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Basic Ferroan Dolomite Forms

Several distinct ferroan dolomite crystal forms occur at the Lamereaux road bridge locality. The most common of these forms are illustrated in figure I to the right. This is an unusual occurrence, as most Ohio localities will produce no more than one or two crystal forms of the species.

The most regularly observed form in Ohio is the simple rhombohedron, labeled A in figure I. Minute examples of these can be found in abundance lining pockets at many dolostone quarries in Western Ohio. The second most common form known as the curved or saddle-shaped rhombohedron, labeled B in figure I, is commonly found at various localities in the state, most notably the C.E. Duff and Son quarry at Huntsville, in Logan County.

While simple rhombohedrons and curved rhombohedrons are fairly common throughout Ohio, dolomite or ferroan dolomite crystals with additional modifications are extremely rare. Two of these modified forms commonly seen at the Lamereaux road bridge locality are labeled C and D in figure I.

The first of these, illustrated as C, is a positive rhombohedron(a) modified by a negative rhombohedron(b). The second, illustrated as D, has an additional positive rhombohedral face(c). These additional faces vary in size as described later in this article.

Curved Rhombohedral Form

Curved Rhombohedral Form

The classic form for dolomite and ferroan dolomite is the curved rhombohedron or saddle shaped crystal group. Crystals of this type are fairly common throughout the Midwest, including certain Ohio localities. At the Lamereaux road bridge locality, the best curved crystals occur regularly in the 5th generation of ferroan dolomite growth. Since the majority of pockets in septaria do not show this later generation, curved crystals are not as common.

The 4th and the rarely present 6th generations of ferroan dolomite growth also produce micro curved crystals which often create a pseudo-like shell around earlier generations of ferroan dolomite or other minerals. Earlier generations do not tend to produce curved crystals as commonly.

A clear example of a slightly curved crystal is displayed in the photo to the upper left. The cause for this curvature is not fully understood, but the effect is always amplified in larger crystals. The reason for this is that smaller crystals will group themselves at a slightly offset angle to each other to form the larger saddle shaped crystals.

This property is best displayed in the photo to the upper right. Notice the size of the individual rhombohedrons in comparison to the overall size of the saddle shape groups. The pearl-like luster of the species is best seen on these curved crystals when a sheen of light sweeps across the curved faces.

The photo to the lower left exhibits a typical example of this effect. Curved crystals are frequently twinned, and occasionally form individual flower-like crystal groups. These are possibly the most attractive of the ferroan dolomite crystals to be found at the locality. This is especially true when they occur as isolated individuals on earlier generations of ferroan dolomite of a different color. A classic example of this is displayed in the photo to the lower right.

Negative Rhombohedral Modification

negative Rhombohedral Form

As mentioned previously, dolomite or ferroan dolomite crystals with any modification to the rhombohedron are generally rare. One of the modifications which does occur at the Lamereaux road bridge site with some frequency is the negative rhombohedral modification.

This form is illustrated as crystal C in figure I. The ‘b” faces represent the negative rhombohedral modification. In this description, it is assumed that the basic crystal form is a positive rhombohedron. This type of modification adds 6 triangular shaped faces to the main rhombohedron.The photo to the left displays a typical cluster of crystals with the negative rhombohedral modification.

These negative rhombohedral faces are generally crude, rounded and difficult to discern as true faces. It is generally necessary to view these under magnification to identify the modification. A close up of this modification is pictured in the photo to the right. This modification varies in size from very slight to near equal proportion to the main crystal form, but is often distorted by increase in size.

Positive Rhombohedral Modification

The more common modified ferroan dolomite crystal observed at the Lamereaux road bridge locality has both a negative rhombohedral modification and a positive rhombohedral modification. This form is illustrated in figure I as crystal D. While the “b” face represents the negative rhombohedral modification, the “c” face represents the positive rhombohedral modification.

The positive rhombohedral modification adds 6 elongate rhombus shaped faces along the edges of the basic rhombohedron. With the addition of this face and dependant on proportion, the negative rhombohedral modification becomes a five sided face. Like the negative rhombohedral modification, this additional modification varies in size and is generally crude. The photo to the right displays a typical group of ferroan dolomite crystals showing both modifications.

Notice the five sided negative rhombohedral modification. These crystals are from the 3rd generation of ferroan dolomite, as is evident by the rich dark brown color. The 2nd, 3rd and 4th generations tend to produce the most crystals with either modification. No crystals have been observed with the positive rhombohedral modification alone. Ferroan dolomite crystals with additional modifications have not been observed at the locality.

In addition to the 4 basic forms, several distinct habits can be observed in ferroan dolomite crystals from the Lamereaux road bridge locality. Many of these habits are broadly observed in particular generations throughout septaria at the locality, while others appear to be more unique to individual septaria.

It should be noted, that all of the crystals in one pocket of the same generation will typically be of similar form and habit. Figure II to the right displays some of the more common growth habits observed at the locality including convex growth or multiple growth hillocks, concave growth or hopper shape, stacked growth, epitaxial growth, multiple step growth, and twin growth. It is possible for any of the 4 crystal forms to occur in any of these habits.

Couple this with the fact that generations of ferroan dolomite crystals are often affected by the generation which came before and it becomes evident why there are so many varieties of ferroan dolomite to be found at the locality. Other ferroan dolomite habits do exist, but are not as regularly encountered and therefore have not been included in this description.

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