5. Reflected Light Microscopy

Sample preparation: Due to its low hardness, hessite does not take a good polish, consistently showing numerous polishing scratches, much like acanthite.

Its polishing hardness is the lowest among all tellurides and is greater than that of acanthite, slightly lower than that of altaite, and much lower than that of galena and native gold.

PLANE POLARIZED LIGHT – PPL

Reflection color: Gray with a subtle brownish tint.

Compared to the color of galena, the color of hessite is light brown.

Compared to the color of sylvanite, the color of hessite is darker and shows more polishing scratches.

Compared to the color of acanthite, the color of hessite is distinctly brown.

Compared to the color of altaite, the color of hessite ranges from gray-brown to reddish-gray.

Compared to the color of petzite, the color of hessite is pinkish.

Compared to the color of krennerite, the color of hessite is much darker and has an olive-green tint.

Compared to the color of native tellurium, the color of hessite is more gray.

Pleochroism: Very faint, very difficult to observe.

Reflectivity: 38.4 – 42.18%

Bireflectance:  No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Distinct anisotropy between (i) dark orange, brownish, or dark golden-brown and (ii) dark blue, bluish, or dark blue-gray.

It strongly resembles the anisotropy of enargite, but there are far more polishing scratches, and the colors are less vivid and darker.

Mottled extinction due to remnant cubic portions is possible.

Internal reflections:  No.

May be confused with: few other minerals. Hessite recognition is relatively simple, considering paragenesis, low hardness, thin lamellar twins and anisotropy.

Acanthite has weaker anisotropy.

General Characteristics:

Grain shape: Hessite can occur as crystals, but it generally forms loose, rounded aggregates of polygonal or anhedral grains with limited intergrowth. Grain size ranges from 0.5 to 1 mm. Skeletal crystals may be present. Grains often exhibit relicts retaining a cubic structure, which becomes more evident under crossed polars. 

Cleavage is not visible; it only becomes apparent when the mineral is heated above the inversion temperature. 

Polishing scratches, very persistent, are always present and difficult to eliminate due to hessite’s low hardness. 

Fine inversion lamellar twins are almost always present and visible under crossed polars; they form due to the phase change that occurs as the temperature drops. In older literature, these twins are referred to as “lamellar texture.” Twinning is present if the mineral was deposited at high temperatures; it may be absent in low-temperature deposits. 

Mottled extinction may occur due to the presence of relicts retaining a cubic structure. These relicts are isotropic, whereas the monoclinic portions are anisotropic.

Myrmekitic intergrowths with sylvanite may occur. Intergrowths with petzite also occur. 

Very fine veinlets of native gold may occur within hessite aggregates, between the grains. 

Substitutions 1: hessite replaces chalcopyrite, sphalerite, pyrite, native gold, tennantite, and galena. 

Substitutions 2: hessite is replaced by native gold and sylvanite. 

Inclusions 1: hessite occurs as inclusions in petzite and galena, sometimes together with tetradymite. It also occurs as inclusions in pyrite and bornite. 

Inclusions 2: hessite may contain inclusions of gold, silver, pyrite, and altaite. 

Rims of hessite may occur around parkerite. 

Interstitial veinlets of hessite may occur in massive maucherite.