The best website on "Herkimer Diamonds"
This site was last updated May, 2011
Sequence of Mineralization in Herkimer Mining District
Definitions and Scope:
Across the region in New York that hosts "Herkimer Diamonds" there are visible minerals, in addition to the "Herkimer Diamond" quartz. These minerals include dolomite (probably three types), quartz as three different types of druze (coatings of quartz crystals), a black carbon material locally called "anthraxolite" (derived from "degraded oil" and probably of different varieties including anthraxolite, bitumen and pyrobitumen), clay, two sulfide events (pyrite, marcasite, chalcopyrite, sphalerite and galena), and three calcite events (in a variety of forms). These minerals got into the Herkimer host rock because fluids containing disolved chemicals entered the rock and the crystals grew out of those fluids.
The fluids that moved through the Herkimer host rocks left behind "clues" about their behavior. These clues can be found through a careful examination of all the minerals in relation to each other. A history of these fluid events, and the associated mineral formation processes, helps us to understand how Herkimer Diamond deposits were formed. The diagram below is an approximation of the regional events and it is updated when new information is made available (check the date at the top of this page). This mineral sequence has not been described in this much detail in any publication. Please reference this author, W. David Hoisington, if you use this information.
The above diagram is only a GENERAL representation for the entire district. Each of the Herkimer Diamond deposits has its own mineral sequence diagram that is slightly different from the above generalized diagram. Return to the home page (button is at the top of this page will take you back to the home page where you can find the list of Herkimer deposits) to choose which deposit diagram you would like to see (research on this is ongoing). Each Herkimer deposit mineral sequence diagram is like a "fingerprint", a unique signature for that specific deposit. It represents the differences that collectors have seen traveling from one deposit to another. The causes for these differences should be part of a theory description explaining how Herkimer diamond deposits were formed.
A public service project always open to input from the community interested in Herkimer diamonds.
If you would like to see how the signature mineral sequence diagram of a Herkimer deposit is different from the generalized diagram just click below.
Web Diagram of the General Mineralization Sequence for the Herkimer Mining District*
Each named "block" (also underlined if there is a page link) below represents an episode of fluid movement through the Herkimer host rock. Each episode changed the appearence of mineral specimens found, but not all episodes can be found in one specimen. In addition each deposit (or mine) shows the effects of these episodes differently. The "blocks" are stacked with the oldest episode at the bottom and youngest at the top. The blocks have different lengths and this represents duration of the episode. When the lengths of the "blocks" are overlapping it represents an approximation of time when the two episodes were occuring together.
You can click on any named mineral episode "block" in this web diagram and see photos with comments. The blocks are mineral episodes ->->
Please help with picture and sample donations!
We can not complete this research without your help. Please send us a photo, or sample, of any interesting or beautiful mineral feature that you find in the Herkimer District.
First Dolomite clear (small)
Notes about the Above Sequence:
The above mineral sequence diagram represents the most common mineral sequence for the entire Herkimer diamond mine district. It should be considered as an "average" representation. Some notes for the various mineral phases are given below:
First dolomite: This is likely the phase that changed the rock from limestone to dolostone. The crystals are small rhombs and are the most common feature at every Herkimer deposit. When unaltered they are clear or a light beige. The common form can be weathered, rounded, or altered to a tan or light brown.
Second dolomite: This is the "large dolomite" that occurs in some cavities in every deposit. When fresh it is shiny white, sometimes in saddle form. The common form is weathered or altered to a tan or beige.
Early Sulfides: This phase is often of very small pyrite, marcasite and chalcopyrite. This author has seen "ruby" sphalerite from HDM and HBQ. Galena has been reported from the district. Because of the heavy weathering these minerals are often weathered to ochre, resulting is a rust coating over near by minerals. This rust coating is different than original mineral features that impart a similar color. Blue coating from the weathering of the chalocpyrite has also been seen (at TCR and HBQ).
single clear rhombs that are often microscopic
thin coatings and sheets of microsocpic crystals, also rods, plates and tubes
white, clear edges, lusterous, sometimes in weak saddle form
pink, peach, red-brown, black, frequently extreme saddle form
white, pyramidal, often has second quartz coating
clear, smoky, pyramidal or small prismatic (see notes below)
very small Herkimer diamonds, often microscopic
these two often occur together
long prismatic crystals that are usually single terminated
needle like marcasite crystals
larger quartz crystals in a variety of forms
a feature seen in many pockets
nailhead in a variety of forms
Late druze: This is the most common druze in the district and it doesn't form a thick rock contact, but it has a thin contact "rind" (probably responsible for what is called "snowball druze"). This druze is generally clear to smoky, pyramidal (little mountains). Often it has black inclusions.
