Helvine

helvine

genthelvite

dalalite

topaz

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Formula: Be3Mn2+4(SiO4)3S
Tectosilicate (framework silicate), helvine group, beryllium and manganese bearing mineral, forms series with genthelvite and with danalite
Specific gravity: 3.2 to 3.44 measured, 3.23 calculated
Hardness: 6 to 6½
Streak: White
Colour: Golden yellow, brown, red, grey-yellow, yellow-green
Luminescence: Red cathodoluminescence
Solubility: Dissolves slowly in boiling hydrochloric acid (1:1), forming a silica gel and hydrogen sulphide
Common impurities: Al,Fe,Zn,Ca
Environments

Plutonic igneous environments
Pegmatites
Metamorphic environments

Helvine may be locally abundant in granites, granite pegmatites, gneisses, contact zones and skarns. Associated minerals include garnet, magnetite, fluorite, chlorite, diopside and vesuvianite (HOM).

Localities

At Loudervielle, Bagnères-de-Bigorre, Hautes-Pyrénées, Occitanie, France, helvine has been found in a zone of manganiferous mineralisation associated with layers of jasper. Associated minerals include rhodochrosite, rhodonite, quartz, tephroite, spessartine, friedelite, braunite, stilpnomelane, chlorite, alleghanyite, sonolite, hübnerite, alabandite, sphalerite and welinite. The crystals of helvine are greenish lemon-yellow, up to 1 cm in length, and exhibit red cathodoluminescence.
There is a complete solid solution between helvine and danalite, and between danalite and genthelvite. Intermediate compositions between genthelvite and helvine are lacking (CM 32.1.111-120).

There are two co-type localities, the Brüder Lorenz mine and the St. Richard Mine, both at Breitenbrunn, Breitenbrunn, Erzgebirgskreis, Saxony, Germany.

At Lexington mine, Walkerville, Butte Mining District, Silver Bow County, Montana, USA, a specimen of helvine was found, which weighs about four pounds, and is largely rhodonite and rhodochrosite, through which sporadic grains of sulphide minerals are scattered. The sulphides include pyrite, galena and sphalerite, but no alabandite was found. Helvine occurs as sparse lemon-yellow grains up to 2 mm across, and as veinlets about 1 mm wide in the midst of the pink manganese minerals.
In the examined sections, helvine occurs only in or near the rhodonite veinlets with which sulphides are associated, and it was, therefore, one of the last minerals to be deposited (AM 22.6.803–804).

In New Hampshire, USA, helvine group minerals usually occur in NYF (niobium Nb, yttrium Y and fluorine F) pegmatites, although occurrences at the Iron Mountain mine in Bartlett and the Madisin lead mine in Madison are exceptions (R&M 97.3.221-224).

At Grandview Mine, Otero county, New Mexico, USA, helvine has been found in a zinc replacement deposit in limestone. The ore minerals are sphalerite and galena with local rare concentrations of chalcopyrite. Associated minerals include garnet, epidote, serpentine, magnetite, fluorite, pyrite, quartz, calcite and chlorite, with traces of scheelite. The limestone has been recrystallised and locally contains considerable garnet.
In a specimen of fluorite from a vug in the ore body tiny tetrahedrons of yellow helvine were discovered, in part incrusting the surface of the fluorite and in part embedded in it. Sphalerite was also present. The crystals of helvine are up to a millimeter across and smaller ones form clusters of interlocking crystals up to 3 millimeters across, scattered sparsely on the fluorite (AM 33.648-649).

At Discovery Gulch, Iron Mountain No. 2 District, Sierra County, New Mexico, USA, most of the known coarsely crystalline helvine occurs in small bodies of tactite along a contact between recrystallised limestone and a plug of coarsely porphyritic rhyolite. Smaller amounts have been found in tactite lenses that lie in recrystallised limestone at and near contacts with masses of aplite and fine-grained granite. The iron-rich host rock for the helvine is composed of magnetite and fluorite, with minor amounts of biotite, chlorite and diopside. Garnet is rare to absent. The tactite is characteristically layered. Some of the bands consist of solid helvine, but most helvine-bearing layers are aggregates of fluorite and helvine; the intervening layers are solid magnetite or biotite-chlorite aggregates 3 to 10 mm thick. Irregular pods of coarsely crystallized helvine, with maximum dimensions of 1 inch by 3 inches by 4 inches, are present but are not common.
The helvine is amber-brown to mahogany-red with a vitreous to resinous luster. Much of it is stained dark brown or nearly black by iron and manganese oxides. Most of the helvine is massive, but well-formed crystals occur in vugs, commonly associated with quartz and fluorite. Although most of the crystals are less than 4 mm across, some reach 12 mm on an edge (AM 29.169-171).

At the North End area, Iron Mountain No. 2 District, Socorro county, New Mexico, USA, A small quantity of helvine was found that has an index of refraction lower than that of the more typical and more abundant helvine from Discovery Gulch; this indicates a higher manganese and lower iron-plus-zinc content. These lower index grains were found in crushed material and their association is not known (AM 29.169).

At the Miller Mine, Granite Mining District, Beaver County, Utah, USA, the country rock consists of marble and tactite, both of which are intruded by granite dikes. The freshest dike rock consists of quartz, orthoclase and oligoclase, together with minor mica and chlorite. Accessory minerals include magnetite, fluorite and allanite(?).
Helvine and beryl occur in close proximity both in altered dike rock and in sugary-textured marble. Neither has been identified by the writer in tactite. The largest helvine fragments were obtained from brecciated marble on the footwall of the dike, and beryl was found in close association. Helvine was obtained also from altered dike rock on the lowest level of the northeriy shaft. The helvine is in parts of the dike that contain abnormal amounts of mica, a black uranium mineral, and such sulphide minerals as sphalerite, galena and chalcopyrite. Fluorite is common and at places constitutes several per cent of the rock. Topaz was identified in several thin sections, and magnetite locally is relatively abundant. Carbonate minerals are abundant throughout the altered dike. Secondary uranium minerals coat the fractures in the dike at several places.
The helvine occurs as anhedral to subhedral masses as much as 1 inch long in both dike rock and in fractured marble. Vugs in the fractured marble contain minute grains of helvine. The helvine in altered dike rock exhibits a distinct preference for dark-green mica in replacement. In one specimen of altered dike rock, helvine replaces carbonate formed from altered feldspar, and hence it appears that the helvine is later than the general alteration of the dike. The helvine is tawny-colored with a vitreous lustre. It contains small grains of a black, opaque mineral, possibly magnetite, and locally small grains of glassy topaz (AM 47.395-398).

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