (dark-gray), calcite (white), and andradite (light gray). Largest specimen is 11 cm in maximum dimension. Photo by Vic Krantz.		Figure 12-25. Four specimens of calcium-silicate minerals from Sterling Hill, all from the Smithsonian Institution. Top left (#145043) is rhodonite mixed with andradite (both light gray), franklinite (dark gray on right), and calcite (white). Top right (#149014) is amphibole (gray-to-black), and calcite (white). Bottom left (#148372) is diopside (gray in center), rhodonite (light gray at right), franklinite (dark gray large blebs) and garnet (dark gray on left). Bottom right (#144268) is rhodonite (light gray), calcite (white), and pyroxene (dark gray). Largest specimen is 11 cm in maximum dimension. Photo by Vic Krantz.

The partitioning of Zn, Mn, and Fe in the various silicate assemblages is largly unstudied and is an area ripe for investigation. The extant studies of local minerals are of very limited scope; see Johnson (1991, 1994) for a discussion of zinc partitioning in ferromagnesian minerals from other localities.

Textural aspects

The principal textural feature of the calcium silicate units is that they are coarsely crystallized in general. There are no published reports of large-scale special or lamellar textures within the great lenticular units at Franklin. Grain size in these units is generally greater than in the ores, and many crystals are measured in centimeters. Large crystals are not uncommon, especially if much calcite is present, and some exceed 30 cm in size (see the section entitled “Giant crystals”).




	Figure 12-26. Andradite-grossular (white) forms a reaction zone between an intrusive mixture of feldspar (gray) with augite (black), and a willemite-franklinite ore (granular gray) from Franklin. The color-change of ore at such contacts has been ascribed to thermal effects. The visible surface is polished. Specimen is 14 cm in maximum dimension. Smithsonian Institution, #143630. Photo by the author.		Figure 12-27. A Franklin specimen showing silicate minerals invasive to franklinite-willemite ore. Microcline (white) invades franklinite (black); the reactant minerals (gray, right center) are grossular and layer silicates. The visible surface is polished. Area shown is 10 cm in maximum dimension. Smithsonian Institution, #147329. Photo by Vic Krantz.

The basic petrographic relations of the calcium silicate minerals, of zinc minerals in general, and of willemite in particular, are largely unstudied; this is a significant gap in our knowledge of these ore deposits. Studies are underway; the discussion here is, of necessity, abbreviated.




	Figure 12-28. A superb example of a coarsely-banded reaction-zone between calcium-silicate minerals and common Franklin ore. Typical granular willemite+franklinite+calcite ore is at far left. To its immediate right is calcite-free ore, followed by an interrupted band of tephroite+willemite+gahnite (dark gray) cut by a younger calcite (white) vein. These are followed by a band of andradite+rhodonite (light gray) and finally at the right by a zone of microcline (white-to-light gray, granular). This feldspar zone contains minor bustamite and fluorapatite and, at the far right, andradite with dark brown diopside. Some such specimens may show all or but part of this sequence, and it may be spatially compressed or expanded. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #R19228. Photo by Vic Krantz.

At Franklin, large-scale reactions between ore and calcium silicates, and among the calcium silicates, are moderately common; some of these were described by Frondel and Baum (1974). In the most common and largest-scale case, hardystonite occurs commonly, but not exclusively, in large amounts between calcite-rich ore units and calcite-poor ore units. Such reaction-areas also form around marble-hosted xenoliths or boudins of gneiss, and at ore-calcium silicate contacts, which may be mineralogically simple or very complex. These, or ore contacts with pegmatites, are usually characterized by the presence of much andradite and rhodonite, and both minerals also occur free of such contacts. The general example, only partially evident in some specimens, is one in which ore contacts potassic feldspar. The reactants are in zones commonly consisting of willemite-franklinite ore, tephroite+ willemite+gahnite, andradite+rhodonite, and microcline containing andradite, diopside, and minor fluorapatite and bustamite (Figure 12-28).

At Franklin, centimeter-thick reaction rims, coronal structures, and symplectites in hand-specimens are uncommon to rare, but striking when found (Figures 12-26 through 12-34). On a small scale, franklinite in particular is commonly rimmed by fine-grained andradite-grossular, which formed by the reaction of franklinite with calcium silicate solutions. If the host rock is zinc-rich, franklinite may be rimmed by willemite. Although such willemite-rimming is locally abundant in the presence of hardystonite, it is generally less common than rimming by andradite. In manganese-rich rocks, commonly characterized by the presence of tephroite, glaucochroite, rhodonite, or sonolite, franklinite may be rimmed by leucophoenicite. Much work remains to be done.