Early druze: This white druze, pyramidal, often forms the base for the later druze episodes. It is characterized by having a rock contact "rind" that is a clearly visible, often with microsopic laminations.
Baby floater druze: This is a coating of small to microscopic Herkimer diamonds. It has almost no contact "rind" with the host rock. This druze is probably more common in the district than has been realized previously, but weathering has made it hard to find. It is found over the top of, and mixed in with, the late prismatic druze. It is also found as inclusions in Herkimer diamonds, sometimes with "melted" edges. The fact that it is absent in some larger crystal pockets may be due to weathering, as these tiny crystals are found in the "screened mud" of most pockets. It is likely that there is a close association of this baby floater phase with the formation of Herkimer diamonds.
First calcite: This is the most common calcite in the district. The difficulty is that the typical "nailhead" (disk shapped) form can be modified. Only part of the disk may appear, looking at just the edge of the crystal. It can be elongated out from the middle of the crystal, making a fatter disc, and sometimes it can be stretched making elongated prismatic crystals. First calcite as rhombs seems to be more rare for the district. Scalenohedrons are very rare.
Prismatic druze quartz: This phase seems to occur in every Herkimer mine the district, but it has a large variation from mine to mine. It frequently has black inclusions, and sometimes a brown film over phantoms. It can also show zonation on cross-section or have a Herkimer diamond growth over the top, making a scepter. This prismatic druze does occur interwined with the Baby floater druze at Ace of Diamonds where the earlier druze is absent. This reults in a unique looking druze. As with the Baby floater druze there is almost no contact rind. Some times this phase alters the previous quartz phases and may include hydrothermal alteration (like at the TCR location) that leaches the host rock turning it into a very light rock that is only a skeleton of quartz veins.
Web page author - W. David Hoisington, Ph.D.
Breccia features (fractured rock) occur before many of the mineral episodes and this has yet to be investigated in detail.
Third calcite: This is not easy to recognize until you see it for the first time. It occurs a rounded bumps and it can be as a coating over any earlier mineral. Weathered it is very pitted, nearly unrecognizable.
There is a final weathering event that has, in some places, severely altered the minerals listed above.
Herkimer Diamonds: The more this research project unfolds the more we discover that there are subtle variations in the crystal habits of Herkimer diamonds across the district. What has been common knowledge is that there is a wide range of shape, clarity, size, intergrowth growth patterns and skeletal features. But as you will see on this website the diversity is surprising, and more research is need to help explain this diversity.
Second sulphides: These are basically marcasite (iron and sulphur). Despite the intense weathering that turns these into rust, they are frequently preserved inside first phase calcite crystals as needle sprays. Where the weathing is not severe (and people have not scrubbed them away) they can be found coating second and third phase dolomite. They can also occur inside Herkimer diamonds.
Third dolomite: This also "large dolomite" that occurs in some cavities in every deposit, and it can also be tan from weathering, but inside it will be often be black, not white, or pale tan. It can also weather to a peach or even brick red color. The black dolomite is seldom seen in its unaltered state. It may sometime be found as pink - but is a strong saddle form.
*A map showing the location of the district can be found on the front page.
Important Note: The minerals listed in the above sequence occur in zones, or layers. For a discussion of this click on the following link:
You can compare these diagrams to each other, and to the general diagram and look to see the differences. This is what makes each Herkimer diamond deposit unique, and also similar.
Hydrocarbon/Clay: There is a major hydrocarbon event that is in place before the Herkimer diamond event. The amount of associated white/pale yellow clay (not brown mud) varies across the district. The intensity of this event varies across a single deposit, but it is consided to be one of the critical events in forming the Herkimer diamond crystals.
Early Hydrocarbon: There appears to be an early hydrocarbon event that overlaps with both the late druze and the prismatic quartz events. Its intensity varies across the district giving rise to different features.
overlaping with the druze events
Second calcite: This calcite is easy to recognize once you have seen how different it is in form from the first phase calcite. It occurs as small clusters and fans of crystals ranging from white to amber.
This theory about the mineral sequence, with all the supportive documentation on this website, is not published elsewhere. Please reference the author, date (top of the page), and the website.