In general, such coronal structures in primary ore represent high-temperature, localized effects. In one lower-temperature case, the minehillite assemblage, apparent reaction-areas containing native lead represent frontal-advance areas of large-scale replacements in which the replacing minerals cannot accommodate Pb derived from replaced, primary, Pb-bearing silicates such as hardystonite. Tephroite is a common constituent of ore-silicate reactions involving Mn-rich phases, such as bustamite and some rhodonite, and it may form two-member symplectic intergrowths with willemite, hardystonite, or other minerals. Although the minerals in such symplectites vary, tephroite and willemite are common components. One of the least common of such intergrowths consists of green willemite in white hardystonite, with willemite rimmed on three sides by a coronal symplectite of zincite and hardystonite.




	Figure 12-29. Franklin specimen showing reaction of willemite-franklinite ore (left) with calcium silicates. This is a compressed-area replication of the zones shown in figure 12-28. Specimen is 10 cm in maximum dimension. Franklin Mineral Museum, unnumbered. Photo by the author.		Figure 12-30. Franklin bustamite (white-to-gray, at right) in contact with franklinite-willemite ore (black). The gray reaction-zone between these minerals is a symplectite of impure tephroite and willemite. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #164034. Photo by the author.




	Figure 12-31. Franklin microcline (white) with mica (black) in contact with impure caswellite (a mica replaced by garnet, dark gray); the reaction zone is grossular garnet. Few such specimens were retained, in part because of the enormous grain size of many such assemblages. Specimen is 13 cm in maximum dimension. Smithsonian Institution, #R19215. Photo by Vic Krantz.		Figure 12-32. Calcite (white) containing large fragments of willemite (light gray) is shown in contact with granular Franklin ore composed of franklinite and willemite. At the interface is a reaction zone comprised of a symplectite of tephroite and platy zincite. Specimen is 12 cm in maximum dimension. Privately owned. Photo by the author.

At Sterling Hill, rimming-assemblages and overgrowth-assemblages also are common in the calcium silicate units. Reilly (1983) noted epidote, spessartine, chlorite, andradite, amphibole, phlogopite, and tremolite as minerals which rim others, commonly pyroxenes. Johnson (1990) also illustrated a number of textures from Sterling Hill. A reaction-rim assemblage of very broad extent occurred at Sterling Hill, where tephroite crystals from 5 mm up to 15 cm are commonly rimmed by dark-brown sonolite (Figures 12-33 and 12-34). Rimming and zoned mineralization is also common, indeed abundant, in the north orebody at Sterling Hill, where pyrochroite is rimmed by sussexite, and it in turn by calcite; barite also rims sussexite here. These north orebody assemblages remain unstudied.

Alterations and replacements

Alterations and replacements of primary calcium silicates and other minerals are common at both deposits, but have been little studied; Ries and Bowen (1922) offered many early insights. In some cases such alterations generate relict pseudomorphs, as in the case of the direct replacement of hardystonite by esperite; rhodonite by serpentine; willemite by tephroite, serpentine, or talc; hendricksite or micas by garnet (caswellite) or vesuvianite; and others. Tephroite may alter to manganese oxides, a white silica residue, or both. Weathering effects are discussed separately below. In terms of preserved hand-specimens, free-standing, crystal-shaped pseudomorphs are not nearly as common here as they are at some other ore deposits; indeed, they are rare.




	Figure 12-33. Choice Sterling Hill specimen illustrating tephroite (gray) rimmed by sonolite (nearly black), associated with zincite (large blackish masses at left), franklinite (black euhedral crystals), and calcite (white). The visible surface is polished. Specimen is 12 cm in maximum dimension. Smithsonian Institution, #12965. Photo by the author.		Figure 12-34. Tephroite (medium gray) rimmed by sonolite (nearly black), with franklinite crystals (black, isolated, and equant) in calcite (white), from the 700 level, Sterling Hill. The visible surface is polished. Specimen is 7 cm in maximum dimension. Privately owned. Photo by the author.

Many assemblages result from in-place hydrothermal alteration. Examples at Franklin include the hydration of hardystonite to form clinohedrite, the hydration of esperite and glaucochroite to form willemite, larsenite and hodgkinsonite, and others. In a vuggy assemblage, johannsenite forms epitactically on rhodonite. Many other examples are given below in specific sections.

Johnson (1990) reported a few replacement textures in Sterling Hill calcium-silicate units and in the marble. Among them are: tremolite after diopside, tremolite+biotite+sphalerite after diopside, epidote after garnet, biotite+amphibole after hercynite/gahnite+clinopyroxenes, and very complex intergrowths of clinohumite+humite after tephroite. The replacement textures in some Sterling Hill willemites were described by Makovicky and Skinner (1990) and Davis (1993); additional discussion is provided herein at the end of the willemite section. The most extensive alteration and replacement zone at the deposits is the mud zone at Sterling Hill, discussed herein under saprolites and chalcophanite.

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